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OPENVPN(8) System Manager's Manual OPENVPN(8)

NAME
openvpn - Secure IP tunnel daemon

SYNOPSIS
openvpn [ options ... ]
openvpn --help

INTRODUCTION
OpenVPN is an open source VPN daemon by James Yonan. Because OpenVPN
tries to be a universal VPN tool offering a great deal of flexibility,
there are a lot of options on this manual page. If you're new to Open-
VPN, you might want to skip ahead to the examples section where you
will see how to construct simple VPNs on the command line without even
needing a configuration file.

Also note that there's more documentation and examples on the OpenVPN
web site: https://openvpn.net/

And if you would like to see a shorter version of this manual, see the
openvpn usage message which can be obtained by running openvpn without
any parameters.

DESCRIPTION
OpenVPN is a robust and highly flexible VPN daemon. OpenVPN supports
SSL/TLS security, ethernet bridging, TCP or UDP tunnel transport
through proxies or NAT, support for dynamic IP addresses and DHCP,
scalability to hundreds or thousands of users, and portability to most
major OS platforms.

OpenVPN is tightly bound to the OpenSSL library, and derives much of
its crypto capabilities from it.

OpenVPN supports conventional encryption using a pre-shared secret key
(Static Key mode) or public key security (SSL/TLS mode) using client &
server certificates. OpenVPN also supports non-encrypted TCP/UDP tun-
nels.

OpenVPN is designed to work with the TUN/TAP virtual networking inter-
face that exists on most platforms.

Overall, OpenVPN aims to offer many of the key features of IPSec but
with a relatively lightweight footprint.

OPTIONS
OpenVPN allows any option to be placed either on the command line or in
a configuration file. Though all command line options are preceded by a
double-leading-dash ("--"), this prefix can be removed when an option
is placed in a configuration file.

Generic Options
This section covers generic options which are accessible regardless of
which mode OpenVPN is configured as.

--help Show options.

--auth-nocache
Don't cache --askpass or --auth-user-pass username/passwords in
virtual memory.

If specified, this directive will cause OpenVPN to immediately
forget username/password inputs after they are used. As a
result, when OpenVPN needs a username/password, it will prompt
for input from stdin, which may be multiple times during the
duration of an OpenVPN session.

When using --auth-nocache in combination with a user/password
file and --chroot or --daemon, make sure to use an absolute
path.

This directive does not affect the --http-proxy username/pass-
word. It is always cached.

--cd dir
Change directory to dir prior to reading any files such as con-
figuration files, key files, scripts, etc. dir should be an
absolute path, with a leading "/", and without any references to
the current directory such as . or ...

This option is useful when you are running OpenVPN in --daemon
mode, and you want to consolidate all of your OpenVPN control
files in one location.

--chroot dir
Chroot to dir after initialization. --chroot essentially rede-
fines dir as being the top level directory tree (/). OpenVPN
will therefore be unable to access any files outside this tree.
This can be desirable from a security standpoint.

Since the chroot operation is delayed until after initializa-
tion, most OpenVPN options that reference files will operate in
a pre-chroot context.

In many cases, the dir parameter can point to an empty direc-
tory, however complications can result when scripts or restarts
are executed after the chroot operation.

Note: The SSL library will probably need /dev/urandom to be
available inside the chroot directory dir. This is because SSL
libraries occasionally need to collect fresh randomness. Newer
linux kernels and some BSDs implement a getrandom() or geten-
tropy() syscall that removes the need for /dev/urandom to be
available.

--compat-mode version
This option provides a convenient way to alter the defaults of
OpenVPN to be more compatible with the version version speci-
fied. All of the changes this option applies can also be
achieved using individual configuration options.

The version specified with this option is the version of OpenVPN
peer OpenVPN should try to be compatible with. In general Open-
VPN should be compatible with the last two previous version
without this option. E.g. OpenVPN 2.6.0 should be compatible
with 2.5.x and 2.4.x without this option. However, there might
be some edge cases that still require this option even in these
cases.

Note: Using this option reverts defaults to no longer recom-
mended values and should be avoided if possible.

The following table details what defaults are changed depending
on the version specified.

o 2.5.x or lower: --allow-compression asym is automatically
added to the configuration if no other compression options are
present.

o 2.4.x or lower: The cipher in --cipher is appended to
--data-ciphers.

o 2.3.x or lower: --data-cipher-fallback is automatically added
with the same cipher as --cipher.

o 2.3.6 or lower: --tls-version-min 1.0 is added to the configu-
ration when --tls-version-min is not explicitly set.

If not required, this is option should be avoided. Setting this
option can lower security or disable features like data-channel
offloading.

--config file
Load additional config options from file where each line corre-
sponds to one command line option, but with the leading --
removed.

If --config file is the only option to the openvpn command, the
--config can be removed, and the command can be given as openvpn
file

Note that configuration files can be nested to a reasonable
depth.

Double quotation or single quotation characters ("", '') can be
used to enclose single parameters containing whitespace, and "#"
or ";" characters in the first column can be used to denote com-
ments.

Note that OpenVPN 2.0 and higher performs backslash-based shell
escaping for characters not in single quotations, so the follow-
ing mappings should be observed:

\\ Maps to a single backslash character (\).
\" Pass a literal doublequote character ("), don't
interpret it as enclosing a parameter.
\[SPACE] Pass a literal space or tab character, don't
interpret it as a parameter delimiter.

For example on Windows, use double backslashes to represent
pathnames:

secret "c:\\OpenVPN\\secret.key"

For examples of configuration files, see
https://openvpn.net/community-resources/how-to/

Here is an example configuration file:

#
# Sample OpenVPN configuration file for
# using a pre-shared static key.
#
# '#' or ';' may be used to delimit comments.

# Use a dynamic tun device.
dev tun

# Our remote peer
remote mypeer.mydomain

# 10.1.0.1 is our local VPN endpoint
# 10.1.0.2 is our remote VPN endpoint
ifconfig 10.1.0.1 10.1.0.2

# Our pre-shared static key
secret static.key

--daemon progname
Become a daemon after all initialization functions are com-
pleted.

Valid syntaxes:

daemon
daemon progname

This option will cause all message and error output to be sent
to the syslog file (such as /var/log/messages), except for the
output of scripts and ifconfig commands, which will go to
/dev/null unless otherwise redirected. The syslog redirection
occurs immediately at the point that --daemon is parsed on the
command line even though the daemonization point occurs later.
If one of the --log options is present, it will supersede syslog
redirection.

The optional progname parameter will cause OpenVPN to report its
program name to the system logger as progname. This can be use-
ful in linking OpenVPN messages in the syslog file with specific
tunnels. When unspecified, progname defaults to openvpn.

When OpenVPN is run with the --daemon option, it will try to
delay daemonization until the majority of initialization func-
tions which are capable of generating fatal errors are complete.
This means that initialization scripts can test the return sta-
tus of the openvpn command for a fairly reliable indication of
whether the command has correctly initialized and entered the
packet forwarding event loop.

In OpenVPN, the vast majority of errors which occur after ini-
tialization are non-fatal.

Note: as soon as OpenVPN has daemonized, it can not ask for
usernames, passwords, or key pass phrases anymore. This has cer-
tain consequences, namely that using a password-protected pri-
vate key will fail unless the --askpass option is used to tell
OpenVPN to ask for the pass phrase (this requirement is new in
v2.3.7, and is a consequence of calling daemon() before initial-
izing the crypto layer).

Further, using --daemon together with --auth-user-pass (entered
on console) and --auth-nocache will fail as soon as key renego-
tiation (and reauthentication) occurs.

--disable-dco
Disable "data channel offload" (DCO).

On Linux don't use the ovpn-dco device driver, but rather rely
on the legacy tun module.

You may want to use this option if your server needs to allow
clients older than version 2.4 to connect.

--disable-occ
DEPRECATED Disable "options consistency check" (OCC) in configu-
rations that do not use TLS.

Don't output a warning message if option inconsistencies are
detected between peers. An example of an option inconsistency
would be where one peer uses --dev tun while the other peer uses
--dev tap.

Use of this option is discouraged, but is provided as a tempo-
rary fix in situations where a recent version of OpenVPN must
connect to an old version.

--engine engine-name
Enable OpenSSL hardware-based crypto engine functionality.

Valid syntaxes:

engine
engine engine-name

If engine-name is specified, use a specific crypto engine. Use
the --show-engines standalone option to list the crypto engines
which are supported by OpenSSL.

--fast-io
(Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding a
call to poll/epoll/select prior to the write operation. The pur-
pose of such a call would normally be to block until the device
or socket is ready to accept the write. Such blocking is unnec-
essary on some platforms which don't support write blocking on
UDP sockets or TUN/TAP devices. In such cases, one can optimize
the event loop by avoiding the poll/epoll/select call, improving
CPU efficiency by 5% to 10%.

This option can only be used on non-Windows systems, when
--proto udp is specified, and when --shaper is NOT specified.

--group group
Similar to the --user option, this option changes the group ID
of the OpenVPN process to group after initialization.

--ignore-unknown-option args
Valid syntax:

ignore-unknown-options opt1 opt2 opt3 ... optN

When one of options opt1 ... optN is encountered in the configu-
ration file the configuration file parsing does not fail if this
OpenVPN version does not support the option. Multiple
--ignore-unknown-option options can be given to support a larger
number of options to ignore.

This option should be used with caution, as there are good secu-
rity reasons for having OpenVPN fail if it detects problems in a
config file. Having said that, there are valid reasons for
wanting new software features to gracefully degrade when encoun-
tered by older software versions.

--ignore-unknown-option is available since OpenVPN 2.3.3.

--iproute cmd
Set alternate command to execute instead of default iproute2
command. May be used in order to execute OpenVPN in unprivi-
leged environment.

--keying-material-exporter args
Save Exported Keying Material [RFC5705] of len bytes (must be
between 16 and 4095 bytes) using label in environment
(exported_keying_material) for use by plugins in OPENVPN_PLUG-
IN_TLS_FINAL callback.

Valid syntax:

keying-material-exporter label len

Note that exporter labels have the potential to collide with
existing PRF labels. In order to prevent this, labels MUST begin
with EXPORTER.

--mlock
Disable paging by calling the POSIX mlockall function. Requires
that OpenVPN be initially run as root (though OpenVPN can subse-
quently downgrade its UID using the --user option).

Using this option ensures that key material and tunnel data are
never written to disk due to virtual memory paging operations
which occur under most modern operating systems. It ensures that
even if an attacker was able to crack the box running OpenVPN,
he would not be able to scan the system swap file to recover
previously used ephemeral keys, which are used for a period of
time governed by the --reneg options (see below), then are dis-
carded.

The downside of using --mlock is that it will reduce the amount
of physical memory available to other applications.

The limit on how much memory can be locked and how that limit is
enforced are OS-dependent. On Linux the default limit that an
unprivileged process may lock (RLIMIT_MEMLOCK) is low, and if
privileges are dropped later, future memory allocations will
very likely fail. The limit can be increased using ulimit or
systemd directives depending on how OpenVPN is started.

If the platform has the getrlimit(2) system call, OpenVPN will
check for the amount of mlock-able memory before calling mlock-
all(2), and tries to increase the limit to 100 MB if less than
this is configured. 100 Mb is somewhat arbitrary - it is enough
for a moderately-sized OpenVPN deployment, but the memory usage
might go beyond that if the number of concurrent clients is
high.

--nice n
Change process priority after initialization (n greater than 0
is lower priority, n less than zero is higher priority).

--persist-key
Don't re-read key files across SIGUSR1 or --ping-restart.

This option can be combined with --user to allow restarts trig-
gered by the SIGUSR1 signal. Normally if you drop root privi-
leges in OpenVPN, the daemon cannot be restarted since it will
now be unable to re-read protected key files.

This option solves the problem by persisting keys across SIGUSR1
resets, so they don't need to be re-read.

--providers providers
Load the list of (OpenSSL) providers. This is mainly useful for
using an external provider for key management like tpm2-openssl
or to load the legacy provider with

--providers legacy default

Behaviour of changing this option between SIGHUP might not be
well behaving. If you need to change/add/remove this option,
fully restart OpenVPN.

--remap-usr1 signal
Control whether internally or externally generated SIGUSR1 sig-
nals are remapped to SIGHUP (restart without persisting state)
or SIGTERM (exit).

signal can be set to SIGHUP or SIGTERM. By default, no remapping
occurs.

--script-security level
This directive offers policy-level control over OpenVPN's usage
of external programs and scripts. Lower level values are more
restrictive, higher values are more permissive. Settings for
level:

0 Strictly no calling of external programs.

1 (Default) Only call built-in executables such as ifcon-
fig, ip, route, or netsh.

2 Allow calling of built-in executables and user-defined
scripts.

3 Allow passwords to be passed to scripts via environmental
variables (potentially unsafe).

OpenVPN releases before v2.3 also supported a method flag which
indicated how OpenVPN should call external commands and scripts.
This could be either execve or system. As of OpenVPN 2.3, this
flag is no longer accepted. In most *nix environments the
execve() approach has been used without any issues.

Some directives such as --up allow options to be passed to the
external script. In these cases make sure the script name does
not contain any spaces or the configuration parser will choke
because it can't determine where the script name ends and script
options start.

To run scripts in Windows in earlier OpenVPN versions you needed
to either add a full path to the script interpreter which can
parse the script or use the system flag to run these scripts. As
of OpenVPN 2.3 it is now a strict requirement to have full path
to the script interpreter when running non-executables files.
This is not needed for executable files, such as .exe, .com,
.bat or .cmd files. For example, if you have a Visual Basic
script, you must use this syntax now:

--up 'C:\\Windows\\System32\\wscript.exe C:\\Program\ Files\\OpenVPN\\config\\my-up-script.vbs'

Please note the single quote marks and the escaping of the back-
slashes (\) and the space character.

The reason the support for the system flag was removed is due to
the security implications with shell expansions when executing
scripts via the system() call.

--setcon context
Apply SELinux context after initialization. This essentially
provides the ability to restrict OpenVPN's rights to only net-
work I/O operations, thanks to SELinux. This goes further than
--user and --chroot in that those two, while being great secu-
rity features, unfortunately do not protect against privilege
escalation by exploitation of a vulnerable system call. You can
of course combine all three, but please note that since setcon
requires access to /proc you will have to provide it inside the
chroot directory (e.g. with mount --bind).

Since the setcon operation is delayed until after initializa-
tion, OpenVPN can be restricted to just network-related system
calls, whereas by applying the context before startup (such as
the OpenVPN one provided in the SELinux Reference Policies) you
will have to allow many things required only during initializa-
tion.

Like with chroot, complications can result when scripts or
restarts are executed after the setcon operation, which is why
you should really consider using the --persist-key and --per-
sist-tun options.

--status args
Write operational status to file every n seconds. n defaults to
60 if not specified.

Valid syntaxes:

status file
status file n

Status can also be written to the syslog by sending a SIGUSR2
signal.

With multi-client capability enabled on a server, the status
file includes a list of clients and a routing table. The output
format can be controlled by the --status-version option in that
case.

For clients or instances running in point-to-point mode, it will
contain the traffic statistics.

--status-version n
Set the status file format version number to n.

This only affects the status file on servers with multi-client
capability enabled. Valid status version values:

1 Traditional format (default). The client list contains
the following fields comma-separated: Common Name, Real
Address, Bytes Received, Bytes Sent, Connected Since.

2 A more reliable format for external processing. Compared
to version 1, the client list contains some additional
fields: Virtual Address, Virtual IPv6 Address, Username,
Client ID, Peer ID, Data Channel Cipher. Future versions
may extend the number of fields.

3 Identical to 2, but fields are tab-separated.

--test-crypto
Do a self-test of OpenVPN's crypto options by encrypting and
decrypting test packets using the data channel encryption
options specified above. This option does not require a peer to
function, and therefore can be specified without --dev or
--remote.

The typical usage of --test-crypto would be something like this:

openvpn --test-crypto --secret key

openvpn --test-crypto --secret key --verb 9

This option is very useful to test OpenVPN after it has been
ported to a new platform, or to isolate problems in the com-
piler, OpenSSL crypto library, or OpenVPN's crypto code. Since
it is a self-test mode, problems with encryption and authentica-
tion can be debugged independently of network and tunnel issues.

--tmp-dir dir
Specify a directory dir for temporary files. This directory will
be used by openvpn processes and script to communicate temporary
data with openvpn main process. Note that the directory must be
writable by the OpenVPN process after it has dropped it's root
privileges.

This directory will be used by in the following cases:

o --client-connect scripts and OPENVPN_PLUGIN_CLIENT_CONNECT
plug-in hook to dynamically generate client-specific configu-
ration client_connect_config_file and return success/failure
via client_connect_deferred_file when using deferred client
connect method

o OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plug-in hooks returns
success/failure via auth_control_file when using deferred auth
method and pending authentication via pending_auth_file.

--use-prediction-resistance
Enable prediction resistance on mbed TLS's RNG.

Enabling prediction resistance causes the RNG to reseed in each
call for random. Reseeding this often can quickly deplete the
kernel entropy pool.

If you need this option, please consider running a daemon that
adds entropy to the kernel pool.

--user user
Change the user ID of the OpenVPN process to user after initial-
ization, dropping privileges in the process. This option is use-
ful to protect the system in the event that some hostile party
was able to gain control of an OpenVPN session. Though OpenVPN's
security features make this unlikely, it is provided as a second
line of defense.

By setting user to an unprivileged user dedicated to run open-
vpn, the hostile party would be limited in what damage they
could cause. Of course once you take away privileges, you cannot
return them to an OpenVPN session. This means, for example, that
if you want to reset an OpenVPN daemon with a SIGUSR1 signal
(for example in response to a DHCP reset), you should make use
of one or more of the --persist options to ensure that OpenVPN
doesn't need to execute any privileged operations in order to
restart (such as re-reading key files or running ifconfig on the
TUN device).

NOTE: Previous versions of openvpn used nobody as the example
unpriviledged user. It is not recommended to actually use that
user since it is usually used by other system services already.
Always create a dedicated user for openvpn.

--writepid file
Write OpenVPN's main process ID to file.

Log options
--echo parms
Echo parms to log output.

Designed to be used to send messages to a controlling applica-
tion which is receiving the OpenVPN log output.

--errors-to-stderr
Output errors to stderr instead of stdout unless log output is
redirected by one of the --log options.

--log file
Output logging messages to file, including output to std-
out/stderr which is generated by called scripts. If file already
exists it will be truncated. This option takes effect immedi-
ately when it is parsed in the command line and will supersede
syslog output if --daemon is also specified. This option is per-
sistent over the entire course of an OpenVPN instantiation and
will not be reset by SIGHUP, SIGUSR1, or --ping-restart.

Note that on Windows, when OpenVPN is started as a service, log-
ging occurs by default without the need to specify this option.

--log-append file
Append logging messages to file. If file does not exist, it
will be created. This option behaves exactly like --log except
that it appends to rather than truncating the log file.

--machine-readable-output
Always write timestamps and message flags to log messages, even
when they otherwise would not be prefixed. In particular, this
applies to log messages sent to stdout.

--mute n
Log at most n consecutive messages in the same category. This is
useful to limit repetitive logging of similar message types.

--mute-replay-warnings
Silence the output of replay warnings, which are a common false
alarm on WiFi networks. This option preserves the security of
the replay protection code without the verbosity associated with
warnings about duplicate packets.

--suppress-timestamps
Avoid writing timestamps to log messages, even when they other-
wise would be prepended. In particular, this applies to log mes-
sages sent to stdout.

--syslog progname
Direct log output to system logger, but do not become a daemon.
See --daemon directive above for description of progname parame-
ter.

--verb n
Set output verbosity to n (default 1). Each level shows all info
from the previous levels. Level 3 is recommended if you want a
good summary of what's happening without being swamped by out-
put.

0 No output except fatal errors.

1 to 4 Normal usage range.

5 Outputs R and W characters to the console for each packet
read and write, uppercase is used for TCP/UDP packets and
lowercase is used for TUN/TAP packets.

6 to 11
Debug info range (see errlevel.h in the source code for
additional information on debug levels).

Protocol options
Options in this section affect features available in the OpenVPN wire
protocol. Many of these options also define the encryption options of
the data channel in the OpenVPN wire protocol. These options must be
configured in a compatible way between both the local and remote side.

--allow-compression mode
As described in the --compress option, compression is a poten-
tially dangerous option. This option allows controlling the be-
haviour of OpenVPN when compression is used and allowed.

Valid syntaxes:

allow-compression
allow-compression mode

The mode argument can be one of the following values:

asym OpenVPN will only decompress downlink packets but not
compress uplink packets. This also allows migrating to
disable compression when changing both server and client
configurations to remove compression at the same time is
not a feasible option.

no (default)
OpenVPN will refuse any compression. If data-channel
offloading is enabled, OpenVPN will additionally also
refuse compression framing (stub).

yes OpenVPN will send and receive compressed packets.

--auth alg
Authenticate data channel packets and (if enabled) tls-auth con-
trol channel packets with HMAC using message digest algorithm
alg. (The default is SHA1 ). HMAC is a commonly used message
authentication algorithm (MAC) that uses a data string, a secure
hash algorithm and a key to produce a digital signature.

The OpenVPN data channel protocol uses encrypt-then-mac (i.e.
first encrypt a packet then HMAC the resulting ciphertext),
which prevents padding oracle attacks.

If an AEAD cipher mode (e.g. GCM) is chosen then the specified
--auth algorithm is ignored for the data channel and the authen-
tication method of the AEAD cipher is used instead. Note that
alg still specifies the digest used for tls-auth.

In static-key encryption mode, the HMAC key is included in the
key file generated by --genkey. In TLS mode, the HMAC key is
dynamically generated and shared between peers via the TLS con-
trol channel. If OpenVPN receives a packet with a bad HMAC it
will drop the packet. HMAC usually adds 16 or 20 bytes per
packet. Set alg=none to disable authentication.

For more information on HMAC see
http://www.cs.ucsd.edu/users/mihir/papers/hmac.html

--cipher alg
This option should not be used any longer in TLS mode and still
exists for two reasons:

o compatibility with old configurations still carrying it
around;

o allow users connecting to OpenVPN peers older than 2.6.0 to
have --cipher configured the same way as the remote counter-
part. This can avoid MTU/frame size warnings.

Before 2.4.0, this option was used to select the cipher to be
configured on the data channel, however, later versions usually
ignored this directive in favour of a negotiated cipher. Start-
ing with 2.6.0, this option is always ignored in TLS mode when
it comes to configuring the cipher and will only control the
cipher for --secret pre-shared-key mode (note: this mode is dep-
recated and strictly not recommended).

If you wish to specify the cipher to use on the data channel,
please see --data-ciphers (for regular negotiation) and
--data-ciphers-fallback (for a fallback option when the negotia-
tion cannot take place because the other peer is old or has
negotiation disabled).

To see ciphers that are available with OpenVPN, use the
--show-ciphers option.

Set alg to none to disable encryption.

--compress algorithm
DEPRECATED Enable a compression algorithm. Compression is gener-
ally not recommended. VPN tunnels which use compression are sus-
ceptible to the VORALCE attack vector. See also the migrate
parameter below.

The algorithm parameter may be lzo, lz4, lz4-v2, stub, stub-v2,
migrate or empty. LZO and LZ4 are different compression algo-
rithms, with LZ4 generally offering the best performance with
least CPU usage.

The lz4-v2 and stub-v2 variants implement a better framing that
does not add overhead when packets cannot be compressed. All
other variants always add one extra framing byte compared to no
compression framing.

Especially stub-v2 is essentially identical to no compression
and no compression framing as its header indicates IP version 5
in a tun setup and can (ab)used to complete disable compression
to clients. (See the migrate option below)

If the algorithm parameter is stub, stub-v2 or empty, compres-
sion will be turned off, but the packet framing for compression
will still be enabled, allowing a different setting to be pushed
later. Additionally, stub and stub-v2 wil disable announcing
lzo and lz4 compression support via IV_ variables to the server.

Note: the stub (or empty) option is NOT compatible with the
older option --comp-lzo no.

Using migrate as compression algorithm enables a special migra-
tion mode. It allows migration away from the --com-
press/--comp-lzo options to no compression. This option sets
the server to no compression mode and the server behaves identi-
cal to a server without a compression option for all clients
without a compression in their config. However, if a client is
detected that indicates that compression is used (via OCC), the
server will automatically add --push compress stub-v2 to the
client specific configuration if supported by the client and
otherwise switch to comp-lzo no and add --push comp-lzo to the
client specific configuration.

*Security Considerations*

Compression and encryption is a tricky combination. If an
attacker knows or is able to control (parts of) the plain-text
of packets that contain secrets, the attacker might be able to
extract the secret if compression is enabled. See e.g. the CRIME
and BREACH attacks on TLS and VORACLE on VPNs which also lever-
age to break encryption. If you are not entirely sure that the
above does not apply to your traffic, you are advised to not
enable compression.

--comp-lzo mode
DEPRECATED Enable LZO compression algorithm. Compression is
generally not recommended. VPN tunnels which uses compression
are suspectible to the VORALCE attack vector.

Use LZO compression -- may add up to 1 byte per packet for
incompressible data. mode may be yes, no, or adaptive (default).

In a server mode setup, it is possible to selectively turn com-
pression on or off for individual clients.

First, make sure the client-side config file enables selective
compression by having at least one --comp-lzo directive, such as
--comp-lzo no. This will turn off compression by default, but
allow a future directive push from the server to dynamically
change the on/off/adaptive setting.

Next in a --client-config-dir file, specify the compression set-
ting for the client, for example:

comp-lzo yes
push "comp-lzo yes"

The first line sets the comp-lzo setting for the server side of
the link, the second sets the client side.

--comp-noadapt
DEPRECATED When used in conjunction with --comp-lzo, this option
will disable OpenVPN's adaptive compression algorithm. Normally,
adaptive compression is enabled with --comp-lzo.

Adaptive compression tries to optimize the case where you have
compression enabled, but you are sending predominantly incom-
pressible (or pre-compressed) packets over the tunnel, such as
an FTP or rsync transfer of a large, compressed file. With adap-
tive compression, OpenVPN will periodically sample the compres-
sion process to measure its efficiency. If the data being sent
over the tunnel is already compressed, the compression effi-
ciency will be very low, triggering openvpn to disable compres-
sion for a period of time until the next re-sample test.

--key-direction
Alternative way of specifying the optional direction parameter
for the --tls-auth and --secret options. Useful when using
inline files (See section on inline files).

--data-ciphers cipher-list
Restrict the allowed ciphers to be negotiated to the ciphers in
cipher-list. cipher-list is a colon-separated list of ciphers,
and defaults to AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305 when
Chacha20-Poly1305 is available and otherwise
AES-256-GCM:AES-128-GCM.

For servers, the first cipher from cipher-list that is also sup-
ported by the client will be pushed to clients that support
cipher negotiation.

For more details see the chapter on Data channel cipher negotia-
tion. Especially if you need to support clients with OpenVPN
versions older than 2.4!

Starting with OpenVPN 2.6 a cipher can be prefixed with a ? to
mark it as optional. This allows including ciphers in the list
that may not be available on all platforms. E.g.
AES-256-GCM:AES-128-GCM:?CHACHA20-POLY1305 would only enable
Chacha20-Poly1305 if the underlying SSL library (and its config-
uration) supports it.

Cipher negotiation is enabled in client-server mode only. I.e.
if --mode is set to server (server-side, implied by setting
--server ), or if --pull is specified (client-side, implied by
setting --client).

If no common cipher is found during cipher negotiation, the con-
nection is terminated. To support old clients/old servers that
do not provide any cipher negotiation support see
--data-ciphers-fallback.

If --compat-mode is set to a version older than 2.5.0 the cipher
specified by --cipher will be appended to --data-ciphers if not
already present.

This list is restricted to be 127 chars long after conversion to
OpenVPN ciphers.

This option was called --ncp-ciphers in OpenVPN 2.4 but has been
renamed to --data-ciphers in OpenVPN 2.5 to more accurately
reflect its meaning.

--data-ciphers-fallback alg
Configure a cipher that is used to fall back to if we could not
determine which cipher the peer is willing to use.

This option should only be needed to connect to peers that are
running OpenVPN 2.3 or older versions, and have been configured
with --enable-small (typically used on routers or other embedded
devices).

--secret args
DEPRECATED Enable Static Key encryption mode (non-TLS). Use
pre-shared secret file which was generated with --genkey.

Valid syntaxes:

secret file
secret file direction

The optional direction parameter enables the use of 4 distinct
keys (HMAC-send, cipher-encrypt, HMAC-receive, cipher-decrypt),
so that each data flow direction has a different set of HMAC and
cipher keys. This has a number of desirable security properties
including eliminating certain kinds of DoS and message replay
attacks.

When the direction parameter is omitted, 2 keys are used bidi-
rectionally, one for HMAC and the other for encryption/decryp-
tion.

The direction parameter should always be complementary on either
side of the connection, i.e. one side should use 0 and the other
should use 1, or both sides should omit it altogether.

The direction parameter requires that file contains a 2048 bit
key. While pre-1.5 versions of OpenVPN generate 1024 bit key
files, any version of OpenVPN which supports the direction
parameter, will also support 2048 bit key file generation using
the --genkey option.

Static key encryption mode has certain advantages, the primary
being ease of configuration.

There are no certificates or certificate authorities or compli-
cated negotiation handshakes and protocols. The only requirement
is that you have a pre-existing secure channel with your peer
(such as ssh) to initially copy the key. This requirement, along
with the fact that your key never changes unless you manually
generate a new one, makes it somewhat less secure than TLS mode
(see below). If an attacker manages to steal your key, every-
thing that was ever encrypted with it is compromised. Contrast
that to the perfect forward secrecy features of TLS mode (using
Diffie Hellman key exchange), where even if an attacker was able
to steal your private key, he would gain no information to help
him decrypt past sessions.

Another advantageous aspect of Static Key encryption mode is
that it is a handshake-free protocol without any distinguishing
signature or feature (such as a header or protocol handshake
sequence) that would mark the ciphertext packets as being gener-
ated by OpenVPN. Anyone eavesdropping on the wire would see
nothing but random-looking data.

--tran-window n
Transition window -- our old key can live this many seconds
after a new a key renegotiation begins (default 3600 seconds).
This feature allows for a graceful transition from old to new
key, and removes the key renegotiation sequence from the criti-
cal path of tunnel data forwarding.

Client Options
The client options are used when connecting to an OpenVPN server con-
figured to use --server, --server-bridge, or --mode server in its con-
figuration.

--allow-pull-fqdn
Allow client to pull DNS names from server (rather than being
limited to IP address) for --ifconfig, --route, and
--route-gateway.

--allow-recursive-routing
When this option is set, OpenVPN will not drop incoming tun
packets with same destination as host.

--auth-token token
This is not an option to be used directly in any configuration
files, but rather push this option from a --client-connect
script or a --plugin which hooks into the OPENVPN_PLUG-
IN_CLIENT_CONNECT or OPENVPN_PLUGIN_CLIENT_CONNECT_V2 calls.
This option provides a possibility to replace the clients pass-
word with an authentication token during the lifetime of the
OpenVPN client.

Whenever the connection is renegotiated and the
--auth-user-pass-verify script or --plugin making use of the
OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY hook is triggered, it will
pass over this token as the password instead of the password the
user provided. The authentication token can only be reset by a
full reconnect where the server can push new options to the
client. The password the user entered is never preserved once an
authentication token has been set. If the OpenVPN server side
rejects the authentication token then the client will receive an
AUTH_FAILED and disconnect.

The purpose of this is to enable two factor authentication meth-
ods, such as HOTP or TOTP, to be used without needing to
retrieve a new OTP code each time the connection is renegoti-
ated. Another use case is to cache authentication data on the
client without needing to have the users password cached in mem-
ory during the life time of the session.

To make use of this feature, the --client-connect script or
--plugin needs to put

push "auth-token UNIQUE_TOKEN_VALUE"

into the file/buffer for dynamic configuration data. This will
then make the OpenVPN server to push this value to the client,
which replaces the local password with the UNIQUE_TOKEN_VALUE.

Newer clients (2.4.7+) will fall back to the original password
method after a failed auth. Older clients will keep using the
token value and react according to --auth-retry

--auth-token-user base64username
Companion option to --auth-token. This options allows one to
override the username used by the client when reauthenticating
with the auth-token. It also allows one to use --auth-token in
setups that normally do not use username and password.

The username has to be base64 encoded.

--auth-user-pass
Authenticate with server using username/password.

Valid syntaxes:

auth-user-pass
auth-user-pass up

If up is present, it must be a file containing username/password
on 2 lines. If the password line is missing, OpenVPN will prompt
for one.

If up is omitted, username/password will be prompted from the
console.

The server configuration must specify an --auth-user-pass-verify
script to verify the username/password provided by the client.

--auth-retry type
Controls how OpenVPN responds to username/password verification
errors such as the client-side response to an AUTH_FAILED mes-
sage from the server or verification failure of the private key
password.

Normally used to prevent auth errors from being fatal on the
client side, and to permit username/password requeries in case
of error.

An AUTH_FAILED message is generated by the server if the client
fails --auth-user-pass authentication, or if the server-side
--client-connect script returns an error status when the client
tries to connect.

type can be one of:

none Client will exit with a fatal error (this is the
default).

nointeract
Client will retry the connection without requerying for
an --auth-user-pass username/password. Use this option
for unattended clients.

interact
Client will requery for an --auth-user-pass user-
name/password and/or private key password before attempt-
ing a reconnection.

Note that while this option cannot be pushed, it can be con-
trolled from the management interface.

--client
A helper directive designed to simplify the configuration of
OpenVPN's client mode. This directive is equivalent to:

pull
tls-client

--client-nat args
This pushable client option sets up a stateless one-to-one NAT
rule on packet addresses (not ports), and is useful in cases
where routes or ifconfig settings pushed to the client would
create an IP numbering conflict.

Examples:

client-nat snat 192.168.0.0/255.255.0.0
client-nat dnat 10.64.0.0/255.255.0.0

network/netmask (for example 192.168.0.0/255.255.0.0) defines
the local view of a resource from the client perspective, while
alias/netmask (for example 10.64.0.0/255.255.0.0) defines the
remote view from the server perspective.

Use snat (source NAT) for resources owned by the client and dnat
(destination NAT) for remote resources.

Set --verb 6 for debugging info showing the transformation of
src/dest addresses in packets.

--connect-retry args
Wait n seconds between connection attempts (default 1).
Repeated reconnection attempts are slowed down after 5 retries
per remote by doubling the wait time after each unsuccessful
attempt.

Valid syntaxes:

connect retry n
connect retry n max

If the optional argument max is specified, the maximum wait time
in seconds gets capped at that value (default 300).

--connect-retry-max n
n specifies the number of times each --remote or <connection>
entry is tried. Specifying n as 1 would try each entry exactly
once. A successful connection resets the counter. (default
unlimited).

--connect-timeout n
See --server-poll-timeout.

--dns args
Client DNS configuration to be used with the connection.

Valid syntaxes:

dns search-domains domain [domain ...]
dns server n address addr[:port] [addr[:port] ...]
dns server n resolve-domains domain [domain ...]
dns server n dnssec yes|optional|no
dns server n transport DoH|DoT|plain
dns server n sni server-name

The --dns search-domains directive takes one or more domain
names to be added as DNS domain suffixes. If it is repeated mul-
tiple times within a configuration the domains are appended,
thus e.g. domain names pushed by a server will amend locally
defined ones.

The --dns server directive is used to configure DNS server n.
The server id n must be a value between -128 and 127. For pushed
DNS server options it must be between 0 and 127. The server id
is used to group options and also for ordering the list of con-
figured DNS servers; lower numbers come first. DNS servers being
pushed to a client replace already configured DNS servers with
the same server id.

The address option configures the IPv4 and / or IPv6 address(es)
of the DNS server. Up to eight addresses can be specified per
DNS server. Optionally a port can be appended after a colon.
IPv6 addresses need to be enclosed in brackets if a port is
appended.

The resolve-domains option takes one or more DNS domains used to
define a split-dns or dns-routing setup, where only the given
domains are resolved by the server. Systems which do not support
fine grained DNS domain configuration will ignore this setting.

The dnssec option is used to configure validation of DNSSEC
records. While the exact semantics may differ for resolvers on
different systems, yes likely makes validation mandatory, no
disables it, and optional uses it opportunistically.

The transport option enables DNS-over-HTTPS (DoH) or
DNS-over-TLS (DoT) for a DNS server. The sni option can be used
with them to specify the server-name for TLS server name indica-
tion.

Each server has to have at least one address configured for a
configuration to be valid. All the other options can be omitted.

Note that not all options may be supported on all platforms. As
soon support for different systems is implemented, information
will be added here how unsupported options are treated.

The --dns option will eventually obsolete the --dhcp-option
directive. Until then it will replace configuration at the
places --dhcp-option puts it, so that --dns overrides
--dhcp-option. Thus, --dns can be used today to migrate from
--dhcp-option.

--explicit-exit-notify n
In UDP client mode or point-to-point mode, send server/peer an
exit notification if tunnel is restarted or OpenVPN process is
exited. In client mode, on exit/restart, this option will tell
the server to immediately close its client instance object
rather than waiting for a timeout.

If both server and client support sending this message using the
control channel, the message will be sent as control-channel
message. Otherwise the message is sent as data-channel message,
which will be ignored by data-channel offloaded peers.

The n parameter (default 1 if not present) controls the maximum
number of attempts that the client will try to resend the exit
notification message if messages are sent in data-channel mode.

In UDP server mode, send RESTART control channel command to con-
nected clients. The n parameter (default 1 if not present) con-
trols client behavior. With n = 1 client will attempt to recon-
nect to the same server, with n = 2 client will advance to the
next server.

OpenVPN will not send any exit notifications unless this option
is enabled.

--inactive args
Causes OpenVPN to exit after n seconds of inactivity on the
TUN/TAP device. The time length of inactivity is measured since
the last incoming or outgoing tunnel packet. The default value
is 0 seconds, which disables this feature.

Valid syntaxes:

inactive n
inactive n bytes

If the optional bytes parameter is included, exit if less than
bytes of combined in/out traffic are produced on the tun/tap
device in n seconds.

In any case, OpenVPN's internal ping packets (which are just
keepalives) and TLS control packets are not considered "activ-
ity", nor are they counted as traffic, as they are used inter-
nally by OpenVPN and are not an indication of actual user activ-
ity.

--proto-force p
When iterating through connection profiles, only consider pro-
files using protocol p (tcp | udp).

Note that this specifically only filters by the transport layer
protocol, i.e. UDP or TCP. This does not affect whether IPv4 or
IPv6 is used as IP protocol.

For implementation reasons the option accepts the 4 and 6 suf-
fixes when specifying the protocol (i.e. udp4 / udp6 / tcp4 /
tcp6). However, these behave the same as without the suffix and
should be avoided to prevent confusion.

--pull This option must be used on a client which is connecting to a
multi-client server. It indicates to OpenVPN that it should
accept options pushed by the server, provided they are part of
the legal set of pushable options (note that the --pull option
is implied by --client ).

In particular, --pull allows the server to push routes to the
client, so you should not use --pull or --client in situations
where you don't trust the server to have control over the
client's routing table.

--pull-filter args
Filter options on the client pushed by the server to the client.

Valid syntaxes:

pull-filter accept text
pull-filter ignore text
pull-filter reject text

Filter options received from the server if the option starts
with text. The action flag accept allows the option, ignore
removes it and reject flags an error and triggers a SIGUSR1
restart. The filters may be specified multiple times, and each
filter is applied in the order it is specified. The filtering of
each option stops as soon as a match is found. Unmatched options
are accepted by default.

Prefix comparison is used to match text against the received
option so that

pull-filter ignore "route"

would remove all pushed options starting with route which would
include, for example, route-gateway. Enclose text in quotes to
embed spaces.

pull-filter accept "route 192.168.1."
pull-filter ignore "route "

would remove all routes that do not start with 192.168.1.

Note that reject may result in a repeated cycle of failure and
reconnect, unless multiple remotes are specified and connection
to the next remote succeeds. To silently ignore an option pushed
by the server, use ignore.

--push-peer-info
Push additional information about the client to server. The fol-
lowing data is always pushed to the server:

IV_VER=<version>
The client OpenVPN version

IV_LZO_STUB=1
If client was built with LZO stub capability

IV_LZ4=1
If the client supports LZ4 compressions.

IV_PROTO
Details about protocol extensions that the peer supports.
The variable is a bitfield and the bits are defined as
follows (starting a bit 0 for the first (unused) bit:

o bit 1: The peer supports peer-id floating mechanism

o bit 2: The client expects a push-reply and the server
may send this reply without waiting for a push-request
first.

o bit 3: The client is capable of doing key derivation
using RFC5705 key material exporter.

o bit 4: The client is capable of accepting additional
arguments to the AUTH_PENDING message.

IV_NCP=2
Negotiable ciphers, client supports --cipher pushed by
the server, a value of 2 or greater indicates client sup-
ports AES-GCM-128 and AES-GCM-256.

IV_CIPHERS=<ncp-ciphers>
The client announces the list of supported ciphers con-
figured with the --data-ciphers option to the server.

IV_MTU=<max_mtu>
The client announces the support of pushable MTU and the
maximum MTU it is willing to accept.

IV_GUI_VER=<gui_id> <version>
The UI version of a UI if one is running, for example
de.blinkt.openvpn 0.5.47 for the Android app.

IV_SSO=[crtext,][openurl,][proxy_url]
Additional authentication methods supported by the
client. This may be set by the client UI/GUI using
--setenv

When --push-peer-info is enabled the additional information con-
sists of the following data:

IV_HWADDR=<string>
This is intended to be a unique and persistent ID of the
client. The string value can be any readable ASCII
string up to 64 bytes. OpenVPN 2.x and some other imple-
mentations use the MAC address of the client's interface
used to reach the default gateway. If this string is gen-
erated by the client, it should be consistent and pre-
served across independent session and preferably
re-installations and upgrades.

IV_SSL=<version string>
The ssl version used by the client, e.g. OpenSSL 1.0.2f
28 Jan 2016.

IV_PLAT_VER=x.y
The version of the operating system, e.g. 6.1 for Windows
7.

UV_<name>=<value>
Client environment variables whose names start with UV_

--remote args
Remote host name or IP address, port and protocol.

Valid syntaxes:

remote host
remote host port
remote host port proto

The port and proto arguments are optional. The OpenVPN client
will try to connect to a server at host:port. The proto argu-
ment indicates the protocol to use when connecting with the
remote, and may be tcp or udp. To enforce IPv4 or IPv6 connec-
tions add a 4 or 6 suffix; like udp4 / udp6 / tcp4 / tcp6.

On the client, multiple --remote options may be specified for
redundancy, each referring to a different OpenVPN server, in the
order specified by the list of --remote options. Specifying mul-
tiple --remote options for this purpose is a special case of the
more general connection-profile feature. See the <connection>
documentation below.

The client will move on to the next host in the list, in the
event of connection failure. Note that at any given time, the
OpenVPN client will at most be connected to one server.

Examples:

remote server1.example.net
remote server1.example.net 1194
remote server2.example.net 1194 tcp

Note: Since UDP is connectionless, connection failure is
defined by the --ping and --ping-restart options.

Also, if you use multiple --remote options, AND you are
dropping root privileges on the client with --user and/or
--group AND the client is running a non-Windows OS, if
the client needs to switch to a different server, and
that server pushes back different TUN/TAP or route set-
tings, the client may lack the necessary privileges to
close and reopen the TUN/TAP interface. This could cause
the client to exit with a fatal error.

If --remote is unspecified, OpenVPN will listen for packets from
any IP address, but will not act on those packets unless they
pass all authentication tests. This requirement for authentica-
tion is binding on all potential peers, even those from known
and supposedly trusted IP addresses (it is very easy to forge a
source IP address on a UDP packet).

When used in TCP mode, --remote will act as a filter, rejecting
connections from any host which does not match host.

If host is a DNS name which resolves to multiple IP addresses,
OpenVPN will try them in the order that the system getaddrinfo()
presents them, so priorization and DNS randomization is done by
the system library. Unless an IP version is forced by the proto-
col specification (4/6 suffix), OpenVPN will try both IPv4 and
IPv6 addresses, in the order getaddrinfo() returns them.

--remote-random
When multiple --remote address/ports are specified, or if con-
nection profiles are being used, initially randomize the order
of the list as a kind of basic load-balancing measure.

--remote-random-hostname
Prepend a random string (6 bytes, 12 hex characters) to hostname
to prevent DNS caching. For example, "foo.bar.gov" would be mod-
ified to "<random-chars>.foo.bar.gov".

--resolv-retry n
If hostname resolve fails for --remote, retry resolve for n sec-
onds before failing.

Set n to infinite to retry indefinitely.

By default, --resolv-retry infinite is enabled. You can disable
by setting n=0.

--single-session
After initially connecting to a remote peer, disallow any new
connections. Using this option means that a remote peer cannot
connect, disconnect, and then reconnect.

If the daemon is reset by a signal or --ping-restart, it will
allow one new connection.

--single-session can be used with --ping-exit or --inactive to
create a single dynamic session that will exit when finished.

--server-poll-timeout n
When connecting to a remote server do not wait for more than n
seconds for a response before trying the next server. The
default value is 120. This timeout includes proxy and TCP con-
nect timeouts.

--static-challenge args
Enable static challenge/response protocol

Valid syntax:

static-challenge text echo

The text challenge text is presented to the user which describes
what information is requested. The echo flag indicates if the
user's input should be echoed on the screen. Valid echo values
are 0 or 1.

See management-notes.txt in the OpenVPN distribution for a
description of the OpenVPN challenge/response protocol.

--show-proxy-settings
Show sensed HTTP or SOCKS proxy settings. Currently, only Win-
dows clients support this option.

--http-proxy args
Connect to remote host through an HTTP proxy. This requires at
least an address server and port argument. If HTTP
Proxy-Authenticate is required, a file name to an authfile file
containing a username and password on 2 lines can be given, or
stdin to prompt from console. Its content can also be specified
in the config file with the --http-proxy-user-pass option. (See
section on inline files)

The last optional argument is an auth-method which should be one
of none, basic, or ntlm.

HTTP Digest authentication is supported as well, but only via
the auto or auto-nct flags (below). This must replace the auth-
file argument.

The auto flag causes OpenVPN to automatically determine the
auth-method and query stdin or the management interface for
username/password credentials, if required. This flag exists on
OpenVPN 2.1 or higher.

The auto-nct flag (no clear-text auth) instructs OpenVPN to
automatically determine the authentication method, but to reject
weak authentication protocols such as HTTP Basic Authentication.

Examples:

http-proxy proxy.example.net 3128
http-proxy proxy.example.net 3128 authfile.txt
http-proxy proxy.example.net 3128 stdin
http-proxy proxy.example.net 3128 auto basic
http-proxy proxy.example.net 3128 auto-nct ntlm

--http-proxy-option args
Set extended HTTP proxy options. Requires an option type as
argument and an optional parameter to the type. Repeat to set
multiple options.

VERSION version
Set HTTP version number to version (default 1.0).

AGENT user-agent
Set HTTP "User-Agent" string to user-agent.

CUSTOM-HEADER name content
Adds the custom Header with name as name and content as
the content of the custom HTTP header.

Examples:

http-proxy-option VERSION 1.1
http-proxy-option AGENT OpenVPN/2.4
http-proxy-option X-Proxy-Flag some-flags

--socks-proxy args
Connect to remote host through a Socks5 proxy. A required
server argument is needed. Optionally a port (default 1080) and
authfile can be given. The authfile is a file containing a
username and password on 2 lines, or stdin can be used to prompt
from console.

Server Options
Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is sup-
ported, and can be enabled with the --mode server option. In server
mode, OpenVPN will listen on a single port for incoming client connec-
tions. All client connections will be routed through a single tun or
tap interface. This mode is designed for scalability and should be able
to support hundreds or even thousands of clients on sufficiently fast
hardware. SSL/TLS authentication must be used in this mode.

--auth-gen-token args
Returns an authentication token to successfully authenticated
clients.

Valid syntax:

auth-gen-token [lifetime] [renewal-time] [external-auth]

After successful user/password authentication, the OpenVPN
server will with this option generate a temporary authentication
token and push that to the client. On the following renegotia-
tions, the OpenVPN client will pass this token instead of the
users password. On the server side the server will do the token
authentication internally and it will NOT do any additional
authentications against configured external user/password
authentication mechanisms.

The tokens implemented by this mechanism include an initial
timestamp and a renew timestamp and are secured by HMAC.

The lifetime argument defines how long the generated token is
valid. The lifetime is defined in seconds. If lifetime is not
set or it is set to 0, the token will never expire.

If renewal-time is not set it defaults to reneg-sec.

The token will expire either after the configured lifetime of
the token is reached or after not being renewed for more than 2
* renewal-time seconds. Clients will be sent renewed tokens on
every TLS renegotiation. If renewal-time is lower than reneg-sec
the server will push an updated temporary authentication token
every reneweal-time seconds. This is done to invalidate a token
if a client is disconnected for a sufficiently long time, while
at the same time permitting much longer token lifetimes for
active clients.

This feature is useful for environments which are configured to
use One Time Passwords (OTP) as part of the user/password
authentications and that authentication mechanism does not
implement any auth-token support.

When the external-auth keyword is present the normal authentica-
tion method will always be called even if auth-token succeeds.
Normally other authentications method are skipped if auth-token
verification succeeds or fails.

This option postpones this decision to the external authentica-
tion methods and checks the validity of the account and do other
checks.

In this mode the environment will have a session_id variable
that holds the session id from auth-gen-token. Also an environ-
ment variable session_state is present. This variable indicates
whether the auth-token has succeeded or not. It can have the
following values:

Initial
No token from client.

Authenticated
Token is valid and not expired.

Expired
Token is valid but has expired.

Invalid
Token is invalid (failed HMAC or wrong length)

AuthenticatedEmptyUser / ExpiredEmptyUser
The token is not valid with the username sent from the
client but would be valid (or expired) if we assume an
empty username was used instead. These two cases are a
workaround for behaviour in OpenVPN 3. If this work-
around is not needed these two cases should be handled in
the same way as Invalid.

Warning: Use this feature only if you want your authentication
method called on every verification. Since the external authen-
tication is called it needs to also indicate a success or fail-
ure of the authentication. It is strongly recommended to return
an authentication failure in the case of the Invalid/Expired
auth-token with the external-auth option unless the client could
authenticate in another acceptable way (e.g. client certifi-
cate), otherwise returning success will lead to authentication
bypass (as does returning success on a wrong password from a
script).

--auth-gen-token-secret file
Specifies a file that holds a secret for the HMAC used in
--auth-gen-token If file is not present OpenVPN will generate a
random secret on startup. This file should be used if auth-token
should validate after restarting a server or if client should be
able to roam between multiple OpenVPN servers with their
auth-token.

--auth-user-pass-optional
Allow connections by clients that do not specify a user-
name/password. Normally, when --auth-user-pass-verify or --man-
agement-client-auth are specified (or an authentication plugin
module), the OpenVPN server daemon will require connecting
clients to specify a username and password. This option makes
the submission of a username/password by clients optional, pass-
ing the responsibility to the user-defined authentication mod-
ule/script to accept or deny the client based on other factors
(such as the setting of X509 certificate fields). When this
option is used, and a connecting client does not submit a user-
name/password, the user-defined authentication module/script
will see the username and password as being set to empty strings
(""). The authentication module/script MUST have logic to detect
this condition and respond accordingly.

--ccd-exclusive
Require, as a condition of authentication, that a connecting
client has a --client-config-dir file.

--client-config-dir dir
Specify a directory dir for custom client config files. After a
connecting client has been authenticated, OpenVPN will look in
this directory for a file having the same name as the client's
X509 common name. If a matching file exists, it will be opened
and parsed for client-specific configuration options. If no
matching file is found, OpenVPN will instead try to open and
parse a default file called "DEFAULT", which may be provided but
is not required. Note that the configuration files must be read-
able by the OpenVPN process after it has dropped it's root priv-
ileges.

This file can specify a fixed IP address for a given client
using --ifconfig-push, as well as fixed subnets owned by the
client using --iroute.

One of the useful properties of this option is that it allows
client configuration files to be conveniently created, edited,
or removed while the server is live, without needing to restart
the server.

The following options are legal in a client-specific context:
--push, --push-reset, --push-remove, --iroute, --ifconfig-push,
--vlan-pvid and --config.

--client-to-client
Because the OpenVPN server mode handles multiple clients through
a single tun or tap interface, it is effectively a router. The
--client-to-client flag tells OpenVPN to internally route
client-to-client traffic rather than pushing all client-origi-
nating traffic to the TUN/TAP interface.

When this option is used, each client will "see" the other
clients which are currently connected. Otherwise, each client
will only see the server. Don't use this option if you want to
firewall tunnel traffic using custom, per-client rules.

Please note that when using data channel offload this option has
no effect. Packets are always sent to the tunnel interface and
then routed based on the system routing table.

--disable
Disable a particular client (based on the common name) from con-
necting. Don't use this option to disable a client due to key
or password compromise. Use a CRL (certificate revocation list)
instead (see the --crl-verify option).

This option must be associated with a specific client instance,
which means that it must be specified either in a client
instance config file using --client-config-dir or dynamically
generated using a --client-connect script.

--connect-freq args
Allow a maximum of n new connections per sec seconds from
clients.

Valid syntax:

connect-freq n sec

This is designed to contain DoS attacks which flood the server
with connection requests using certificates which will ulti-
mately fail to authenticate.

This limit applies after --connect-freq-initial and only applies
to client that have completed the three-way handshake or client
that use --tls-crypt-v2 without cookie support (allow-noncookie
argument to --tls-crypt-v2).

This is an imperfect solution however, because in a real DoS
scenario, legitimate connections might also be refused.

For the best protection against DoS attacks in server mode, use
--proto udp and either --tls-auth or --tls-crypt.

--connect-freq-initial args
(UDP only) Allow a maximum of n initial connection packet
responses per sec seconds from the OpenVPN server to clients.

Valid syntax:

connect-freq-initial n sec

OpenVPN starting at 2.6 is very efficient in responding to ini-
tial connection packets. When not limiting the initial responses
an OpenVPN daemon can be abused in reflection attacks. This
option is designed to limit the rate OpenVPN will respond to
initial attacks.

Connection attempts that complete the initial three-way hand-
shake will not be counted against the limit. The default is to
allow 100 initial connection per 10s.

--duplicate-cn
Allow multiple clients with the same common name to concurrently
connect. In the absence of this option, OpenVPN will disconnect
a client instance upon connection of a new client having the
same common name.

--ifconfig-pool args
Set aside a pool of subnets to be dynamically allocated to con-
necting clients, similar to a DHCP server.

Valid syntax:

ifconfig-pool start-IP end-IP [netmask]

For tun-style tunnels, each client will be given a /30 subnet
(for interoperability with Windows clients). For tap-style tun-
nels, individual addresses will be allocated, and the optional
netmask parameter will also be pushed to clients.

--ifconfig-ipv6-pool args
Specify an IPv6 address pool for dynamic assignment to clients.

Valid args:

ifconfig-ipv6-pool ipv6addr/bits

The pool starts at ipv6addr and matches the offset determined
from the start of the IPv4 pool. If the host part of the given
IPv6 address is 0, the pool starts at ipv6addr +1.

--ifconfig-pool-persist args
Persist/unpersist ifconfig-pool data to file, at seconds inter-
vals (default 600), as well as on program startup and shutdown.

Valid syntax:

ifconfig-pool-persist file [seconds]

The goal of this option is to provide a long-term association
between clients (denoted by their common name) and the virtual
IP address assigned to them from the ifconfig-pool. Maintaining
a long-term association is good for clients because it allows
them to effectively use the --persist-tun option.

file is a comma-delimited ASCII file, formatted as <Com-
mon-Name>,<IP-address>.

If seconds = 0, file will be treated as read-only. This is use-
ful if you would like to treat file as a configuration file.

Note that the entries in this file are treated by OpenVPN as
suggestions only, based on past associations between a common
name and IP address. They do not guarantee that the given com-
mon name will always receive the given IP address. If you want
guaranteed assignment, use --ifconfig-push

--ifconfig-push args
Push virtual IP endpoints for client tunnel, overriding the
--ifconfig-pool dynamic allocation.

Valid syntax:

ifconfig-push local remote-netmask [alias]

The parameters local and remote-netmask are set according to the
--ifconfig directive which you want to execute on the client
machine to configure the remote end of the tunnel. Note that the
parameters local and remote-netmask are from the perspective of
the client, not the server. They may be DNS names rather than IP
addresses, in which case they will be resolved on the server at
the time of client connection.

The optional alias parameter may be used in cases where NAT
causes the client view of its local endpoint to differ from the
server view. In this case local/remote-netmask will refer to the
server view while alias/remote-netmask will refer to the client
view.

Remember also to include a --route directive in the main OpenVPN
config file which encloses local, so that the kernel will know
to route it to the server's TUN/TAP interface.

OpenVPN's internal client IP address selection algorithm works
as follows:

1. Use --client-connect script generated file for static IP
(first choice).

2. Use --client-config-dir file for static IP (next choice).

3. Use --ifconfig-pool allocation for dynamic IP (last choice).

--ifconfig-ipv6-push args
for --client-config-dir per-client static IPv6 interface config-
uration, see --client-config-dir and --ifconfig-push for more
details.

Valid syntax:

ifconfig-ipv6-push ipv6addr/bits ipv6remote

--multihome
Configure a multi-homed UDP server. This option needs to be used
when a server has more than one IP address (e.g. multiple inter-
faces, or secondary IP addresses), and is not using --local to
force binding to one specific address only. This option will add
some extra lookups to the packet path to ensure that the UDP
reply packets are always sent from the address that the client
is talking to. This is not supported on all platforms, and it
adds more processing, so it's not enabled by default.

Notes:

o This option is only relevant for UDP servers.

o If you do an IPv6+IPv4 dual-stack bind on a Linux
machine with multiple IPv4 address, connections to IPv4
addresses will not work right on kernels before 3.15,
due to missing kernel support for the IPv4-mapped case
(some distributions have ported this to earlier kernel
versions, though).

--iroute args
Generate an internal route to a specific client. The netmask
parameter, if omitted, defaults to 255.255.255.255.

Valid syntax:

iroute network [netmask]

This directive can be used to route a fixed subnet from the
server to a particular client, regardless of where the client is
connecting from. Remember that you must also add the route to
the system routing table as well (such as by using the --route
directive). The reason why two routes are needed is that the
--route directive routes the packet from the kernel to OpenVPN.
Once in OpenVPN, the --iroute directive routes to the specific
client.

However, when using DCO, the --iroute directive is usually
enough for DCO to fully configure the routing table. The extra
--route directive is required only if the expected behaviour is
to route the traffic for a specific network to the VPN interface
also when the responsible client is not connected (traffic will
then be dropped).

This option must be specified either in a client instance config
file using --client-config-dir or dynamically generated using a
--client-connect script.

The --iroute directive also has an important interaction with
--push "route ...". --iroute essentially defines a subnet which
is owned by a particular client (we will call this client A). If
you would like other clients to be able to reach A's subnet, you
can use --push "route ..." together with --client-to-client to
effect this. In order for all clients to see A's subnet, OpenVPN
must push this route to all clients EXCEPT for A, since the sub-
net is already owned by A. OpenVPN accomplishes this by not not
pushing a route to a client if it matches one of the client's
iroutes.

--iroute-ipv6 args
for --client-config-dir per-client static IPv6 route configura-
tion, see --iroute for more details how to setup and use this,
and how --iroute and --route interact.

Valid syntax:

iroute-ipv6 ipv6addr/bits

--max-clients n
Limit server to a maximum of n concurrent clients.

--max-routes-per-client n
Allow a maximum of n internal routes per client (default 256).
This is designed to help contain DoS attacks where an authenti-
cated client floods the server with packets appearing to come
from many unique MAC addresses, forcing the server to deplete
virtual memory as its internal routing table expands. This
directive can be used in a --client-config-dir file or auto-gen-
erated by a --client-connect script to override the global value
for a particular client.

Note that this directive affects OpenVPN's internal routing ta-
ble, not the kernel routing table.

--opt-verify
DEPRECATED Clients that connect with options that are incompati-
ble with those of the server will be disconnected.

Options that will be compared for compatibility include
dev-type, link-mtu, tun-mtu, proto, ifconfig, comp-lzo, frag-
ment, keydir, cipher, auth, keysize, secret, no-replay,
tls-auth, key-method, tls-server and tls-client.

This option requires that --disable-occ NOT be used.

--port-share args
Share OpenVPN TCP with another service

Valid syntax:

port-share host port [dir]

When run in TCP server mode, share the OpenVPN port with another
application, such as an HTTPS server. If OpenVPN senses a con-
nection to its port which is using a non-OpenVPN protocol, it
will proxy the connection to the server at host:port. Currently
only designed to work with HTTP/HTTPS, though it would be theo-
retically possible to extend to other protocols such as ssh.

dir specifies an optional directory where a temporary file with
name N containing content C will be dynamically generated for
each proxy connection, where N is the source IP:port of the
client connection and C is the source IP:port of the connection
to the proxy receiver. This directory can be used as a dictio-
nary by the proxy receiver to determine the origin of the con-
nection. Each generated file will be automatically deleted when
the proxied connection is torn down.

Not implemented on Windows.

--push option
Push a config file option back to the client for remote execu-
tion. Note that option must be enclosed in double quotes ("").
The client must specify --pull in its config file. The set of
options which can be pushed is limited by both feasibility and
security. Some options such as those which would execute scripts
are banned, since they would effectively allow a compromised
server to execute arbitrary code on the client. Other options
such as TLS or MTU parameters cannot be pushed because the
client needs to know them before the connection to the server
can be initiated.

This is a partial list of options which can currently be pushed:
--route, --route-gateway, --route-delay, --redirect-gateway,
--ip-win32, --dhcp-option, --dns, --inactive, --ping,
--ping-exit, --ping-restart, --setenv, --auth-token, --per-
sist-key, --persist-tun, --echo, --comp-lzo, --socket-flags,
--sndbuf, --rcvbuf, --session-timeout

--push-remove opt
Selectively remove all --push options matching "opt" from the
option list for a client. opt is matched as a substring against
the whole option string to-be-pushed to the client, so
--push-remove route would remove all --push route ... and --push
route-ipv6 ... statements, while --push-remove "route-ipv6
2001:" would only remove IPv6 routes for 2001:... networks.

--push-remove can only be used in a client-specific context,
like in a --client-config-dir file, or --client-connect script
or plugin -- similar to --push-reset, just more selective.

NOTE: to change an option, --push-remove can be used to first
remove the old value, and then add a new --push option with the
new value.

NOTE 2: due to implementation details, 'ifconfig' and 'ifcon-
fig-ipv6' can only be removed with an exact match on the option
( push-remove ifconfig), no substring matching and no matching
on the IPv4/IPv6 address argument is possible.

--push-reset
Don't inherit the global push list for a specific client
instance. Specify this option in a client-specific context such
as with a --client-config-dir configuration file. This option
will ignore --push options at the global config file level.

NOTE: --push-reset is very thorough: it will remove almost all
options from the list of to-be-pushed options. In many cases,
some of these options will need to be re-configured afterwards -
specifically, --topology subnet and --route-gateway will get
lost and this will break client configs in many cases. Thus,
for most purposes, --push-remove is better suited to selectively
remove push options for individual clients.

--server args
A helper directive designed to simplify the configuration of
OpenVPN's server mode. This directive will set up an OpenVPN
server which will allocate addresses to clients out of the given
network/netmask. The server itself will take the .1 address of
the given network for use as the server-side endpoint of the
local TUN/TAP interface. If the optional nopool flag is given,
no dynamic IP address pool will prepared for VPN clients.

Valid syntax:

server network netmask [nopool]

For example, --server 10.8.0.0 255.255.255.0 expands as follows:

mode server
tls-server
push "topology [topology]"

if dev tun AND (topology == net30 OR topology == p2p):
ifconfig 10.8.0.1 10.8.0.2
if !nopool:
ifconfig-pool 10.8.0.4 10.8.0.251
route 10.8.0.0 255.255.255.0
if client-to-client:
push "route 10.8.0.0 255.255.255.0"
else if topology == net30:
push "route 10.8.0.1"

if dev tap OR (dev tun AND topology == subnet):
ifconfig 10.8.0.1 255.255.255.0
if !nopool:
ifconfig-pool 10.8.0.2 10.8.0.253 255.255.255.0
push "route-gateway 10.8.0.1"
if route-gateway unset:
route-gateway 10.8.0.2

Don't use --server if you are ethernet bridging. Use
--server-bridge instead.

--server-bridge args
A helper directive similar to --server which is designed to sim-
plify the configuration of OpenVPN's server mode in ethernet
bridging configurations.

Valid syntaxes:

server-bridge gateway netmask pool-start-IP pool-end-IP
server-bridge [nogw]

If --server-bridge is used without any parameters, it will
enable a DHCP-proxy mode, where connecting OpenVPN clients will
receive an IP address for their TAP adapter from the DHCP server
running on the OpenVPN server-side LAN. Note that only clients
that support the binding of a DHCP client with the TAP adapter
(such as Windows) can support this mode. The optional nogw flag
(advanced) indicates that gateway information should not be
pushed to the client.

To configure ethernet bridging, you must first use your OS's
bridging capability to bridge the TAP interface with the ether-
net NIC interface. For example, on Linux this is done with the
brctl tool, and with Windows XP it is done in the Network Con-
nections Panel by selecting the ethernet and TAP adapters and
right-clicking on "Bridge Connections".

Next you you must manually set the IP/netmask on the bridge
interface. The gateway and netmask parameters to
--server-bridge can be set to either the IP/netmask of the
bridge interface, or the IP/netmask of the default gate-
way/router on the bridged subnet.

Finally, set aside a IP range in the bridged subnet, denoted by
pool-start-IP and pool-end-IP, for OpenVPN to allocate to con-
necting clients.

For example, server-bridge 10.8.0.4 255.255.255.0 10.8.0.128
10.8.0.254 expands as follows:

mode server
tls-server

ifconfig-pool 10.8.0.128 10.8.0.254 255.255.255.0
push "route-gateway 10.8.0.4"

In another example, --server-bridge (without parameters) expands
as follows:

mode server
tls-server

push "route-gateway dhcp"

Or --server-bridge nogw expands as follows:

mode server
tls-server

--server-ipv6 args
Convenience-function to enable a number of IPv6 related options
at once, namely --ifconfig-ipv6, --ifconfig-ipv6-pool and --push
tun-ipv6.

Valid syntax:

server-ipv6 ipv6addr/bits

Pushing of the --tun-ipv6 directive is done for older clients
which require an explicit --tun-ipv6 in their configuration.

--stale-routes-check args
Remove routes which haven't had activity for n seconds (i.e. the
ageing time). This check is run every t seconds (i.e. check
interval).

Valid syntax:

stale-routes-check n [t]

If t is not present it defaults to n.

This option helps to keep the dynamic routing table small. See
also --max-routes-per-client

--username-as-common-name
Use the authenticated username as the common-name, rather than
the common-name from the client certificate. Requires that some
form of --auth-user-pass verification is in effect. As the
replacement happens after --auth-user-pass verification, the
verification script or plugin will still receive the common-name
from the certificate.

The common_name environment variable passed to scripts and plug-
ins invoked after authentication (e.g, client-connect script)
and file names parsed in client-config directory will match the
username.

--verify-client-cert mode
Specify whether the client is required to supply a valid cer-
tificate.

Possible mode options are:

none A client certificate is not required. the client needs to
authenticate using username/password only. Be aware that
using this directive is less secure than requiring cer-
tificates from all clients.

If you use this directive, the entire responsibility of
authentication will rest on your --auth-user-pass-verify
script, so keep in mind that bugs in your script could
potentially compromise the security of your VPN.

--verify-client-cert none is functionally equivalent to
--client-cert-not-required.

optional
A client may present a certificate but it is not required
to do so. When using this directive, you should also use
a --auth-user-pass-verify script to ensure that clients
are authenticated using a certificate, a username and
password, or possibly even both.

Again, the entire responsibility of authentication will
rest on your --auth-user-pass-verify script, so keep in
mind that bugs in your script could potentially compro-
mise the security of your VPN.

require
This is the default option. A client is required to
present a certificate, otherwise VPN access is refused.

If you don't use this directive (or use --verify-client-cert
require) but you also specify an --auth-user-pass-verify script,
then OpenVPN will perform double authentication. The client cer-
tificate verification AND the --auth-user-pass-verify script
will need to succeed in order for a client to be authenticated
and accepted onto the VPN.

--vlan-tagging
Server-only option. Turns the OpenVPN server instance into a
switch that understands VLAN-tagging, based on IEEE 802.1Q.

The server TAP device and each of the connecting clients is seen
as a port of the switch. All client ports are in untagged mode
and the server TAP device is VLAN-tagged, untagged or accepts
both, depending on the --vlan-accept setting.

Ethernet frames with a prepended 802.1Q tag are called "tagged".
If the VLAN Identifier (VID) field in such a tag is non-zero,
the frame is called "VLAN-tagged". If the VID is zero, but the
Priority Control Point (PCP) field is non-zero, the frame is
called "prio-tagged". If there is no 802.1Q tag, the frame is
"untagged".

Using the --vlan-pvid v option once per client (see
--client-config-dir), each port can be associated with a certain
VID. Packets can only be forwarded between ports having the
same VID. Therefore, clients with differing VIDs are completely
separated from one-another, even if --client-to-client is acti-
vated.

The packet filtering takes place in the OpenVPN server. Clients
should not have any VLAN tagging configuration applied.

The --vlan-tagging option is off by default. While turned off,
OpenVPN accepts any Ethernet frame and does not perform any spe-
cial processing for VLAN-tagged packets.

This option can only be activated in --dev tap mode.

--vlan-accept args
Configure the VLAN tagging policy for the server TAP device.

Valid syntax:

vlan-accept all|tagged|untagged

The following modes are available:

tagged Admit only VLAN-tagged frames. Only VLAN-tagged packets
are accepted, while untagged or priority-tagged packets
are dropped when entering the server TAP device.

untagged
Admit only untagged and prio-tagged frames. VLAN-tagged
packets are not accepted, while untagged or prior-
ity-tagged packets entering the server TAP device are
tagged with the value configured for the global
--vlan-pvid setting.

all (default)
Admit all frames. All packets are admitted and then
treated like untagged or tagged mode respectively.

Note: Some vendors refer to switch ports running in tagged mode
as "trunk ports" and switch ports running in untagged
mode as "access ports".

Packets forwarded from clients to the server are VLAN-tagged
with the originating client's PVID, unless the VID matches the
global --vlan-pvid, in which case the tag is removed.

If no PVID is configured for a given client (see --vlan-pvid)
packets are tagged with 1 by default.

--vlan-pvid v
Specifies which VLAN identifier a "port" is associated with.
Only valid when --vlan-tagging is specified.

In the client context, the setting specifies which VLAN ID a
client is associated with. In the global context, the VLAN ID of
the server TAP device is set. The latter only makes sense for
--vlan-accept untagged and --vlan-accept all modes.

Valid values for v go from 1 through to 4094. The global value
defaults to 1. If no --vlan-pvid is specified in the client con-
text, the global value is inherited.

In some switch implementations, the PVID is also referred to as
"Native VLAN".

ENCRYPTION OPTIONS
SSL Library information
--show-ciphers
(Standalone) Show all cipher algorithms to use with the --cipher
option.

--show-digests
(Standalone) Show all message digest algorithms to use with the
--auth option.

--show-tls
(Standalone) Show all TLS ciphers supported by the crypto
library. OpenVPN uses TLS to secure the control channel, over
which the keys that are used to protect the actual VPN traffic
are exchanged. The TLS ciphers will be sorted from highest pref-
erence (most secure) to lowest.

Be aware that whether a cipher suite in this list can actually
work depends on the specific setup of both peers (e.g. both
peers must support the cipher, and an ECDSA cipher suite will
not work if you are using an RSA certificate, etc.).

--show-engines
(Standalone) Show currently available hardware-based crypto
acceleration engines supported by the OpenSSL library.

--show-groups
(Standalone) Show all available elliptic curves/groups to use
with the --ecdh-curve and tls-groups options.

Generating key material
--genkey args
(Standalone) Generate a key to be used of the type keytype. if
keyfile is left out or empty the key will be output on stdout.
See the following sections for the different keytypes.

Valid syntax:

--genkey keytype keyfile

Valid keytype arguments are:

secret Standard OpenVPN shared secret keys

tls-crypt Alias for secret

tls-auth Alias for secret

auth-token Key used for --auth-gen-token-key

tls-crypt-v2-server TLS Crypt v2 server key

tls-crypt-v2-client TLS Crypt v2 client key

Examples:

$ openvpn --genkey secret shared.key
$ openvpn --genkey tls-crypt shared.key
$ openvpn --genkey tls-auth shared.key
$ openvpn --genkey tls-crypt-v2-server v2crypt-server.key
$ openvpn --tls-crypt-v2 v2crypt-server.key --genkey tls-crypt-v2-client v2crypt-client-1.key

o Generating Shared Secret Keys Generate a shared secret, for
use with the --secret, --tls-auth or --tls-crypt options.

Syntax:

$ openvpn --genkey secret|tls-crypt|tls-auth keyfile

The key is saved in keyfile. All three variants (--secret,
tls-crypt and tls-auth) generate the same type of key. The
aliases are added for convenience.

If using this for --secret, this file must be shared with the
peer over a pre-existing secure channel such as scp(1).

o Generating TLS Crypt v2 Server key Generate a --tls-crypt-v2
key to be used by an OpenVPN server. The key is stored in
keyfile.

Syntax:

--genkey tls-crypt-v2-server keyfile

o Generating TLS Crypt v2 Client key Generate a --tls-crypt-v2
key to be used by OpenVPN clients. The key is stored in key-
file.

Syntax

--genkey tls-crypt-v2-client keyfile [metadata]

If supplied, include the supplied metadata in the wrapped
client key. This metadata must be supplied in base64-encoded
form. The metadata must be at most 733 bytes long (980 charac-
ters in base64, though note that 980 base64 characters can
encode more than 733 bytes).

If no metadata is supplied, OpenVPN will use a 64-bit unix
timestamp representing the current time in UTC, encoded in
network order, as metadata for the generated key.

A tls-crypt-v2 client key is wrapped using a server key. To
generate a client key, the user must therefore supply the
server key using the --tls-crypt-v2 option.

Servers can use --tls-crypt-v2-verify to specify a metadata
verification command.

o Generate Authentication Token key Generate a new secret that
can be used with --auth-gen-token-secret

Syntax:

--genkey auth-token [keyfile]

Note: This file should be kept secret to the server as anyone
that has access to this file will be able to generate
auth tokens that the OpenVPN server will accept as
valid.

Data Channel Renegotiation
When running OpenVPN in client/server mode, the data channel will use a
separate ephemeral encryption key which is rotated at regular inter-
vals.

--reneg-bytes n
Renegotiate data channel key after n bytes sent or received
(disabled by default with an exception, see below). OpenVPN
allows the lifetime of a key to be expressed as a number of
bytes encrypted/decrypted, a number of packets, or a number of
seconds. A key renegotiation will be forced if any of these
three criteria are met by either peer.

If using ciphers with cipher block sizes less than 128-bits,
--reneg-bytes is set to 64MB by default, unless it is explicitly
disabled by setting the value to 0, but this is HIGHLY DISCOUR-
AGED as this is designed to add some protection against the
SWEET32 attack vector. For more information see the --cipher
option.

--reneg-pkts n
Renegotiate data channel key after n packets sent and received
(disabled by default).

--reneg-sec args
Renegotiate data channel key after at most max seconds (default
3600) and at least min seconds (default is 90% of max for
servers, and equal to max for clients).

reneg-sec max [min]

The effective --reneg-sec value used is per session pseudo-uni-
form-randomized between min and max.

With the default value of 3600 this results in an effective per
session value in the range of 3240 .. 3600 seconds for servers,
or just 3600 for clients.

When using dual-factor authentication, note that this default
value may cause the end user to be challenged to reauthorize
once per hour.

Also, keep in mind that this option can be used on both the
client and server, and whichever uses the lower value will be
the one to trigger the renegotiation. A common mistake is to set
--reneg-sec to a higher value on either the client or server,
while the other side of the connection is still using the
default value of 3600 seconds, meaning that the renegotiation
will still occur once per 3600 seconds. The solution is to
increase --reneg-sec on both the client and server, or set it to
0 on one side of the connection (to disable), and to your chosen
value on the other side.

TLS Mode Options
TLS mode is the most powerful crypto mode of OpenVPN in both security
and flexibility. TLS mode works by establishing control and data chan-
nels which are multiplexed over a single TCP/UDP port. OpenVPN initi-
ates a TLS session over the control channel and uses it to exchange
cipher and HMAC keys to protect the data channel. TLS mode uses a
robust reliability layer over the UDP connection for all control chan-
nel communication, while the data channel, over which encrypted tunnel
data passes, is forwarded without any mediation. The result is the best
of both worlds: a fast data channel that forwards over UDP with only
the overhead of encrypt, decrypt, and HMAC functions, and a control
channel that provides all of the security features of TLS, including
certificate-based authentication and Diffie Hellman forward secrecy.

To use TLS mode, each peer that runs OpenVPN should have its own local
certificate/key pair (--cert and --key), signed by the root certificate
which is specified in --ca.

When two OpenVPN peers connect, each presents its local certificate to
the other. Each peer will then check that its partner peer presented a
certificate which was signed by the master root certificate as speci-
fied in --ca.

If that check on both peers succeeds, then the TLS negotiation will
succeed, both OpenVPN peers will exchange temporary session keys, and
the tunnel will begin passing data.

The OpenVPN project provides a set of scripts for managing RSA certifi-
cates and keys: https://github.com/OpenVPN/easy-rsa

--askpass file
Get certificate password from console or file before we daemo-
nize.

Valid syntaxes:

askpass
askpass file

For the extremely security conscious, it is possible to protect
your private key with a password. Of course this means that
every time the OpenVPN daemon is started you must be there to
type the password. The --askpass option allows you to start
OpenVPN from the command line. It will query you for a password
before it daemonizes. To protect a private key with a password
you should omit the -nodes option when you use the openssl com-
mand line tool to manage certificates and private keys.

If file is specified, read the password from the first line of
file. Keep in mind that storing your password in a file to a
certain extent invalidates the extra security provided by using
an encrypted key.

--ca file
Certificate authority (CA) file in .pem format, also referred to
as the root certificate. This file can have multiple certifi-
cates in .pem format, concatenated together. You can construct
your own certificate authority certificate and private key by
using a command such as:

openssl req -nodes -new -x509 -keyout ca.key -out ca.crt

Then edit your openssl.cnf file and edit the certificate vari-
able to point to your new root certificate ca.crt.

For testing purposes only, the OpenVPN distribution includes a
sample CA certificate (ca.crt). Of course you should never use
the test certificates and test keys distributed with OpenVPN in
a production environment, since by virtue of the fact that they
are distributed with OpenVPN, they are totally insecure.

--capath dir
Directory containing trusted certificates (CAs and CRLs). Not
available with mbed TLS.

CAs in the capath directory are expected to be named <hash>.<n>.
CRLs are expected to be named <hash>.r<n>. See the -CApath
option of openssl verify, and the -hash option of openssl x509,
openssl crl and X509_LOOKUP_hash_dir()(3) for more information.

Similar to the --crl-verify option, CRLs are not mandatory -
OpenVPN will log the usual warning in the logs if the relevant
CRL is missing, but the connection will be allowed.

--cert file
Local peer's signed certificate in .pem format -- must be signed
by a certificate authority whose certificate is in --ca file.
Each peer in an OpenVPN link running in TLS mode should have its
own certificate and private key file. In addition, each certifi-
cate should have been signed by the key of a certificate author-
ity whose public key resides in the --ca certificate authority
file. You can easily make your own certificate authority (see
above) or pay money to use a commercial service such as
thawte.com (in which case you will be helping to finance the
world's second space tourist :). To generate a certificate, you
can use a command such as:

openssl req -nodes -new -keyout mycert.key -out mycert.csr

If your certificate authority private key lives on another
machine, copy the certificate signing request (mycert.csr) to
this other machine (this can be done over an insecure channel
such as email). Now sign the certificate with a command such as:

openssl ca -out mycert.crt -in mycert.csr

Now copy the certificate (mycert.crt) back to the peer which
initially generated the .csr file (this can be over a public
medium). Note that the openssl ca command reads the location of
the certificate authority key from its configuration file such
as /usr/share/ssl/openssl.cnf -- note also that for certificate
authority functions, you must set up the files index.txt (may be
empty) and serial (initialize to 01).

--crl-verify args
Check peer certificate against a Certificate Revocation List.

Valid syntax:

crl-verify file/directory flag

Examples:

crl-verify crl-file.pem
crl-verify /etc/openvpn/crls dir

A CRL (certificate revocation list) is used when a particular
key is compromised but when the overall PKI is still intact.

Suppose you had a PKI consisting of a CA, root certificate, and
a number of client certificates. Suppose a laptop computer con-
taining a client key and certificate was stolen. By adding the
stolen certificate to the CRL file, you could reject any connec-
tion which attempts to use it, while preserving the overall
integrity of the PKI.

The only time when it would be necessary to rebuild the entire
PKI from scratch would be if the root certificate key itself was
compromised.

The option is not mandatory - if the relevant CRL is missing,
OpenVPN will log a warning in the logs - e.g.

VERIFY WARNING: depth=0, unable to get certificate CRL

but the connection will be allowed. If the optional dir flag is
specified, enable a different mode where the crl-verify is
pointed at a directory containing files named as revoked serial
numbers (the files may be empty, the contents are never read).
If a client requests a connection, where the client certificate
serial number (decimal string) is the name of a file present in
the directory, it will be rejected.

Note: As the crl file (or directory) is read every time a peer
connects, if you are dropping root privileges with
--user, make sure that this user has sufficient privi-
leges to read the file.

--dh file
File containing Diffie Hellman parameters in .pem format
(required for --tls-server only).

Set file to none to disable Diffie Hellman key exchange (and use
ECDH only). Note that this requires peers to be using an SSL
library that supports ECDH TLS cipher suites (e.g. OpenSSL
1.0.1+, or mbed TLS 2.0+).

Use openssl dhparam -out dh2048.pem 2048 to generate 2048-bit DH
parameters. Diffie Hellman parameters may be considered public.

--ecdh-curve name
Specify the curve to use for elliptic curve Diffie Hellman.
Available curves can be listed with --show-curves. The specified
curve will only be used for ECDH TLS-ciphers.

This option is not supported in mbed TLS builds of OpenVPN.

--extra-certs file
Specify a file containing one or more PEM certs (concatenated
together) that complete the local certificate chain.

This option is useful for "split" CAs, where the CA for server
certs is different than the CA for client certs. Putting certs
in this file allows them to be used to complete the local cer-
tificate chain without trusting them to verify the peer-submit-
ted certificate, as would be the case if the certs were placed
in the ca file.

--hand-window n
Handshake Window -- the TLS-based key exchange must finalize
within n seconds of handshake initiation by any peer (default 60
seconds). If the handshake fails we will attempt to reset our
connection with our peer and try again. Even in the event of
handshake failure we will still use our expiring key for up to
--tran-window seconds to maintain continuity of transmission of
tunnel data.

The --hand-window parameter also controls the amount of time
that the OpenVPN client repeats the pull request until it times
out.

--key file
Local peer's private key in .pem format. Use the private key
which was generated when you built your peer's certificate (see
--cert file above).

--pkcs12 file
Specify a PKCS #12 file containing local private key, local cer-
tificate, and root CA certificate. This option can be used
instead of --ca, --cert, and --key. Not available with mbed
TLS.

--remote-cert-eku oid
Require that peer certificate was signed with an explicit
extended key usage.

This is a useful security option for clients, to ensure that the
host they connect to is a designated server.

The extended key usage should be encoded in oid notation, or
OpenSSL symbolic representation.

--remote-cert-ku key-usage
Require that peer certificate was signed with an explicit
key-usage.

If present in the certificate, the keyUsage value is validated
by the TLS library during the TLS handshake. Specifying this
option without arguments requires this extension to be present
(so the TLS library will verify it).

If key-usage is a list of usage bits, the keyUsage field must
have at least the same bits set as the bits in one of the values
supplied in the key-usage list.

The key-usage values in the list must be encoded in hex, e.g.

remote-cert-ku a0

--remote-cert-tls type
Require that peer certificate was signed with an explicit key
usage and extended key usage based on RFC3280 TLS rules.

Valid syntaxes:

remote-cert-tls server
remote-cert-tls client

This is a useful security option for clients, to ensure that the
host they connect to is a designated server. Or the other way
around; for a server to verify that only hosts with a client
certificate can connect.

The --remote-cert-tls client option is equivalent to

remote-cert-ku
remote-cert-eku "TLS Web Client Authentication"

The --remote-cert-tls server option is equivalent to

remote-cert-ku
remote-cert-eku "TLS Web Server Authentication"

This is an important security precaution to protect against a
man-in-the-middle attack where an authorized client attempts to
connect to another client by impersonating the server. The
attack is easily prevented by having clients verify the server
certificate using any one of --remote-cert-tls, --ver-
ify-x509-name, --peer-fingerprint or --tls-verify.

--tls-auth args
Add an additional layer of HMAC authentication on top of the TLS
control channel to mitigate DoS attacks and attacks on the TLS
stack.

Valid syntaxes:

tls-auth file
tls-auth file 0
tls-auth file 1

In a nutshell, --tls-auth enables a kind of "HMAC firewall" on
OpenVPN's TCP/UDP port, where TLS control channel packets bear-
ing an incorrect HMAC signature can be dropped immediately with-
out response.

file (required) is a file in OpenVPN static key format which can
be generated by --genkey.

Older versions (up to OpenVPN 2.3) supported a freeform
passphrase file. This is no longer supported in newer versions
(v2.4+).

See the --secret option for more information on the optional
direction parameter.

--tls-auth is recommended when you are running OpenVPN in a mode
where it is listening for packets from any IP address, such as
when --remote is not specified, or --remote is specified with
--float.

The rationale for this feature is as follows. TLS requires a
multi-packet exchange before it is able to authenticate a peer.
During this time before authentication, OpenVPN is allocating
resources (memory and CPU) to this potential peer. The potential
peer is also exposing many parts of OpenVPN and the OpenSSL
library to the packets it is sending. Most successful network
attacks today seek to either exploit bugs in programs (such as
buffer overflow attacks) or force a program to consume so many
resources that it becomes unusable. Of course the first line of
defense is always to produce clean, well-audited code. OpenVPN
has been written with buffer overflow attack prevention as a top
priority. But as history has shown, many of the most widely used
network applications have, from time to time, fallen to buffer
overflow attacks.

So as a second line of defense, OpenVPN offers this special
layer of authentication on top of the TLS control channel so
that every packet on the control channel is authenticated by an
HMAC signature and a unique ID for replay protection. This sig-
nature will also help protect against DoS (Denial of Service)
attacks. An important rule of thumb in reducing vulnerability to
DoS attacks is to minimize the amount of resources a potential,
but as yet unauthenticated, client is able to consume.

--tls-auth does this by signing every TLS control channel packet
with an HMAC signature, including packets which are sent before
the TLS level has had a chance to authenticate the peer. The
result is that packets without the correct signature can be
dropped immediately upon reception, before they have a chance to
consume additional system resources such as by initiating a TLS
handshake. --tls-auth can be strengthened by adding the
--replay-persist option which will keep OpenVPN's replay protec-
tion state in a file so that it is not lost across restarts.

It should be emphasized that this feature is optional and that
the key file used with --tls-auth gives a peer nothing more than
the power to initiate a TLS handshake. It is not used to encrypt
or authenticate any tunnel data.

Use --tls-crypt instead if you want to use the key file to not
only authenticate, but also encrypt the TLS control channel.

--tls-groups list
A list of allowable groups/curves in order of preference.

Set the allowed elliptic curves/groups for the TLS session.
These groups are allowed to be used in signatures and key
exchange.

mbedTLS currently allows all known curves per default.

OpenSSL 1.1+ restricts the list per default to

"X25519:secp256r1:X448:secp521r1:secp384r1".

If you use certificates that use non-standard curves, you might
need to add them here. If you do not force the ecdh curve by
using --ecdh-curve, the groups for ecdh will also be picked from
this list.

OpenVPN maps the curve name secp256r1 to prime256v1 to allow
specifying the same tls-groups option for mbedTLS and OpenSSL.

Warning: this option not only affects elliptic curve certifi-
cates but also the key exchange in TLS 1.3 and using this option
improperly will disable TLS 1.3.

--tls-cert-profile profile
Set the allowed cryptographic algorithms for certificates
according to profile.

The following profiles are supported:

insecure
Identical for mbed TLS to legacy

legacy (default)
SHA1 and newer, RSA 2048-bit+, any elliptic curve.

preferred
SHA2 and newer, RSA 2048-bit+, any elliptic curve.

suiteb SHA256/SHA384, ECDSA with P-256 or P-384.

This option is only fully supported for mbed TLS builds. OpenSSL
builds use the following approximation:

insecure
sets "security level 0"

legacy (default)
sets "security level 1"

preferred
sets "security level 2"

suiteb sets "security level 3" and --tls-cipher "SUITEB128".

OpenVPN will migrate to 'preferred' as default in the future.
Please ensure that your keys already comply.

WARNING: --tls-ciphers, --tls-ciphersuites and tls-groups
These options are expert features, which - if used correctly -
can improve the security of your VPN connection. But it is also
easy to unwittingly use them to carefully align a gun with your
foot, or just break your connection. Use with care!

--tls-cipher l
A list l of allowable TLS ciphers delimited by a colon (":").

These setting can be used to ensure that certain cipher suites
are used (or not used) for the TLS connection. OpenVPN uses TLS
to secure the control channel, over which the keys that are used
to protect the actual VPN traffic are exchanged.

The supplied list of ciphers is (after potential OpenSSL/IANA
name translation) simply supplied to the crypto library. Please
see the OpenSSL and/or mbed TLS documentation for details on the
cipher list interpretation.

For OpenSSL, the --tls-cipher is used for TLS 1.2 and below.

Use --show-tls to see a list of TLS ciphers supported by your
crypto library.

The default for --tls-cipher is to use mbed TLS's default cipher
list when using mbed TLS or
DEFAULT:!EXP:!LOW:!MEDIUM:!kDH:!kECDH:!DSS:!PSK:!SRP:!kRSA when
using OpenSSL.

--tls-ciphersuites l
Same as --tls-cipher but for TLS 1.3 and up. mbed TLS has no TLS
1.3 support yet and only the --tls-cipher setting is used.

The default for --tls-ciphersuites is to use the crypto
library's default.

--tls-client
Enable TLS and assume client role during TLS handshake.

--tls-crypt keyfile
Encrypt and authenticate all control channel packets with the
key from keyfile. (See --tls-auth for more background.)

Encrypting (and authenticating) control channel packets:

o provides more privacy by hiding the certificate used for the
TLS connection,

o makes it harder to identify OpenVPN traffic as such,

o provides "poor-man's" post-quantum security, against attackers
who will never know the pre-shared key (i.e. no forward
secrecy).

In contrast to --tls-auth, --tls-crypt does not require the user
to set --key-direction.

Security Considerations

All peers use the same --tls-crypt pre-shared group key to
authenticate and encrypt control channel messages. To ensure
that IV collisions remain unlikely, this key should not be used
to encrypt more than 2^48 client-to-server or 2^48
server-to-client control channel messages. A typical initial
negotiation is about 10 packets in each direction. Assuming both
initial negotiation and renegotiations are at most 2^16 (65536)
packets (to be conservative), and (re)negotiations happen each
minute for each user (24/7), this limits the tls-crypt key life-
time to 8171 years divided by the number of users. So a setup
with 1000 users should rotate the key at least once each eight
years. (And a setup with 8000 users each year.)

If IV collisions were to occur, this could result in the secu-
rity of --tls-crypt degrading to the same security as using
--tls-auth. That is, the control channel still benefits from
the extra protection against active man-in-the-middle-attacks
and DoS attacks, but may no longer offer extra privacy and
post-quantum security on top of what TLS itself offers.

For large setups or setups where clients are not trusted, con-
sider using --tls-crypt-v2 instead. That uses per-client unique
keys, and thereby improves the bounds to 'rotate a client key at
least once per 8000 years'.

--tls-crypt-v2 keyfile
Valid syntax:

tls-crypt-v2 keyfile
tls-crypt-v2 keyfile force-cookie
tls-crypt-v2 keyfile allow-noncookie

Use client-specific tls-crypt keys.

For clients, keyfile is a client-specific tls-crypt key. Such a
key can be generated using the --genkey tls-crypt-v2-client
option.

For servers, keyfile is used to unwrap client-specific keys sup-
plied by the client during connection setup. This key must be
the same as the key used to generate the client-specific key
(see --genkey tls-crypt-v2-client).

On servers, this option can be used together with the --tls-auth
or --tls-crypt option. In that case, the server will detect
whether the client is using client-specific keys, and automati-
cally select the right mode.

The optional parameters force-cookie allows only tls-crypt-v2
clients that support a cookie based stateless three way hand-
shake that avoids replay attacks and state exhaustion on the
server side (OpenVPN 2.6 and later). The option allow-noncookie
explicitly allows older tls-crypt-v2 clients. The default is
(currently) allow-noncookie.

--tls-crypt-v2-verify cmd
Run command cmd to verify the metadata of the client-specific
tls-crypt-v2 key of a connecting client. This allows server
administrators to reject client connections, before exposing the
TLS stack (including the notoriously dangerous X.509 and ASN.1
stacks) to the connecting client.

OpenVPN supplies the following environment variables to the com-
mand:

o script_type is set to tls-crypt-v2-verify

o metadata_type is set to 0 if the metadata was user supplied,
or 1 if it's a 64-bit unix timestamp representing the key cre-
ation time.

o metadata_file contains the filename of a temporary file that
contains the client metadata.

The command can reject the connection by exiting with a non-zero
exit code.

--tls-exit
Exit on TLS negotiation failure.

--tls-export-cert directory
Store the certificates the clients use upon connection to this
directory. This will be done before --tls-verify is called. The
certificates will use a temporary name and will be deleted when
the tls-verify script returns. The file name used for the cer-
tificate is available via the peer_cert environment variable.

--tls-server
Enable TLS and assume server role during TLS handshake. Note
that OpenVPN is designed as a peer-to-peer application. The des-
ignation of client or server is only for the purpose of negoti-
ating the TLS control channel.

--tls-timeout n
Packet retransmit timeout on TLS control channel if no acknowl-
edgment from remote within n seconds (default 2). When OpenVPN
sends a control packet to its peer, it will expect to receive an
acknowledgement within n seconds or it will retransmit the
packet, subject to a TCP-like exponential backoff algorithm.
This parameter only applies to control channel packets. Data
channel packets (which carry encrypted tunnel data) are never
acknowledged, sequenced, or retransmitted by OpenVPN because the
higher level network protocols running on top of the tunnel such
as TCP expect this role to be left to them.

--tls-version-min args
Sets the minimum TLS version we will accept from the peer
(default in 2.6.0 and later is "1.2").

Valid syntax:

tls-version-min version ['or-highest']

Examples for version include 1.0, 1.1, or 1.2. If or-highest is
specified and version is not recognized, we will only accept the
highest TLS version supported by the local SSL implementation.

--tls-version-max version
Set the maximum TLS version we will use (default is the highest
version supported). Examples for version include 1.0, 1.1, or
1.2.

--verify-hash args
DEPRECATED Specify SHA1 or SHA256 fingerprint for level-1 cert.

Valid syntax:

verify-hash hash [algo]

The level-1 cert is the CA (or intermediate cert) that signs the
leaf certificate, and is one removed from the leaf certificate
in the direction of the root. When accepting a connection from a
peer, the level-1 cert fingerprint must match hash or certifi-
cate verification will fail. Hash is specified as XX:XX:... For
example:

AD:B0:95:D8:09:C8:36:45:12:A9:89:C8:90:09:CB:13:72:A6:AD:16

The algo flag can be either SHA1 or SHA256. If not provided, it
defaults to SHA1.

This option can also be inlined

<verify-hash>
00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff
11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00
</verify-hash>

If the option is inlined, algo is always SHA256.

--peer-fingerprint args
Specify a SHA256 fingerprint or list of SHA256 fingerprints
to verify the peer certificate against. The peer certificate
must match one of the fingerprint or certificate verification
will fail. The option can also be inlined

Valid syntax:

peer-fingerprint AD:B0:95:D8:09:...

or inline:

<peer-fingerprint>
00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff
11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff:00
</peer-fingerprint>

When the --peer-fingerprint option is used, specifying a CA with
--ca or --capath is optional. This allows the he --peer-finger-
print to be used as alternative to a PKI with self-signed cer-
tificates for small setups. See the examples section for such a
setup.

--verify-x509-name args
Accept connections only if a host's X.509 name is equal to name.
The remote host must also pass all other tests of verification.

Valid syntax:

verify-x509 name type

Which X.509 name is compared to name depends on the setting of
type. type can be subject to match the complete subject DN
(default), name to match a subject RDN or name-prefix to match a
subject RDN prefix. Which RDN is verified as name depends on the
--x509-username-field option. But it defaults to the common name
(CN), e.g. a certificate with a subject DN

C=KG, ST=NA, L=Bishkek, CN=Server-1

would be matched by:

verify-x509-name 'C=KG, ST=NA, L=Bishkek, CN=Server-1'
verify-x509-name Server-1 name
verify-x509-name Server- name-prefix

The last example is useful if you want a client to only accept
connections to Server-1, Server-2, etc.

--verify-x509-name is a useful replacement for the --tls-verify
option to verify the remote host, because --verify-x509-name
works in a --chroot environment without any dependencies.

Using a name prefix is a useful alternative to managing a CRL
(Certificate Revocation List) on the client, since it allows the
client to refuse all certificates except for those associated
with designated servers.

NOTE: Test against a name prefix only when you are using Open-
VPN with a custom CA certificate that is under your con-
trol. Never use this option with type name-prefix when
your client certificates are signed by a third party,
such as a commercial web CA.

--x509-track attribute
Save peer X509 attribute value in environment for use by plugins
and management interface. Prepend a + to attribute to save val-
ues from full cert chain. Values will be encoded as
X509_<depth>_<attribute>=<value>. Multiple --x509-track options
can be defined to track multiple attributes.

--x509-username-field args
Fields in the X.509 certificate subject to be used as the user-
name (default CN). If multiple fields are specified their values
will be concatenated into the one username using _ symbol as a
separator.

Valid syntax:

x509-username-field [ext:]fieldname [[ext:]fieldname...]

Typically, this option is specified with fieldname arguments as
either of the following:

x509-username-field emailAddress
x509-username-field ext:subjectAltName
x509-username-field CN serialNumber

The first example uses the value of the emailAddress attribute
in the certificate's Subject field as the username. The second
example uses the ext: prefix to signify that the X.509 extension
fieldname subjectAltName be searched for an rfc822Name (email)
field to be used as the username. In cases where there are mul-
tiple email addresses in ext:fieldname, the last occurrence is
chosen. The last example uses the value of the CN attribute in
the Subject field, combined with the _ separator and the hexa-
decimal representation of the certificate's serialNumber.

When this option is used, the --verify-x509-name option will
match against the chosen fieldname instead of the Common Name.

Only the subjectAltName and issuerAltName X.509 extensions and
serialNumber X.509 attribute are supported.

Please note: This option has a feature which will convert an
all-lowercase fieldname to uppercase characters, e.g., ou -> OU.
A mixed-case fieldname or one having the ext: prefix will be
left as-is. This automatic upcasing feature is deprecated and
will be removed in a future release.

Non-compliant symbols are being replaced with the _ symbol, same
as the field separator, so concatenating multiple fields with
such or _ symbols can potentially lead to username collisions.

PKCS#11 / SmartCard options
--pkcs11-cert-private args
Set if access to certificate object should be performed after
login. Every provider has its own setting.

Valid syntaxes:

pkcs11-cert-private 0
pkcs11-cert-private 1

--pkcs11-id name
Specify the serialized certificate id to be used. The id can be
gotten by the standalone --show-pkcs11-ids option. See also the
description of --pkcs11-providers option.

--pkcs11-id-management
Acquire PKCS#11 id from management interface. In this case a
NEED-STR 'pkcs11-id-request' real-time message will be trig-
gered, application may use pkcs11-id-count command to retrieve
available number of certificates, and pkcs11-id-get command to
retrieve certificate id and certificate body. See also the
description of --pkcs11-providers option.

--pkcs11-pin-cache seconds
Specify how many seconds the PIN can be cached, the default is
until the token is removed.

--pkcs11-private-mode mode
Specify which method to use in order to perform private key
operations. A different mode can be specified for each
provider. Mode is encoded as hex number, and can be a mask one
of the following:

0 (default) Try to determine automatically.

1 Use sign.

2 Use sign recover.

4 Use decrypt.

8 Use unwrap.

--pkcs11-protected-authentication args
Use PKCS#11 protected authentication path, useful for biometric
and external keypad devices. Every provider has its own setting.

Valid syntaxes:

pkcs11-protected-authentication 0
pkcs11-protected-authentication 1

--pkcs11-providers providers
Specify an RSA Security Inc. PKCS #11 Cryptographic Token Inter-
face (Cryptoki) providers to load. A space-separated list of one
or more provider library names may be specified. This option
along with --pkcs11-id or pkcs11-id-management can be used
instead of --cert and --key or --pkcs12.

If p11-kit is present on the system and was enabled during
build, its p11-kit-proxy.so module will be loaded by default if
either the --pkcs11-id or --pkcs11-id-management options is
present without --pkcs11-providers. If default loading is not
enabled in the build and no providers are specified, the former
options will be ignored.

--show-pkcs11-ids args
(Standalone) Show PKCS#11 token object list.

Valid syntax:

show-pkcs11 [provider] [cert_private]

Specify cert_private as 1 if certificates are stored as private
objects.

If p11-kit is present on the system, the provider argument is
optional; if omitted the default p11-kit-proxy.so module will be
queried.

--verb option can be used BEFORE this option to produce debug-
ging information.

DATA CHANNEL CIPHER NEGOTIATION
OpenVPN 2.4 and higher have the capability to negotiate the data cipher
that is used to encrypt data packets. This section describes the mecha-
nism in more detail and the different backwards compatibility mechanism
with older server and clients.

OpenVPN 2.5 and later behaviour
When both client and server are at least running OpenVPN 2.5, that the
order of the ciphers of the server's --data-ciphers is used to pick the
the data cipher. That means that the first cipher in that list that is
also in the client's --data-ciphers list is chosen. If no common cipher
is found the client is rejected with a AUTH_FAILED message (as seen in
client log):
AUTH: Received control message: AUTH_FAILED,Data channel cipher
negotiation failed (no shared cipher)

OpenVPN 2.5 and later will only allow the ciphers specified in
--data-ciphers. If --data-ciphers is not set the default is
AES-256-GCM:AES-128-GCM. In 2.6 and later the default is changed to
AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305 when Chacha20-Poly1305 is
available.

For backwards compatibility OpenVPN 2.6 and later with --compat-mode
2.4.x (or lower) and OpenVPN 2.5 will automatically add a cipher speci-
fied using the --cipher option to this list.

OpenVPN 2.4 clients
The negotiation support in OpenVPN 2.4 was the first iteration of the
implementation and still had some quirks. Its main goal was "upgrade to
AES-256-GCM when possible". An OpenVPN 2.4 client that is built
against a crypto library that supports AES in GCM mode and does not
have --ncp-disable will always announce support for AES-256-GCM and
AES-128-GCM to a server by sending IV_NCP=2.

This only causes a problem if --ncp-ciphers option has been changed
from the default of AES-256-GCM:AES-128-GCM to a value that does not
include these two ciphers. When an OpenVPN server tries to use
AES-256-GCM or AES-128-GCM the connection will then fail. It is there-
fore recommended to always have the AES-256-GCM and AES-128-GCM ciphers
to the --ncp-ciphers options to avoid this behaviour.

OpenVPN 3 clients
Clients based on the OpenVPN 3.x library (-
https://github.com/openvpn/openvpn3/) do not have a configurable
--ncp-ciphers or --data-ciphers option. Newer versions by default dis-
able legacy AES-CBC, BF-CBC, and DES-CBC ciphers. These clients will
always announce support for all their supported AEAD ciphers
(AES-256-GCM, AES-128-GCM and in newer versions also
Chacha20-Poly1305).

To support OpenVPN 3.x based clients at least one of these ciphers
needs to be included in the server's --data-ciphers option.

OpenVPN 2.3 and older clients (and clients with --ncp-disable)
When a client without cipher negotiation support connects to a server
the cipher specified with the --cipher option in the client configura-
tion must be included in the --data-ciphers option of the server to
allow the client to connect. Otherwise the client will be sent the
AUTH_FAILED message that indicates no shared cipher.

If the client is 2.3 or older and has been configured with the
--enable-small ./configure argument, using data-ciphers-fallback
cipher in the server config file with the explicit cipher used by the
client is necessary.

OpenVPN 2.4 server
When a client indicates support for AES-128-GCM and AES-256-GCM (with
IV_NCP=2) an OpenVPN 2.4 server will send the first cipher of the
--ncp-ciphers to the OpenVPN client regardless of what the cipher is.
To emulate the behaviour of an OpenVPN 2.4 client as close as possible
and have compatibility to a setup that depends on this quirk, adding
AES-128-GCM and AES-256-GCM to the client's --data-ciphers option is
required. OpenVPN 2.5+ will only announce the IV_NCP=2 flag if those
ciphers are present.

OpenVPN 2.3 and older servers (and servers with --ncp-disable)
The cipher used by the server must be included in --data-ciphers to
allow the client connecting to a server without cipher negotiation sup-
port. (For compatibility OpenVPN 2.5 will also accept the cipher set
with --cipher)

If the server is 2.3 or older and has been configured with the
--enable-small ./configure argument, adding --data-ciphers-fallback
cipher to the client config with the explicit cipher used by the server
is necessary.

Blowfish in CBC mode (BF-CBC) deprecation
The --cipher option defaulted to BF-CBC in OpenVPN 2.4 and older ver-
sion. The default was never changed to ensure backwards compatibility.
In OpenVPN 2.5 this behaviour has now been changed so that if the
--cipher is not explicitly set it does not allow the weak BF-CBC cipher
any more and needs to explicitly added as --cipher BFC-CBC or added to
--data-ciphers.

We strongly recommend to switching away from BF-CBC to a more secure
cipher as soon as possible instead.

NETWORK CONFIGURATION
OpenVPN consists of two sides of network configuration. One side is
the link between the local and remote side, the other side is the vir-
tual network adapter (tun/tap device).

Link Options
This link options section covers options related to the connection
between the local and the remote host.

--bind keywords
Bind to local address and port. This is the default unless any
of --proto tcp-client , --http-proxy or --socks-proxy are used.

If the optional ipv6only keyword is present OpenVPN will bind
only to IPv6 (as opposed to IPv6 and IPv4) when a IPv6 socket is
opened.

--float
Allow remote peer to change its IP address and/or port number,
such as due to DHCP (this is the default if --remote is not
used). --float when specified with --remote allows an OpenVPN
session to initially connect to a peer at a known address, how-
ever if packets arrive from a new address and pass all authenti-
cation tests, the new address will take control of the session.
This is useful when you are connecting to a peer which holds a
dynamic address such as a dial-in user or DHCP client.

Essentially, --float tells OpenVPN to accept authenticated pack-
ets from any address, not only the address which was specified
in the --remote option.

--fragment args
Valid syntax:

fragment max
fragment max mtu

Enable internal datagram fragmentation so that no UDP datagrams
are sent which are larger than max bytes.

If the mtu parameter is present the max parameter is interpreted
to include IP and UDP encapsulation overhead. The mtu parameter
is introduced in OpenVPN version 2.6.0.

If the mtu parameter is absent, the max parameter is interpreted
in the same way as the --link-mtu parameter, i.e. the UDP
packet size after encapsulation overhead has been added in, but
not including the UDP header itself.

The --fragment option only makes sense when you are using the
UDP protocol (--proto udp).

--fragment adds 4 bytes of overhead per datagram.

See the --mssfix option below for an important related option to
--fragment.

It should also be noted that this option is not meant to replace
UDP fragmentation at the IP stack level. It is only meant as a
last resort when path MTU discovery is broken. Using this option
is less efficient than fixing path MTU discovery for your IP
link and using native IP fragmentation instead.

Having said that, there are circumstances where using OpenVPN's
internal fragmentation capability may be your only option, such
as tunneling a UDP multicast stream which requires fragmenta-
tion.

--keepalive args
A helper directive designed to simplify the expression of --ping
and --ping-restart.

Valid syntax:

keepalive interval timeout

Send ping once every interval seconds, restart if ping is not
received for timeout seconds.

This option can be used on both client and server side, but it
is enough to add this on the server side as it will push appro-
priate --ping and --ping-restart options to the client. If used
on both server and client, the values pushed from server will
override the client local values.

The timeout argument will be twice as long on the server side.
This ensures that a timeout is detected on client side before
the server side drops the connection.

For example, --keepalive 10 60 expands as follows:

if mode server:
ping 10 # Argument: interval
ping-restart 120 # Argument: timeout*2
push "ping 10" # Argument: interval
push "ping-restart 60" # Argument: timeout
else
ping 10 # Argument: interval
ping-restart 60 # Argument: timeout

--link-mtu n
DEPRECATED Sets an upper bound on the size of UDP packets which
are sent between OpenVPN peers. It's best not to set this param-
eter unless you know what you're doing.

Due to variable header size of IP header (20 bytes for IPv4 and
40 bytes for IPv6) and dynamically negotiated data channel
cipher, this option is not reliable. It is recommended to set
tun-mtu with enough headroom instead.

--local host
Local host name or IP address for bind. If specified, OpenVPN
will bind to this address only. If unspecified, OpenVPN will
bind to all interfaces.

--lport port
Set local TCP/UDP port number or name. Cannot be used together
with --nobind option.

--mark value
Mark encrypted packets being sent with value. The mark value can
be matched in policy routing and packetfilter rules. This option
is only supported in Linux and does nothing on other operating
systems.

--mode m
Set OpenVPN major mode. By default, OpenVPN runs in
point-to-point mode (p2p). OpenVPN 2.0 introduces a new mode
(server) which implements a multi-client server capability.

--mssfix args
Valid syntax:

mssfix max [mtu]

mssfix max [fixed]

mssfix

Announce to TCP sessions running over the tunnel that they
should limit their send packet sizes such that after OpenVPN has
encapsulated them, the resulting UDP packet size that OpenVPN
sends to its peer will not exceed max bytes. The default value
is 1492 mtu. Use 0 as max to disable mssfix.

If the mtu parameter is specified the max value is interpreted
as the resulting packet size of VPN packets including the IP and
UDP header. Support for the mtu parameter was added with Open-
VPN version 2.6.0.

If the mtu parameter is not specified, the max parameter is
interpreted in the same way as the --link-mtu parameter, i.e.
the UDP packet size after encapsulation overhead has been added
in, but not including the UDP header itself. Resulting packet
would be at most 28 bytes larger for IPv4 and 48 bytes for IPv6
(20/40 bytes for IP header and 8 bytes for UDP header). Default
value of 1450 allows OpenVPN packets to be transmitted over IPv4
on a link with MTU 1478 or higher without IP level fragmentation
(and 1498 for IPv6).

If the fixed parameter is specified, OpenVPN will make no
attempt to calculate the VPN encapsulation overhead but instead
will set the MSS to limit the size of the payload IP packets to
the specified number. IPv4 packets will have the MSS value low-
ered to mssfix - 40 and IPv6 packets to mssfix - 60.

if --mssfix is specified is specified without any parameter it
inherits the parameters of --fragment if specified or uses the
default for --mssfix otherwise.

The --mssfix option only makes sense when you are using the UDP
protocol for OpenVPN peer-to-peer communication, i.e. --proto
udp.

--mssfix and --fragment can be ideally used together, where
--mssfix will try to keep TCP from needing packet fragmentation
in the first place, and if big packets come through anyhow (from
protocols other than TCP), --fragment will internally fragment
them.

--max-packet-size, --fragment, and --mssfix are designed to work
around cases where Path MTU discovery is broken on the network
path between OpenVPN peers.

The usual symptom of such a breakdown is an OpenVPN connection
which successfully starts, but then stalls during active usage.

If --fragment and --mssfix are used together, --mssfix will take
its default max parameter from the --fragment max option.

Therefore, one could lower the maximum UDP packet size to 1300
(a good first try for solving MTU-related connection problems)
with the following options:

--tun-mtu 1500 --fragment 1300 --mssfix

If the max-packet-size size option is used in the configuration
it will also act as if mssfix size mtu was specified in the con-
figuration.

--mtu-disc type
Should we do Path MTU discovery on TCP/UDP channel? Only sup-
ported on OSes such as Linux that supports the necessary system
call to set.

Valid types:

no Never send DF (Don't Fragment) frames

maybe Use per-route hints

yes Always DF (Don't Fragment)

--mtu-test
To empirically measure MTU on connection startup, add the
--mtu-test option to your configuration. OpenVPN will send ping
packets of various sizes to the remote peer and measure the
largest packets which were successfully received. The --mtu-test
process normally takes about 3 minutes to complete.

--nobind
Do not bind to local address and port. The IP stack will allo-
cate a dynamic port for returning packets. Since the value of
the dynamic port could not be known in advance by a peer, this
option is only suitable for peers which will be initiating con-
nections by using the --remote option.

--passtos
Set the TOS field of the tunnel packet to what the payload's TOS
is.

--ping n
Ping remote over the TCP/UDP control channel if no packets have
been sent for at least n seconds (specify --ping on both peers
to cause ping packets to be sent in both directions since Open-
VPN ping packets are not echoed like IP ping packets). When used
in one of OpenVPN's secure modes (where --secret, --tls-server
or --tls-client is specified), the ping packet will be crypto-
graphically secure.

This option has two intended uses:

1. Compatibility with stateful firewalls. The periodic ping will
ensure that a stateful firewall rule which allows OpenVPN UDP
packets to pass will not time out.

2. To provide a basis for the remote to test the existence of
its peer using the --ping-exit option.

When using OpenVPN in server mode see also --keepalive.

--ping-exit n
Causes OpenVPN to exit after n seconds pass without reception of
a ping or other packet from remote. This option can be combined
with --inactive, --ping and --ping-exit to create a two-tiered
inactivity disconnect.

For example,

openvpn [options...] --inactive 3600 --ping 10 --ping-exit 60

when used on both peers will cause OpenVPN to exit within 60
seconds if its peer disconnects, but will exit after one hour if
no actual tunnel data is exchanged.

--ping-restart n
Similar to --ping-exit, but trigger a SIGUSR1 restart after n
seconds pass without reception of a ping or other packet from
remote.

This option is useful in cases where the remote peer has a
dynamic IP address and a low-TTL DNS name is used to track the
IP address using a service such as https://www.nsupdate.info/ +
a dynamic DNS client such as ddclient.

If the peer cannot be reached, a restart will be triggered,
causing the hostname used with --remote to be re-resolved (if
--resolv-retry is also specified).

In server mode, --ping-restart, --inactive or any other type of
internally generated signal will always be applied to individual
client instance objects, never to whole server itself. Note also
in server mode that any internally generated signal which would
normally cause a restart, will cause the deletion of the client
instance object instead.

In client mode, the --ping-restart parameter is set to 120 sec-
onds by default. This default will hold until the client pulls a
replacement value from the server, based on the --keepalive set-
ting in the server configuration. To disable the 120 second
default, set --ping-restart 0 on the client.

See the signals section below for more information on SIGUSR1.

Note that the behavior of SIGUSR1 can be modified by the --per-
sist-tun, --persist-key, --persist-local-ip and --per-
sist-remote-ip options.

Also note that --ping-exit and --ping-restart are mutually
exclusive and cannot be used together.

--ping-timer-rem
Run the --ping-exit / --ping-restart timer only if we have a
remote address. Use this option if you are starting the daemon
in listen mode (i.e. without an explicit --remote peer), and you
don't want to start clocking timeouts until a remote peer con-
nects.

--proto p
Use protocol p for communicating with remote host. p can be udp,
tcp-client, or tcp-server. You can also limit OpenVPN to use
only IPv4 or only IPv6 by specifying p as udp4, tcp4-client,
tcp4-server or udp6, tcp6-client, tcp6-server, respectively.

The default protocol is udp when --proto is not specified.

For UDP operation, --proto udp should be specified on both
peers.

For TCP operation, one peer must use --proto tcp-server and the
other must use --proto tcp-client. A peer started with
tcp-server will wait indefinitely for an incoming connection. A
peer started with tcp-client will attempt to connect, and if
that fails, will sleep for 5 seconds (adjustable via the --con-
nect-retry option) and try again infinite or up to N retries
(adjustable via the --connect-retry-max option). Both TCP client
and server will simulate a SIGUSR1 restart signal if either side
resets the connection.

OpenVPN is designed to operate optimally over UDP, but TCP capa-
bility is provided for situations where UDP cannot be used. In
comparison with UDP, TCP will usually be somewhat less efficient
and less robust when used over unreliable or congested networks.

This article outlines some of problems with tunneling IP over
TCP: http://sites.inka.de/sites/bigred/devel/tcp-tcp.html

There are certain cases, however, where using TCP may be advan-
tageous from a security and robustness perspective, such as tun-
neling non-IP or application-level UDP protocols, or tunneling
protocols which don't possess a built-in reliability layer.

--port port
TCP/UDP port number or port name for both local and remote (sets
both --lport and --rport options to given port). The current
default of 1194 represents the official IANA port number assign-
ment for OpenVPN and has been used since version 2.0-beta17.
Previous versions used port 5000 as the default.

--rport port
Set TCP/UDP port number or name used by the --remote option. The
port can also be set directly using the --remote option.

--replay-window args
Modify the replay protection sliding-window size and time win-
dow.

Valid syntaxes:

replay-window n
replay-window n t

Use a replay protection sliding-window of size n and a time win-
dow of t seconds.

By default n is 64 (the IPSec default) and t is 15 seconds.

This option is only relevant in UDP mode, i.e. when either
--proto udp is specified, or no --proto option is specified.

When OpenVPN tunnels IP packets over UDP, there is the possibil-
ity that packets might be dropped or delivered out of order.
Because OpenVPN, like IPSec, is emulating the physical network
layer, it will accept an out-of-order packet sequence, and will
deliver such packets in the same order they were received to the
TCP/IP protocol stack, provided they satisfy several con-
straints.

a. The packet cannot be a replay (unless --no-replay is speci-
fied, which disables replay protection altogether).

b. If a packet arrives out of order, it will only be accepted if
the difference between its sequence number and the highest
sequence number received so far is less than n.

c. If a packet arrives out of order, it will only be accepted if
it arrives no later than t seconds after any packet contain-
ing a higher sequence number.

If you are using a network link with a large pipeline (meaning
that the product of bandwidth and latency is high), you may want
to use a larger value for n. Satellite links in particular often
require this.

If you run OpenVPN at --verb 4, you will see the message
"PID_ERR replay-window backtrack occurred [x]" every time the
maximum sequence number backtrack seen thus far increases. This
can be used to calibrate n.

There is some controversy on the appropriate method of handling
packet reordering at the security layer.

Namely, to what extent should the security layer protect the
encapsulated protocol from attacks which masquerade as the kinds
of normal packet loss and reordering that occur over IP net-
works?

The IPSec and OpenVPN approach is to allow packet reordering
within a certain fixed sequence number window.

OpenVPN adds to the IPSec model by limiting the window size in
time as well as sequence space.

OpenVPN also adds TCP transport as an option (not offered by
IPSec) in which case OpenVPN can adopt a very strict attitude
towards message deletion and reordering: Don't allow it. Since
TCP guarantees reliability, any packet loss or reordering event
can be assumed to be an attack.

In this sense, it could be argued that TCP tunnel transport is
preferred when tunneling non-IP or UDP application protocols
which might be vulnerable to a message deletion or reordering
attack which falls within the normal operational parameters of
IP networks.

So I would make the statement that one should never tunnel a
non-IP protocol or UDP application protocol over UDP, if the
protocol might be vulnerable to a message deletion or reordering
attack that falls within the normal operating parameters of what
is to be expected from the physical IP layer. The problem is
easily fixed by simply using TCP as the VPN transport layer.

--replay-persist file
Persist replay-protection state across sessions using file to
save and reload the state.

This option will keep a disk copy of the current replay protec-
tion state (i.e. the most recent packet timestamp and sequence
number received from the remote peer), so that if an OpenVPN
session is stopped and restarted, it will reject any replays of
packets which were already received by the prior session.

This option only makes sense when replay protection is enabled
(the default) and you are using either --secret (shared-secret
key mode) or TLS mode with --tls-auth.

--session-timeout n
Raises SIGTERM for the client instance after n seconds since the
beginning of the session, forcing OpenVPN to disconnect. In
client mode, OpenVPN will disconnect and exit, while in server
mode all client sessions are terminated.

This option can also be specified in a client instance config
file using --client-config-dir or dynamically generated using a
--client-connect script. In these cases, only the related client
session is terminated.

--socket-flags flags
Apply the given flags to the OpenVPN transport socket. Cur-
rently, only TCP_NODELAY is supported.

The TCP_NODELAY socket flag is useful in TCP mode, and causes
the kernel to send tunnel packets immediately over the TCP con-
nection without trying to group several smaller packets into a
larger packet. This can result in a considerably improvement in
latency.

This option is pushable from server to client, and should be
used on both client and server for maximum effect.

--tcp-nodelay
This macro sets the TCP_NODELAY socket flag on the server as
well as pushes it to connecting clients. The TCP_NODELAY flag
disables the Nagle algorithm on TCP sockets causing packets to
be transmitted immediately with low latency, rather than waiting
a short period of time in order to aggregate several packets
into a larger containing packet. In VPN applications over TCP,
TCP_NODELAY is generally a good latency optimization.

The macro expands as follows:

if mode server:
socket-flags TCP_NODELAY
push "socket-flags TCP_NODELAY"

--max-packet-size size
This option will instruct OpenVPN to try to limit the maximum
on-write packet size by restricting the control channel packet
size and setting --mssfix.

OpenVPN will try to keep its control channel messages below this
size but due to some constraints in the protocol this is not
always possible. If the option is not set, the control packet
maximum size defaults to 1250. The control channel packet size
will be restricted to values between 154 and 2048. The maximum
packet size includes encapsulation overhead like UDP and IP.

In terms of --mssfix it will expand to:

mssfix size mtu

If you need to set --mssfix for data channel and control channel
maximum packet size independently, use --max-packet-size first,
followed by a --mssfix in the configuration.

In general the default size of 1250 should work almost univer-
sally apart from specific corner cases, especially since IPv6
requires a MTU of 1280 or larger.

Virtual Network Adapter (VPN interface)
Options in this section relates to configuration of the virtual tun/tap
network interface, including setting the VPN IP address and network
routing.

--bind-dev device
(Linux only) Set device to bind the server socket to a Virtual
Routing and Forwarding device

--block-ipv6
On the client, instead of sending IPv6 packets over the VPN tun-
nel, all IPv6 packets are answered with an ICMPv6 no route host
message. On the server, all IPv6 packets from clients are
answered with an ICMPv6 no route to host message. This options
is intended for cases when IPv6 should be blocked and other
options are not available. --block-ipv6 will use the remote IPv6
as source address of the ICMPv6 packets if set, otherwise will
use fe80::7 as source address.

For this option to make sense you actually have to route traffic
to the tun interface. The following example config block would
send all IPv6 traffic to OpenVPN and answer all requests with no
route to host, effectively blocking IPv6 (to avoid IPv6 connec-
tions from dual-stacked clients leaking around IPv4-only VPN
services).

Client config

--ifconfig-ipv6 fd15:53b6:dead::2/64 fd15:53b6:dead::1
--redirect-gateway ipv6
--block-ipv6

Server config
Push a "valid" ipv6 config to the client and block on the
server

--push "ifconfig-ipv6 fd15:53b6:dead::2/64 fd15:53b6:dead::1"
--push "redirect-gateway ipv6"
--block-ipv6

Note: this option does not influence traffic sent from the
server towards the client (neither on the server nor on the
client side). This is not seen as necessary, as such traffic
can be most easily avoided by not configuring IPv6 on the server
tun, or setting up a server-side firewall rule.

--dev device
TUN/TAP virtual network device which can be tunX, tapX, null or
an arbitrary name string (X can be omitted for a dynamic
device.)

See examples section below for an example on setting up a TUN
device.

You must use either tun devices on both ends of the connection
or tap devices on both ends. You cannot mix them, as they repre-
sent different underlying network layers:

tun devices encapsulate IPv4 or IPv6 (OSI Layer 3)

tap devices encapsulate Ethernet 802.3 (OSI Layer 2).

Valid syntaxes:

dev tun2
dev tap4
dev ovpn

What happens if the device name is not tun or tap is platform
dependent.

On most platforms, tunN (e.g. tun2, tun30) and tapN (e.g. tap3)
will create a numbered tun/tap interface with the number speci-
fied - this is useful if multiple OpenVPN instances are active,
and the instance-to-device mapping needs to be known. Some
platforms do not support "numbered tap", so trying --dev tap3
will fail.

Arbitrary device names (e.g. --dev tun-home) will only work on
FreeBSD (with the DCO kernel driver for tun devices) and Linux
(for both tun and tap devices, DCO and tun/tap driver).

If such a device name starts with tun or tap (e.g. tun-home),
OpenVPN will choose the right device type automatically. Other-
wise the desired device type needs to be specified with
--dev-type tun or --dev-type tap.

On Windows, only the names tun and tap are supported. Selection
among multiple installed drivers or driver instances is done
with --dev-node and --windows-driver.

--dev-node node
This is a highly system dependent option to influence tun/tap
driver selection.

On Linux, tun/tap devices are created by accessing /dev/net/tun,
and this device name can be changed using --dev-node ....

Under Mac OS X this option can be used to specify the default
tun implementation. Using --dev-node utun forces usage of the
native Darwin tun kernel support. Use --dev-node utunN to select
a specific utun instance. To force using the tun.kext
(/dev/tunX) use --dev-node tun. When not specifying a --dev-node
option openvpn will first try to open utun, and fall back to
tun.kext.

On Windows systems, select the TAP-Win32 adapter which is named
node in the Network Connections Control Panel or the raw GUID of
the adapter enclosed by braces. The --show-adapters option under
Windows can also be used to enumerate all available TAP-Win32
adapters and will show both the network connections control
panel name and the GUID for each TAP-Win32 adapter.

On other platforms, --dev-node node will influence the naming of
the created tun/tap device, if supported on that platform. If
OpenVPN cannot figure out whether node is a TUN or TAP device
based on the name, you should also specify --dev-type tun or
--dev-type tap.

--dev-type device-type
Which device type are we using? device-type should be tun (OSI
Layer 3) or tap (OSI Layer 2). Use this option only if the
TUN/TAP device used with --dev does not begin with tun or tap.

--dhcp-option args
Set additional network parameters on supported platforms. May be
specified on the client or pushed from the server. On Windows
these options are handled by the tap-windows6 driver by default
or directly by OpenVPN if dhcp is disabled or the wintun driver
is in use. The OpenVPN for Android client also handles them
internally.

On all other platforms these options are only saved in the
client's environment under the name foreign_option_{n} before
the --up script is called. A plugin or an --up script must be
used to pick up and interpret these as required. Many Linux dis-
tributions include such scripts and some third-party user inter-
faces such as tunnelblick also come with scripts that process
these options.

Valid syntax:

dhcp-option type [parm]

DOMAIN name
Set Connection-specific DNS Suffix to name.

ADAPTER_DOMAIN_SUFFIX name
Alias to DOMAIN. This is a compatibility option, it
should not be used in new deployments.

DOMAIN-SEARCH name
Add name to the domain search list. Repeat this option
to add more entries. Up to 10 domains are supported.

DNS address
Set primary domain name server IPv4 or IPv6 address.
Repeat this option to set secondary DNS server addresses.

Note: DNS IPv6 servers are currently set using netsh (the
existing DHCP code can only do IPv4 DHCP, and that proto-
col only permits IPv4 addresses anywhere). The option
will be put into the environment, so an --up script could
act upon it if needed.

WINS address
Set primary WINS server address (NetBIOS over TCP/IP Name
Server). Repeat this option to set secondary WINS server
addresses.

NBDD address
Set primary NBDD server address (NetBIOS over TCP/IP
Datagram Distribution Server). Repeat this option to set
secondary NBDD server addresses.

NTP address
Set primary NTP server address (Network Time Protocol).
Repeat this option to set secondary NTP server addresses.

NBT type
Set NetBIOS over TCP/IP Node type. Possible options:

1 b-node (broadcasts)

2 p-node (point-to-point name queries to a WINS
server)

4 m-node (broadcast then query name server)

8 h-node (query name server, then broadcast).

NBS scope-id
Set NetBIOS over TCP/IP Scope. A NetBIOS Scope ID pro-
vides an extended naming service for the NetBIOS over
TCP/IP (Known as NBT) module. The primary purpose of a
NetBIOS scope ID is to isolate NetBIOS traffic on a sin-
gle network to only those nodes with the same NetBIOS
scope ID. The NetBIOS scope ID is a character string that
is appended to the NetBIOS name. The NetBIOS scope ID on
two hosts must match, or the two hosts will not be able
to communicate. The NetBIOS Scope ID also allows comput-
ers to use the same computer name, as they have different
scope IDs. The Scope ID becomes a part of the NetBIOS
name, making the name unique. (This description of Net-
BIOS scopes courtesy of NeonSurge@abyss.com)

DISABLE-NBT
Disable Netbios-over-TCP/IP.

code PROXY_HTTP host port Sets a HTTP proxy that should be
used when connected to the VPN.

This option currently only works on OpenVPN for Android
and requires Android 10 or later.

--ifconfig args
Set TUN/TAP adapter parameters. It requires the IP address of
the local VPN endpoint. For TUN devices in point-to-point mode,
the next argument must be the VPN IP address of the remote VPN
endpoint. For TAP devices, or TUN devices used with --topology
subnet, the second argument is the subnet mask of the virtual
network segment which is being created or connected to.

For TUN devices, which facilitate virtual point-to-point IP con-
nections (when used in --topology net30 or p2p mode), the proper
usage of --ifconfig is to use two private IP addresses which are
not a member of any existing subnet which is in use. The IP
addresses may be consecutive and should have their order
reversed on the remote peer. After the VPN is established, by
pinging rn, you will be pinging across the VPN.

For TAP devices, which provide the ability to create virtual
ethernet segments, or TUN devices in --topology subnet mode
(which create virtual "multipoint networks"), --ifconfig is used
to set an IP address and subnet mask just as a physical ethernet
adapter would be similarly configured. If you are attempting to
connect to a remote ethernet bridge, the IP address and subnet
should be set to values which would be valid on the the bridged
ethernet segment (note also that DHCP can be used for the same
purpose).

This option, while primarily a proxy for the ifconfig(8) com-
mand, is designed to simplify TUN/TAP tunnel configuration by
providing a standard interface to the different ifconfig imple-
mentations on different platforms.

--ifconfig parameters which are IP addresses can also be speci-
fied as a DNS or /etc/hosts file resolvable name.

For TAP devices, --ifconfig should not be used if the TAP inter-
face will be getting an IP address lease from a DHCP server.

Examples:

# tun device in net30/p2p mode
ifconfig 10.8.0.2 10.8.0.1

# tun/tap device in subnet mode
ifconfig 10.8.0.2 255.255.255.0

--ifconfig-ipv6 args
Configure an IPv6 address on the tun device.

Valid syntax:

ifconfig-ipv6 ipv6addr/bits [ipv6remote]

The ipv6addr/bits argument is the IPv6 address to use. The sec-
ond parameter is used as route target for --route-ipv6 if no
gateway is specified.

The --topology option has no influence with --ifconfig-ipv6

--ifconfig-noexec
Don't actually execute ifconfig/netsh commands, instead pass
--ifconfig parameters to scripts using environmental variables.

--ifconfig-nowarn
Don't output an options consistency check warning if the
--ifconfig option on this side of the connection doesn't match
the remote side. This is useful when you want to retain the
overall benefits of the options consistency check (also see
--disable-occ option) while only disabling the ifconfig compo-
nent of the check.

For example, if you have a configuration where the local host
uses --ifconfig but the remote host does not, use --ifcon-
fig-nowarn on the local host.

This option will also silence warnings about potential address
conflicts which occasionally annoy more experienced users by
triggering "false positive" warnings.

--lladdr address
Specify the link layer address, more commonly known as the MAC
address. Only applied to TAP devices.

--persist-tun
Don't close and reopen TUN/TAP device or run up/down scripts
across SIGUSR1 or --ping-restart restarts.

SIGUSR1 is a restart signal similar to SIGHUP, but which offers
finer-grained control over reset options.

--redirect-gateway flags
Automatically execute routing commands to cause all outgoing IP
traffic to be redirected over the VPN. This is a client-side
option.

This option performs three steps:

1. Create a static route for the --remote address which forwards
to the pre-existing default gateway. This is done so that (3)
will not create a routing loop.

2. Delete the default gateway route.

3. Set the new default gateway to be the VPN endpoint address
(derived either from --route-gateway or the second parameter
to --ifconfig when --dev tun is specified).

When the tunnel is torn down, all of the above steps are
reversed so that the original default route is restored.

Option flags:

local Add the local flag if both OpenVPN peers are directly
connected via a common subnet, such as with wireless. The
local flag will cause step (1) above to be omitted.

autolocal
Try to automatically determine whether to enable local
flag above.

def1 Use this flag to override the default gateway by using
0.0.0.0/1 and 128.0.0.0/1 rather than 0.0.0.0/0. This has
the benefit of overriding but not wiping out the original
default gateway.

bypass-dhcp
Add a direct route to the DHCP server (if it is
non-local) which bypasses the tunnel (Available on Win-
dows clients, may not be available on non-Windows
clients).

bypass-dns
Add a direct route to the DNS server(s) (if they are
non-local) which bypasses the tunnel (Available on Win-
dows clients, may not be available on non-Windows
clients).

block-local
Block access to local LAN when the tunnel is active,
except for the LAN gateway itself. This is accomplished
by routing the local LAN (except for the LAN gateway
address) into the tunnel.

ipv6 Redirect IPv6 routing into the tunnel. This works similar
to the def1 flag, that is, more specific IPv6 routes are
added (2000::/4, 3000::/4), covering the whole IPv6 uni-
cast space.

!ipv4 Do not redirect IPv4 traffic - typically used in the flag
pair ipv6 !ipv4 to redirect IPv6-only.

--redirect-private flags
Like --redirect-gateway, but omit actually changing the default
gateway. Useful when pushing private subnets.

--route args
Add route to routing table after connection is established. Mul-
tiple routes can be specified. Routes will be automatically torn
down in reverse order prior to TUN/TAP device close.

Valid syntaxes:

route network/IP
route network/IP netmask
route network/IP netmask gateway
route network/IP netmask gateway metric

This option is intended as a convenience proxy for the route(8)
shell command, while at the same time providing portable seman-
tics across OpenVPN's platform space.

netmask
defaults to 255.255.255.255 when not given

gateway
default taken from --route-gateway or the second parame-
ter to --ifconfig when --dev tun is specified.

metric default taken from --route-metric if set, otherwise 0.

The default can be specified by leaving an option blank or set-
ting it to default.

The network and gateway parameters can also be specified as a
DNS or /etc/hosts file resolvable name, or as one of three spe-
cial keywords:

vpn_gateway
The remote VPN endpoint address (derived either from
--route-gateway or the second parameter to --ifconfig
when --dev tun is specified).

net_gateway
The pre-existing IP default gateway, read from the rout-
ing table (not supported on all OSes).

remote_host
The --remote address if OpenVPN is being run in client
mode, and is undefined in server mode.

--route-delay args
Valid syntaxes:

route-delay
route-delay n
route-delay n w

Delay n seconds (default 0) after connection establishment,
before adding routes. If n is 0, routes will be added immedi-
ately upon connection establishment. If --route-delay is omit-
ted, routes will be added immediately after TUN/TAP device open
and --up script execution, before any --user or --group privi-
lege downgrade (or --chroot execution.)

This option is designed to be useful in scenarios where DHCP is
used to set tap adapter addresses. The delay will give the DHCP
handshake time to complete before routes are added.

On Windows, --route-delay tries to be more intelligent by wait-
ing w seconds (default 30) for the TAP-Win32 adapter to come up
before adding routes.

--route-ipv6 args
Setup IPv6 routing in the system to send the specified IPv6 net-
work into OpenVPN's tun.

Valid syntax:

route-ipv6 ipv6addr/bits [gateway] [metric]

The gateway parameter is only used for IPv6 routes across tap
devices, and if missing, the ipv6remote field from --ifcon-
fig-ipv6 or --route-ipv6-gateway is used.

--route-gateway arg
Specify a default gateway for use with --route.

If dhcp is specified as the parameter, the gateway address will
be extracted from a DHCP negotiation with the OpenVPN
server-side LAN.

Valid syntaxes:

route-gateway gateway
route-gateway dhcp

--route-ipv6-gateway gw
Specify a default gateway gw for use with --route-ipv6.

--route-metric m
Specify a default metric m for use with --route.

--route-noexec
Don't add or remove routes automatically. Instead pass routes to
--route-up script using environmental variables.

--route-nopull
When used with --client or --pull, accept options pushed by
server EXCEPT for routes, block-outside-dns and dhcp options
like DNS servers.

When used on the client, this option effectively bars the server
from adding routes to the client's routing table, however note
that this option still allows the server to set the TCP/IP prop-
erties of the client's TUN/TAP interface.

--topology mode
Configure virtual addressing topology when running in --dev tun
mode. This directive has no meaning in --dev tap mode, which
always uses a subnet topology.

If you set this directive on the server, the --server and
--server-bridge directives will automatically push your chosen
topology setting to clients as well. This directive can also be
manually pushed to clients. Like the --dev directive, this
directive must always be compatible between client and server.

mode can be one of:

net30 Use a point-to-point topology, by allocating one /30 sub-
net per client. This is designed to allow point-to-point
semantics when some or all of the connecting clients
might be Windows systems. This is the default.

p2p Use a point-to-point topology where the remote endpoint
of the client's tun interface always points to the local
endpoint of the server's tun interface. This mode allo-
cates a single IP address per connecting client. Only use
when none of the connecting clients are Windows systems.

subnet Use a subnet rather than a point-to-point topology by
configuring the tun interface with a local IP address and
subnet mask, similar to the topology used in --dev tap
and ethernet bridging mode. This mode allocates a single
IP address per connecting client and works on Windows as
well.

Note: Using --topology subnet changes the interpretation of the
arguments of --ifconfig to mean "address netmask", no longer
"local remote".

--tun-mtu args
Valid syntaxes:

tun-mtu tun-mtu
tun-mtu tun-mtu occ-mtu

Take the TUN device MTU to be tun-mtu and derive the link MTU
from it. In most cases, you will probably want to leave this
parameter set to its default value.

The default for tun-mtu is 1500.

The OCC MTU can be used to avoid warnings about mismatched MTU
from clients. If occ-mtu is not specified, it will to default to
the tun-mtu.

The MTU (Maximum Transmission Units) is the maximum datagram
size in bytes that can be sent unfragmented over a particular
network path. OpenVPN requires that packets on the control and
data channels be sent unfragmented.

MTU problems often manifest themselves as connections which hang
during periods of active usage.

It's best to use the --fragment and/or --mssfix options to deal
with MTU sizing issues.

Note: Depending on the platform, the operating system allows one
to receive packets larger than tun-mtu (e.g. Linux and FreeBSD)
but other platforms (like macOS) limit received packets to the
same size as the MTU.

--tun-max-mtu maxmtu
This configures the maximum MTU size that a server can push to
maxmtu, by configuring the internal buffers to allow at least
this packet size. The default for maxmtu is 1600. Currently,
only increasing beyond 1600 is possible, and attempting to
reduce max-mtu below 1600 will be ignored.

--tun-mtu-extra n
Assume that the TUN/TAP device might return as many as n bytes
more than the --tun-mtu size on read. This parameter defaults to
0, which is sufficient for most TUN devices. TAP devices may
introduce additional overhead in excess of the MTU size, and a
setting of 32 is the default when TAP devices are used. This
parameter only controls internal OpenVPN buffer sizing, so there
is no transmission overhead associated with using a larger
value.

TUN/TAP standalone operations
These two standalone operations will require --dev and optionally
--user and/or --group.

--mktun
(Standalone) Create a persistent tunnel on platforms which sup-
port them such as Linux. Normally TUN/TAP tunnels exist only for
the period of time that an application has them open. This
option takes advantage of the TUN/TAP driver's ability to build
persistent tunnels that live through multiple instantiations of
OpenVPN and die only when they are deleted or the machine is
rebooted.

One of the advantages of persistent tunnels is that they elimi-
nate the need for separate --up and --down scripts to run the
appropriate ifconfig(8) and route(8) commands. These commands
can be placed in the the same shell script which starts or ter-
minates an OpenVPN session.

Another advantage is that open connections through the
TUN/TAP-based tunnel will not be reset if the OpenVPN peer
restarts. This can be useful to provide uninterrupted connectiv-
ity through the tunnel in the event of a DHCP reset of the
peer's public IP address (see the --ipchange option above).

One disadvantage of persistent tunnels is that it is harder to
automatically configure their MTU value (see --link-mtu and
--tun-mtu above).

On some platforms such as Windows, TAP-Win32 tunnels are persis-
tent by default.

--rmtun
(Standalone) Remove a persistent tunnel.

Virtual Routing and Forwarding
Options in this section relates to configuration of virtual routing and
forwarding in combination with the underlying operating system.

As of today this is only supported on Linux, a kernel >= 4.9 is recom-
mended.

This could come in handy when for example the external network should
be only used as a means to connect to some VPN endpoints and all regu-
lar traffic should only be routed through any tunnel(s). This could be
achieved by setting up a VRF and configuring the interface connected to
the external network to be part of the VRF. The examples below will
cover this setup.

Another option would be to put the tun/tap interface into a VRF. This
could be done by an up-script which uses the ip link set command shown
below.

VRF setup with iproute2
Create VRF vrf_external and map it to routing table 1023

ip link add vrf_external type vrf table 1023

Move eth0 into vrf_external

ip link set master vrf_external dev eth0

Any prefixes configured on eth0 will be moved from the :code`main`
routing table into routing table 1023

VRF setup with ifupdown
For Debian based Distributions ifupdown2 provides an almost drop-in
replacement for ifupdown including VRFs and other features. A configu-
ration for an interface eth0 being part of VRF code:vrf_external could
look like this:

auto eth0
iface eth0
address 192.0.2.42/24
address 2001:db8:08:15::42/64
gateway 192.0.2.1
gateway 2001:db8:08:15::1
vrf vrf_external

auto vrf_external
iface vrf_external
vrf-table 1023

OpenVPN configuration
The OpenVPN configuration needs to contain this line:

bind-dev vrf_external

Further reading
Wikipedia has nice page one VRFs:
https://en.wikipedia.org/wiki/Virtual_routing_and_forwarding

This talk from the Network Track of FrOSCon 2018 provides an overview
about advanced layer 2 and layer 3 features of Linux

o Slides:
https://www.slideshare.net/BarbarossaTM/l2l3-fr-fortgeschrittene-helle-und-dunkle-magie-im-linuxnetzwerkstack

o Video (german):
https://media.ccc.de/v/froscon2018-2247-l2_l3_fur_fortgeschrittene_-_helle_und_dunkle_magie_im_linux-netzwerkstack

SCRIPTING INTEGRATION
OpenVPN can execute external scripts in various phases of the lifetime
of the OpenVPN process.

Script Order of Execution
1. --up

Executed after TCP/UDP socket bind and TUN/TAP open.

2. --tls-verify

Executed when we have a still untrusted remote peer.

3. --ipchange

Executed after connection authentication, or remote IP address
change.

4. --client-connect

Executed in --mode server mode immediately after client authentica-
tion.

5. --route-up

Executed after connection authentication, either immediately after,
or some number of seconds after as defined by the --route-delay
option.

6. --route-pre-down

Executed right before the routes are removed.

7. --client-disconnect

Executed in --mode server mode on client instance shutdown.

8. --down

Executed after TCP/UDP and TUN/TAP close.

9. --learn-address

Executed in --mode server mode whenever an IPv4 address/route or
MAC address is added to OpenVPN's internal routing table.

10. --auth-user-pass-verify

Executed in --mode server mode on new client connections, when the
client is still untrusted.

11. --client-crresponse
Execute in --mode server whenever a client sends a CR_RESPONSE
message

SCRIPT HOOKS
--auth-user-pass-verify args
Require the client to provide a username/password (possibly in
addition to a client certificate) for authentication.

Valid syntax:

auth-user-pass-verify cmd method

OpenVPN will run command cmd to validate the username/password
provided by the client.

cmd consists of a path to a script (or executable program),
optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using a backslash, and
should be separated by one or more spaces.

If method is set to via-env, OpenVPN will call cmd with the
environmental variables username and password set to the user-
name/password strings provided by the client. Beware that this
method is insecure on some platforms which make the environment
of a process publicly visible to other unprivileged processes.

If method is set to via-file, OpenVPN will write the username
and password to the first two lines of a temporary file. The
filename will be passed as an argument to cmd, and the file will
be automatically deleted by OpenVPN after the script returns.
The location of the temporary file is controlled by the
--tmp-dir option, and will default to the current directory if
unspecified. For security, consider setting --tmp-dir to a
volatile storage medium such as /dev/shm (if available) to pre-
vent the username/password file from touching the hard drive.

The script should examine the username and password, returning a
success exit code (0) if the client's authentication request is
to be accepted, a failure code (1) to reject the client, or a
that the authentication is deferred (2). If the authentication
is deferred, the script must fork/start a background or another
non-blocking operation to continue the authentication in the
background. When finshing the authentication, a 1 or 0 must be
written to the file specified by the auth_control_file.

If the file specified by auth_failed_reason_file exists and has
non-empty content, the content of this file will be used as
AUTH_FAILED message. To avoid race conditions, this file should
be written before auth_control_file.

This auth fail reason can be something simple like "User has
been permanently disabled" but there are also some special auth
failed messages.

The TEMP message indicates that the authentication temporarily
failed and that the client should continue to retry to connect.
The server can optionally give a user readable message and hint
the client a behavior how to proceed. The keywords of the
AUTH_FAILED,TEMP message are comma separated keys/values and
provide a hint to the client how to proceed. Currently defined
keywords are:

backoff s
instructs the client to wait at least s seconds before
the next connection attempt. If the client already uses a
higher delay for reconnection attempt, the delay will not
be shortened.

advance addr
Instructs the client to reconnect to the next (IP)
address of the current server.

advance remote
Instructs the client to skip the remaining IP addresses
of the current server and instead connect to the next
server specified in the configuration file.

advance no
Instructs the client to retry connecting to the same
server again.

For example, the message TEMP[backoff 42,advance no]: No free IP
addresses indicates that the VPN connection can currently not
succeed and instructs the client to retry in 42 seconds again.

When deferred authentication is in use, the script can also
request pending authentication by writing to the file specified
by the auth_pending_file. The first line must be the timeout in
seconds, the required method on the second line (e.g. crtext)
and third line must be the EXTRA as documented in the
client-pending-auth section of doc/management.txt.

This directive is designed to enable a plugin-style interface
for extending OpenVPN's authentication capabilities.

To protect against a client passing a maliciously formed user-
name or password string, the username string must consist only
of these characters: alphanumeric, underbar ('_'), dash ('-'),
dot ('.'), or at ('@'). The password string can consist of any
printable characters except for CR or LF. Any illegal characters
in either the username or password string will be converted to
underbar ('_').

Care must be taken by any user-defined scripts to avoid creating
a security vulnerability in the way that these strings are han-
dled. Never use these strings in such a way that they might be
escaped or evaluated by a shell interpreter.

For a sample script that performs PAM authentication, see sam-
ple-scripts/auth-pam.pl in the OpenVPN source distribution.

--client-crresponse
Executed when the client sends a text based challenge
response.

Valid syntax:

client-crresponse cmd

OpenVPN will write the response of the client into a temporary
file. The filename will be passed as an argument to cmd, and
the file will be automatically deleted by OpenVPN after the
script returns.

The response is passed as is from the client. The script needs
to check itself if the input is valid, e.g. if the input is
valid base64 encoding.

The script can either directly write the result of the verifica-
tion to auth_control_file or further defer it. See
`--auth-user-pass-verify`` for details.

For a sample script that implement TOTP (RFC 6238) based
two-factor authentication, see sample-scripts/totpauth.py.

--client-connect cmd
Run command cmd on client connection.

The command is passed the common name and IP address of the
just-authenticated client as environmental variables (see envi-
ronmental variable section below). The command is also passed
the pathname of a freshly created temporary file as the last
argument (after any arguments specified in cmd ), to be used by
the command to pass dynamically generated config file directives
back to OpenVPN.

If the script wants to generate a dynamic config file to be
applied on the server when the client connects, it should write
it to the file named by the last argument.

See the --client-config-dir option below for options which can
be legally used in a dynamically generated config file.

Note that the return value of script is significant. If script
returns a non-zero error status, it will cause the client to be
disconnected.

If a --client-connect wants to defer the generating of the con-
figuration then the script needs to use the client_con-
nect_deferred_file and client_connect_config_file environment
variables, and write status accordingly into these files. See
the Environmental Variables section for more details.

--client-disconnect cmd
Like --client-connect but called on client instance shutdown.
Will not be called unless the --client-connect script and plug-
ins (if defined) were previously called on this instance with
successful (0) status returns.

The exception to this rule is if the --client-disconnect command
or plugins are cascaded, and at least one client-connect func-
tion succeeded, then ALL of the client-disconnect functions for
scripts and plugins will be called on client instance object
deletion, even in cases where some of the related client-connect
functions returned an error status.

The --client-disconnect command is not passed any extra argu-
ments (only those arguments specified in cmd, if any).

--down cmd
Run command cmd after TUN/TAP device close (post --user UID
change and/or --chroot ). cmd consists of a path to script (or
executable program), optionally followed by arguments. The path
and arguments may be single- or double-quoted and/or escaped
using a backslash, and should be separated by one or more spa-
ces.

Called with the same parameters and environmental variables as
the --up option above.

Note that if you reduce privileges by using --user and/or
--group, your --down script will also run at reduced privilege.

--down-pre
Call --down cmd/script before, rather than after, TUN/TAP close.

--ipchange cmd
Run command cmd when our remote ip-address is initially authen-
ticated or changes.

When cmd is executed two arguments are appended after any argu-
ments specified in cmd , as follows:

cmd ip address port number

Don't use --ipchange in --mode server mode. Use a --client-con-
nect script instead.

See the Environmental Variables section below for additional
parameters passed as environmental variables.

If you are running in a dynamic IP address environment where the
IP addresses of either peer could change without notice, you can
use this script, for example, to edit the /etc/hosts file with
the current address of the peer. The script will be run every
time the remote peer changes its IP address.

Similarly if our IP address changes due to DHCP, we should con-
figure our IP address change script (see man page for dhcpcd(8))
to deliver a SIGHUP or SIGUSR1 signal to OpenVPN. OpenVPN will
then re-establish a connection with its most recently authenti-
cated peer on its new IP address.

--learn-address cmd
Run command cmd to validate client virtual addresses or routes.

Three arguments will be appended to any arguments in cmd as fol-
lows:

$1 - [operation]
"add", "update", or "delete" based on whether or not the
address is being added to, modified, or deleted from
OpenVPN's internal routing table.

$2 - [address]
The address being learned or unlearned. This can be an
IPv4 address such as "198.162.10.14", an IPv4 subnet such
as "198.162.10.0/24", or an ethernet MAC address (when
--dev tap is being used) such as "00:FF:01:02:03:04".

$3 - [common name]
The common name on the certificate associated with the
client linked to this address. Only present for "add" or
"update" operations, not "delete".

On "add" or "update" methods, if the script returns a failure
code (non-zero), OpenVPN will reject the address and will not
modify its internal routing table.

Normally, the cmd script will use the information provided above
to set appropriate firewall entries on the VPN TUN/TAP inter-
face. Since OpenVPN provides the association between virtual IP
or MAC address and the client's authenticated common name, it
allows a user-defined script to configure firewall access poli-
cies with regard to the client's high-level common name, rather
than the low level client virtual addresses.

--route-up cmd
Run command cmd after routes are added, subject to
--route-delay.

See the Environmental Variables section below for additional
parameters passed as environmental variables.

--route-pre-down cmd
Run command cmd before routes are removed upon disconnection.

See the Environmental Variables section below for additional
parameters passed as environmental variables.

--setenv args
Set a custom environmental variable name=value to pass to
script.

Valid syntaxes:

setenv name value
setenv FORWARD_COMPATIBLE 1
setenv opt config_option

By setting FORWARD_COMPATIBLE to 1, the config file syntax
checking is relaxed so that unknown directives will trigger a
warning but not a fatal error, on the assumption that a given
unknown directive might be valid in future OpenVPN versions.

It is also possible to tag a single directive so as not to trig-
ger a fatal error if the directive isn't recognized. To do this,
prepend the following before the directive: setenv opt

Versions prior to OpenVPN 2.3.3 will always ignore options set
with the setenv opt directive.

See also --ignore-unknown-option

--setenv-safe args
Set a custom environmental variable OPENVPN_name to value to
pass to scripts.

Valid syntaxes:

setenv-safe name value

This directive is designed to be pushed by the server to
clients, and the prepending of OPENVPN_ to the environmental
variable is a safety precaution to prevent a LD_PRELOAD style
attack from a malicious or compromised server.

--tls-verify cmd
Run command cmd to verify the X509 name of a pending TLS connec-
tion that has otherwise passed all other tests of certification
(except for revocation via --crl-verify directive; the revoca-
tion test occurs after the --tls-verify test).

cmd should return 0 to allow the TLS handshake to proceed, or 1
to fail.

When cmd is executed two arguments are appended after any argu-
ments specified in cmd, as follows:

cmd certificate_depth subject

These arguments are, respectively, the current certificate depth
and the X509 subject distinguished name (dn) of the peer.

This feature is useful if the peer you want to trust has a cer-
tificate which was signed by a certificate authority who also
signed many other certificates, where you don't necessarily want
to trust all of them, but rather be selective about which peer
certificate you will accept. This feature allows you to write a
script which will test the X509 name on a certificate and decide
whether or not it should be accepted. For a simple perl script
which will test the common name field on the certificate, see
the file verify-cn in the OpenVPN distribution.

See the Environmental Variables section below for additional
parameters passed as environmental variables.

--up cmd
Run command cmd after successful TUN/TAP device open (pre --user
UID change).

The up command is useful for specifying route commands which
route IP traffic destined for private subnets which exist at the
other end of the VPN connection into the tunnel.

For --dev tun execute as:

cmd tun_dev tun_mtu 0 ifconfig_local_ip ifconfig_remote_ip [init | restart]

For --dev tap execute as:

cmd tap_dev tap_mtu 0 ifconfig_local_ip ifconfig_netmask [init | restart]

See the Environmental Variables section below for additional
parameters passed as environmental variables. The 0 argument
used to be link_mtu which is no longer passed to scripts - to
keep the argument order, it was replaced with 0.

Note that if cmd includes arguments, all OpenVPN-generated argu-
ments will be appended to them to build an argument list with
which the executable will be called.

Typically, cmd will run a script to add routes to the tunnel.

Normally the up script is called after the TUN/TAP device is
opened. In this context, the last command line parameter passed
to the script will be init. If the --up-restart option is also
used, the up script will be called for restarts as well. A
restart is considered to be a partial reinitialization of Open-
VPN where the TUN/TAP instance is preserved (the --persist-tun
option will enable such preservation). A restart can be gener-
ated by a SIGUSR1 signal, a --ping-restart timeout, or a connec-
tion reset when the TCP protocol is enabled with the --proto
option. If a restart occurs, and --up-restart has been speci-
fied, the up script will be called with restart as the last
parameter.

NOTE: On restart, OpenVPN will not pass the full set of envi-
ronment variables to the script. Namely, everything
related to routing and gateways will not be passed, as
nothing needs to be done anyway - all the routing setup
is already in place. Additionally, the up-restart script
will run with the downgraded UID/GID settings (if config-
ured).

The following standalone example shows how the --up script can
be called in both an initialization and restart context. (NOTE:
for security reasons, don't run the following example unless UDP
port 9999 is blocked by your firewall. Also, the example will
run indefinitely, so you should abort with control-c).

openvpn --dev tun --port 9999 --verb 4 --ping-restart 10 \
--up 'echo up' --down 'echo down' --persist-tun \
--up-restart

Note that OpenVPN also provides the --ifconfig option to auto-
matically ifconfig the TUN device, eliminating the need to
define an --up script, unless you also want to configure routes
in the --up script.

If --ifconfig is also specified, OpenVPN will pass the ifconfig
local and remote endpoints on the command line to the --up
script so that they can be used to configure routes such as:

route add -net 10.0.0.0 netmask 255.255.255.0 gw $5

--up-delay
Delay TUN/TAP open and possible --up script execution until
after TCP/UDP connection establishment with peer.

In --proto udp mode, this option normally requires the use of
--ping to allow connection initiation to be sensed in the
absence of tunnel data, since UDP is a "connectionless" proto-
col.

On Windows, this option will delay the TAP-Win32 media state
transitioning to "connected" until connection establishment,
i.e. the receipt of the first authenticated packet from the
peer.

--up-restart
Enable the --up and --down scripts to be called for restarts as
well as initial program start. This option is described more
fully above in the --up option documentation.

String Types and Remapping
In certain cases, OpenVPN will perform remapping of characters in
strings. Essentially, any characters outside the set of permitted char-
acters for each string type will be converted to underbar ('_').

Q: Why is string remapping necessary?
It's an important security feature to prevent the malicious cod-
ing of strings from untrusted sources to be passed as parameters
to scripts, saved in the environment, used as a common name,
translated to a filename, etc.

Q: Can string remapping be disabled?
Yes, by using the --no-name-remapping option, however this
should be considered an advanced option.

Here is a brief rundown of OpenVPN's current string types and the per-
mitted character class for each string:

X509 Names
Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at ('@'),
colon (':'), slash ('/'), and equal ('='). Alphanumeric is
defined as a character which will cause the C library isalnum()
function to return true.

Common Names
Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and at
('@').

--auth-user-pass username
Same as Common Name, with one exception: starting with OpenVPN
2.0.1, the username is passed to the OPENVPN_PLUG-
IN_AUTH_USER_PASS_VERIFY plugin in its raw form, without string
remapping.

--auth-user-pass password
Any "printable" character except CR or LF. Printable is defined
to be a character which will cause the C library isprint() func-
tion to return true.

--client-config-dir filename as derived from common name or`username
Alphanumeric, underbar ('_'), dash ('-'), and dot ('.') except
for "." or ".." as standalone strings. As of v2.0.1-rc6, the at
('@') character has been added as well for compatibility with
the common name character class.

Environmental variable names
Alphanumeric or underbar ('_').

Environmental variable values
Any printable character.

For all cases, characters in a string which are not members of the
legal character class for that string type will be remapped to underbar
('_').

Environmental Variables
Once set, a variable is persisted indefinitely until it is reset by a
new value or a restart,

As of OpenVPN 2.0-beta12, in server mode, environmental variables set
by OpenVPN are scoped according to the client objects they are associ-
ated with, so there should not be any issues with scripts having access
to stale, previously set variables which refer to different client
instances.

bytes_received
Total number of bytes received from client during VPN session.
Set prior to execution of the --client-disconnect script.

bytes_sent
Total number of bytes sent to client during VPN session. Set
prior to execution of the --client-disconnect script.

client_connect_config_file
The path to the configuration file that should be written to by
the --client-connect script (optional, if per-session configura-
tion is desired). This is the same file name as passed via com-
mand line argument on the call to the --client-connect script.

client_connect_deferred_file
This file can be optionally written to in order to to communi-
cate a status code of the --client-connect script or plgin.
Only the first character in the file is relevant. It must be
either 1 to indicate normal script execution, 0 indicates an
error (in the same way that a non zero exit status does) or 2 to
indicate that the script deferred returning the config file.

For deferred (background) handling, the script or plugin MUST
write 2 to the file to indicate the deferral and then return
with exit code 0 to signal deferred handler started OK.

A background process or similar must then take care of writing
the configuration to the file indicated by the client_con-
nect_config_file environment variable and when finished, write
the a 1 to this file (or 0 in case of an error).

The absence of any character in the file when the script fin-
ishes executing is interpreted the same as 1. This allows
scripts that are not written to support the defer mechanism to
be used unmodified.

common_name
The X509 common name of an authenticated client. Set prior to
execution of --client-connect, --client-disconnect and
--auth-user-pass-verify scripts.

config Name of first --config file. Set on program initiation and reset
on SIGHUP.

daemon Set to "1" if the --daemon directive is specified, or "0" other-
wise. Set on program initiation and reset on SIGHUP.

daemon_log_redirect
Set to "1" if the --log or --log-append directives are speci-
fied, or "0" otherwise. Set on program initiation and reset on
SIGHUP.

dev The actual name of the TUN/TAP device, including a unit number
if it exists. Set prior to --up or --down script execution.

dev_idx
On Windows, the device index of the TUN/TAP adapter (to be used
in netsh.exe calls which sometimes just do not work right with
interface names). Set prior to --up or --down script execution.

dns_* The --dns configuration options will be made available to script
execution through this set of environment variables. Variables
appear only if the corresponding option has a value assigned.
For the semantics of each individual variable, please refer to
the documentation for --dns.

dns_search_domain_{n}
dns_server_{n}_address_{m}
dns_server_{n}_port_{m}
dns_server_{n}_resolve_domain_{m}
dns_server_{n}_dnssec
dns_server_{n}_transport
dns_server_{n}_sni

foreign_option_{n}
An option pushed via --push to a client which does not natively
support it, such as --dhcp-option on a non-Windows system, will
be recorded to this environmental variable sequence prior to
--up script execution.

ifconfig_broadcast
The broadcast address for the virtual ethernet segment which is
derived from the --ifconfig option when --dev tap is used. Set
prior to OpenVPN calling the ifconfig or netsh (windows version
of ifconfig) commands which normally occurs prior to --up script
execution.

ifconfig_ipv6_local
The local VPN endpoint IPv6 address specified in the --ifcon-
fig-ipv6 option (first parameter). Set prior to OpenVPN calling
the ifconfig or code:netsh (windows version of ifconfig) com-
mands which normally occurs prior to --up script execution.

ifconfig_ipv6_netbits
The prefix length of the IPv6 network on the VPN interface.
Derived from the /nnn parameter of the IPv6 address in the
--ifconfig-ipv6 option (first parameter). Set prior to OpenVPN
calling the ifconfig or netsh (windows version of ifconfig) com-
mands which normally occurs prior to --up script execution.

ifconfig_ipv6_remote
The remote VPN endpoint IPv6 address specified in the --ifcon-
fig-ipv6 option (second parameter). Set prior to OpenVPN calling
the ifconfig or netsh (windows version of ifconfig) commands
which normally occurs prior to --up script execution.

ifconfig_local
The local VPN endpoint IP address specified in the --ifconfig
option (first parameter). Set prior to OpenVPN calling the
ifconfig or netsh (windows version of ifconfig) commands which
normally occurs prior to --up script execution.

ifconfig_remote
The remote VPN endpoint IP address specified in the --ifconfig
option (second parameter) when --dev tun is used. Set prior to
OpenVPN calling the ifconfig or netsh (windows version of ifcon-
fig) commands which normally occurs prior to --up script execu-
tion.

ifconfig_netmask
The subnet mask of the virtual ethernet segment that is speci-
fied as the second parameter to --ifconfig when --dev tap is
being used. Set prior to OpenVPN calling the ifconfig or netsh
(windows version of ifconfig) commands which normally occurs
prior to --up script execution.

ifconfig_pool_local_ip
The local virtual IP address for the TUN/TAP tunnel taken from
an --ifconfig-push directive if specified, or otherwise from the
ifconfig pool (controlled by the --ifconfig-pool config file
directive). Only set for --dev tun tunnels. This option is set
on the server prior to execution of the --client-connect and
--client-disconnect scripts.

ifconfig_pool_netmask
The virtual IP netmask for the TUN/TAP tunnel taken from an
--ifconfig-push directive if specified, or otherwise from the
ifconfig pool (controlled by the --ifconfig-pool config file
directive). Only set for --dev tap tunnels. This option is set
on the server prior to execution of the --client-connect and
--client-disconnect scripts.

ifconfig_pool_remote_ip
The remote virtual IP address for the TUN/TAP tunnel taken from
an --ifconfig-push directive if specified, or otherwise from the
ifconfig pool (controlled by the --ifconfig-pool config file
directive). This option is set on the server prior to execution
of the --client-connect and --client-disconnect scripts.

link_mtu
No longer passed to scripts since OpenVPN 2.6.0. Used to be the
maximum packet size (not including the IP header) of tunnel data
in UDP tunnel transport mode.

local The --local parameter. Set on program initiation and reset on
SIGHUP.

local_port
The local port number or name, specified by --port or --lport.
Set on program initiation and reset on SIGHUP.

password
The password provided by a connecting client. Set prior to
--auth-user-pass-verify script execution only when the via-env
modifier is specified, and deleted from the environment after
the script returns.

proto The --proto parameter. Set on program initiation and reset on
SIGHUP.

remote_{n}
The --remote parameter. Set on program initiation and reset on
SIGHUP.

remote_port_{n}
The remote port number, specified by --port or --rport. Set on
program initiation and reset on SIGHUP.

route_net_gateway
The pre-existing default IP gateway in the system routing table.
Set prior to --up script execution.

route_vpn_gateway
The default gateway used by --route options, as specified in
either the --route-gateway option or the second parameter to
--ifconfig when --dev tun is specified. Set prior to --up script
execution.

route_{parm}_{n}
A set of variables which define each route to be added, and are
set prior to --up script execution.

parm will be one of network, netmask", gateway, or metric.

n is the OpenVPN route number, starting from 1.

If the network or gateway are resolvable DNS names, their IP
address translations will be recorded rather than their names as
denoted on the command line or configuration file.

route_ipv6_{parm}_{n}
A set of variables which define each IPv6 route to be added, and
are set prior to --up script execution.

parm will be one of network, gateway or metric. route_ipv6_net-
work_{n} contains netmask as /nnn, unlike IPv4 where it is
passed in a separate environment variable.

n is the OpenVPN route number, starting from 1.

If the network or gateway are resolvable DNS names, their IP
address translations will be recorded rather than their names as
denoted on the command line or configuration file.

peer_cert
Temporary file name containing the client certificate upon con-
nection. Useful in conjunction with --tls-verify.

script_context
Set to "init" or "restart" prior to up/down script execution.
For more information, see documentation for --up.

script_type
Prior to execution of any script, this variable is set to the
type of script being run. It can be one of the following: up,
down, ipchange, route-up, tls-verify, auth-user-pass-verify,
client-connect, client-disconnect or learn-address. Set prior to
execution of any script.

signal The reason for exit or restart. Can be one of sigusr1, sighup,
sigterm, sigint, inactive (controlled by --inactive option),
ping-exit (controlled by --ping-exit option), ping-restart (con-
trolled by --ping-restart option), connection-reset (triggered
on TCP connection reset), error or unknown (unknown signal).
This variable is set just prior to down script execution.

time_ascii
Client connection timestamp, formatted as a human-readable time
string. Set prior to execution of the --client-connect script.

time_duration
The duration (in seconds) of the client session which is now
disconnecting. Set prior to execution of the --client-disconnect
script.

time_unix
Client connection timestamp, formatted as a unix integer
date/time value. Set prior to execution of the --client-connect
script.

tls_digest_{n} / tls_digest_sha256_{n}
Contains the certificate SHA1 / SHA256 fingerprint, where n is
the verification level. Only set for TLS connections. Set prior
to execution of --tls-verify script.

tls_id_{n}
A series of certificate fields from the remote peer, where n is
the verification level. Only set for TLS connections. Set prior
to execution of --tls-verify script.

tls_serial_{n}
The serial number of the certificate from the remote peer, where
n is the verification level. Only set for TLS connections. Set
prior to execution of --tls-verify script. This is in the form
of a decimal string like "933971680", which is suitable for
doing serial-based OCSP queries (with OpenSSL, do not prepend
"0x" to the string) If something goes wrong while reading the
value from the certificate it will be an empty string, so your
code should check that. See the contrib/OCSP_check/OCSP_check.sh
script for an example.

tls_serial_hex_{n}
Like tls_serial_{n}, but in hex form (e.g. 12:34:56:78:9A).

tun_mtu
The MTU of the TUN/TAP device. Set prior to --up or --down
script execution.

trusted_ip / trusted_ip6)
Actual IP address of connecting client or peer which has been
authenticated. Set prior to execution of --ipchange,
--client-connect and --client-disconnect scripts. If using ipv6
endpoints (udp6, tcp6), trusted_ip6 will be set instead.

trusted_port
Actual port number of connecting client or peer which has been
authenticated. Set prior to execution of --ipchange,
--client-connect and --client-disconnect scripts.

untrusted_ip / untrusted_ip6
Actual IP address of connecting client or peer which has not
been authenticated yet. Sometimes used to nmap the connecting
host in a --tls-verify script to ensure it is firewalled prop-
erly. Set prior to execution of --tls-verify and
--auth-user-pass-verify scripts. If using ipv6 endpoints (udp6,
tcp6), untrusted_ip6 will be set instead.

untrusted_port
Actual port number of connecting client or peer which has not
been authenticated yet. Set prior to execution of --tls-verify
and --auth-user-pass-verify scripts.

username
The username provided by a connecting client. Set prior to
--auth-user-pass-verify script execution only when the via-env
modifier is specified.

X509_{n}_{subject_field}
An X509 subject field from the remote peer certificate, where n
is the verification level. Only set for TLS connections. Set
prior to execution of --tls-verify script. This variable is sim-
ilar to tls_id_{n} except the component X509 subject fields are
broken out, and no string remapping occurs on these field values
(except for remapping of control characters to "_"). For exam-
ple, the following variables would be set on the OpenVPN server
using the sample client certificate in sample-keys (client.crt).
Note that the verification level is 0 for the client certificate
and 1 for the CA certificate.

X509_0_emailAddress=me@myhost.mydomain
X509_0_CN=Test-Client
X509_0_O=OpenVPN-TEST
X509_0_ST=NA
X509_0_C=KG
X509_1_emailAddress=me@myhost.mydomain
X509_1_O=OpenVPN-TEST
X509_1_L=BISHKEK
X509_1_ST=NA
X509_1_C=KG

Management Interface Options
OpenVPN provides a feature rich socket based management interface for
both server and client mode operations.

--management args
Enable a management server on a socket-name Unix socket on those
platforms supporting it, or on a designated TCP port.

Valid syntaxes:

management socket-name unix #
management socket-name unix pw-file # (recommended)
management IP port # (INSECURE)
management IP port pw-file #

pw-file, if specified, is a password file where the password
must be on first line. Instead of a filename it can use the key-
word stdin which will prompt the user for a password to use when
OpenVPN is starting.

For unix sockets, the default behaviour is to create a unix
domain socket that may be connected to by any process. Use the
--management-client-user and --management-client-group direc-
tives to restrict access.

The management interface provides a special mode where the TCP
management link can operate over the tunnel itself. To enable
this mode, set IP to tunnel. Tunnel mode will cause the manage-
ment interface to listen for a TCP connection on the local VPN
address of the TUN/TAP interface.

*BEWARE* of enabling the management interface over TCP. In these
cases you should ALWAYS make use of pw-file to password protect
the management interface. Any user who can connect to this TCP
IP:port will be able to manage and control (and interfere with)
the OpenVPN process. It is also strongly recommended to set IP
to 127.0.0.1 (localhost) to restrict accessibility of the man-
agement server to local clients.

While the management port is designed for programmatic control
of OpenVPN by other applications, it is possible to telnet to
the port, using a telnet client in "raw" mode. Once connected,
type help for a list of commands.

For detailed documentation on the management interface, see the
management-notes.txt file in the management folder of the Open-
VPN source distribution.

--management-client
Management interface will connect as a TCP/unix domain client to
IP:port specified by --management rather than listen as a TCP
server or on a unix domain socket.

If the client connection fails to connect or is disconnected, a
SIGTERM signal will be generated causing OpenVPN to quit.

--management-client-auth
Gives management interface client the responsibility to authen-
ticate clients after their client certificate has been verified.
See management-notes.txt in OpenVPN distribution for detailed
notes.

--management-client-group g
When the management interface is listening on a unix domain
socket, only allow connections from group g.

--management-client-user u
When the management interface is listening on a unix domain
socket, only allow connections from user u.

--management-external-cert certificate-hint
Allows usage for external certificate instead of --cert option
(client-only). certificate-hint is an arbitrary string which is
passed to a management interface client as an argument of
NEED-CERTIFICATE notification. Requires --management-exter-
nal-key.

--management-external-key args
Allows usage for external private key file instead of --key
option (client-only).

Valid syntaxes:

management-external-key
management-external-key nopadding
management-external-key pkcs1
management-external-key pss

or any combination like:

management-external-key nopadding pkcs1
management-external-key pkcs1 pss

The optional parameters nopadding pkcs1 and pss signal support
for different padding algorithms. See doc/mangement-notes.txt
for a complete description of this feature.

--management-forget-disconnect
Make OpenVPN forget passwords when management session discon-
nects.

This directive does not affect the --http-proxy username/pass-
word. It is always cached.

--management-hold
Start OpenVPN in a hibernating state, until a client of the man-
agement interface explicitly starts it with the hold release
command.

--management-log-cache n
Cache the most recent n lines of log file history for usage by
the management channel.

--management-query-passwords
Query management channel for private key password and
--auth-user-pass username/password. Only query the management
channel for inputs which ordinarily would have been queried from
the console.

--management-query-proxy
Query management channel for proxy server information for a spe-
cific --remote (client-only).

--management-query-remote
Allow management interface to override --remote directives
(client-only).

--management-signal
Send SIGUSR1 signal to OpenVPN if management session discon-
nects. This is useful when you wish to disconnect an OpenVPN
session on user logoff. For --management-client this option is
not needed since a disconnect will always generate a SIGTERM.

--management-up-down
Report tunnel up/down events to management interface.

Plug-in Interface Options
OpenVPN can be extended by loading external plug-in modules at runtime.
These plug-ins must be prebuilt and adhere to the OpenVPN Plug-In API.

--plugin args
Loads an OpenVPN plug-in module.

Valid syntax:

plugin module-name
plugin module-name "arguments"

The module-name needs to be the first argument, indicating the
plug-in to load. The second argument is an optional init string
which will be passed directly to the plug-in. If the init con-
sists of multiple arguments it must be enclosed in double-quotes
("). Multiple plugin modules may be loaded into one OpenVPN
process.

The module-name argument can be just a filename or a filename
with a relative or absolute path. The format of the filename and
path defines if the plug-in will be loaded from a default
plug-in directory or outside this directory.

--plugin path Effective directory used
===================== =============================
myplug.so DEFAULT_DIR/myplug.so
subdir/myplug.so DEFAULT_DIR/subdir/myplug.so
./subdir/myplug.so CWD/subdir/myplug.so
/usr/lib/my/plug.so /usr/lib/my/plug.so

DEFAULT_DIR is replaced by the default plug-in directory, which
is configured at the build time of OpenVPN. CWD is the current
directory where OpenVPN was started or the directory OpenVPN
have switched into via the --cd option before the --plugin
option.

For more information and examples on how to build OpenVPN
plug-in modules, see the README file in the plugin folder of the
OpenVPN source distribution.

If you are using an RPM install of OpenVPN, the actual plugin
modules are in /usr/lib64/openvpn/plugins and the documentation
is in /usr/share/doc/packages/openvpn/README.<plugin-name>.

Multiple plugin modules can be cascaded, and modules can be used
in tandem with scripts. The modules will be called by OpenVPN in
the order that they are declared in the config file. If both a
plugin and script are configured for the same callback, the
script will be called last. If the return code of the mod-
ule/script controls an authentication function (such as tls-ver-
ify, auth-user-pass-verify, or client-connect), then every mod-
ule and script must return success (0) in order for the connec-
tion to be authenticated.

WARNING:
Plug-ins may do deferred execution, meaning the plug-in
will return the control back to the main OpenVPN process
and provide the plug-in result later on via a different
thread or process. OpenVPN does NOT support multiple
authentication plug-ins where more than one plugin tries
to do deferred authentication. If this behaviour is
detected, OpenVPN will shut down upon first authentica-
tion.

Windows-Specific Options
--allow-nonadmin TAP-adapter
(Standalone) Set TAP-adapter to allow access from non-adminis-
trative accounts. If TAP-adapter is omitted, all TAP adapters on
the system will be configured to allow non-admin access. The
non-admin access setting will only persist for the length of
time that the TAP-Win32 device object and driver remain loaded,
and will need to be re-enabled after a reboot, or if the driver
is unloaded and reloaded. This directive can only be used by an
administrator.

--block-outside-dns
Block DNS servers on other network adapters to prevent DNS
leaks. This option prevents any application from accessing TCP
or UDP port 53 except one inside the tunnel. It uses Windows
Filtering Platform (WFP) and works on Windows Vista or later.

This option is considered unknown on non-Windows platforms and
unsupported on Windows XP, resulting in fatal error. You may
want to use --setenv opt or --ignore-unknown-option (not suit-
able for Windows XP) to ignore said error. Note that pushing
unknown options from server does not trigger fatal errors.

--cryptoapicert select-string
(Windows/OpenSSL Only) Load the certificate and private key from
the Windows Certificate System Store.

Use this option instead of --cert and --key.

This makes it possible to use any smart card, supported by Win-
dows, but also any kind of certificate, residing in the Cert
Store, where you have access to the private key. This option has
been tested with a couple of different smart cards (GemSAFE,
Cryptoflex, and Swedish Post Office eID) on the client side, and
also an imported PKCS12 software certificate on the server side.

To select a certificate, based on a substring search in the cer-
tificate's subject:

cryptoapicert "SUBJ:Peter Runestig"

To select a certificate, based on certificate's thumbprint (SHA1
hash):

cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."

The thumbprint hex string can easily be copy-and-pasted from the
Windows Certificate Store GUI. The embedded spaces in the hex
string are optional.

To select a certificate based on a substring in certificate's
issuer name:

cryptoapicert "ISSUER:Sample CA"

The first non-expired certificate found in the user's store or
the machine store that matches the select-string is used.

--dhcp-release
Ask Windows to release the TAP adapter lease on shutdown. This
option has no effect now, as it is enabled by default starting
with OpenVPN 2.4.1.

--dhcp-renew
Ask Windows to renew the TAP adapter lease on startup. This
option is normally unnecessary, as Windows automatically trig-
gers a DHCP renegotiation on the TAP adapter when it comes up,
however if you set the TAP-Win32 adapter Media Status property
to "Always Connected", you may need this flag.

--ip-win32 method
When using --ifconfig on Windows, set the TAP-Win32 adapter IP
address and netmask using method. Don't use this option unless
you are also using --ifconfig.

manual Don't set the IP address or netmask automatically.
Instead output a message to the console telling the user
to configure the adapter manually and indicating the
IP/netmask which OpenVPN expects the adapter to be set
to.

dynamic [offset] [lease-time]
Automatically set the IP address and netmask by replying
to DHCP query messages generated by the kernel. This
mode is probably the "cleanest" solution for setting the
TCP/IP properties since it uses the well-known DHCP pro-
tocol. There are, however, two prerequisites for using
this mode:

1. The TCP/IP properties for the TAP-Win32 adapter must
be set to "Obtain an IP address automatically", and

2. OpenVPN needs to claim an IP address in the subnet for
use as the virtual DHCP server address.

By default in --dev tap mode, OpenVPN will take the nor-
mally unused first address in the subnet. For example, if
your subnet is 192.168.4.0 netmask 255.255.255.0, then
OpenVPN will take the IP address 192.168.4.0 to use as
the virtual DHCP server address. In --dev tun mode,
OpenVPN will cause the DHCP server to masquerade as if it
were coming from the remote endpoint.

The optional offset parameter is an integer which is >
-256 and < 256 and which defaults to 0. If offset is pos-
itive, the DHCP server will masquerade as the IP address
at network address + offset. If offset is negative, the
DHCP server will masquerade as the IP address at broad-
cast address + offset.

The Windows ipconfig /all command can be used to show
what Windows thinks the DHCP server address is. OpenVPN
will "claim" this address, so make sure to use a free
address. Having said that, different OpenVPN instantia-
tions, including different ends of the same connection,
can share the same virtual DHCP server address.

The lease-time parameter controls the lease time of the
DHCP assignment given to the TAP-Win32 adapter, and is
denoted in seconds. Normally a very long lease time is
preferred because it prevents routes involving the
TAP-Win32 adapter from being lost when the system goes to
sleep. The default lease time is one year.

netsh Automatically set the IP address and netmask using the
Windows command-line "netsh" command. This method appears
to work correctly on Windows XP but not Windows 2000.

ipapi Automatically set the IP address and netmask using the
Windows IP Helper API. This approach does not have ideal
semantics, though testing has indicated that it works
okay in practice. If you use this option, it is best to
leave the TCP/IP properties for the TAP-Win32 adapter in
their default state, i.e. "Obtain an IP address automati-
cally."

adaptive (Default)
Try dynamic method initially and fail over to netsh if
the DHCP negotiation with the TAP-Win32 adapter does not
succeed in 20 seconds. Such failures have been known to
occur when certain third-party firewall packages
installed on the client machine block the DHCP negotia-
tion used by the TAP-Win32 adapter. Note that if the
netsh failover occurs, the TAP-Win32 adapter TCP/IP prop-
erties will be reset from DHCP to static, and this will
cause future OpenVPN startups using the adaptive mode to
use netsh immediately, rather than trying dynamic first.

To "unstick" the adaptive mode from using netsh, run
OpenVPN at least once using the dynamic mode to restore
the TAP-Win32 adapter TCP/IP properties to a DHCP config-
uration.

--pause-exit
Put up a "press any key to continue" message on the console
prior to OpenVPN program exit. This option is automatically used
by the Windows explorer when OpenVPN is run on a configuration
file using the right-click explorer menu.

--register-dns
Run ipconfig /flushdns and ipconfig /registerdns on connection
initiation. This is known to kick Windows into recognizing
pushed DNS servers.

--route-method m
Which method m to use for adding routes on Windows?

adaptive (default)
Try IP helper API first. If that fails, fall back to the
route.exe shell command.

ipapi Use IP helper API.

exe Call the route.exe shell command.

--service args
Should be used when OpenVPN is being automatically executed by
another program in such a context that no interaction with the
user via display or keyboard is possible.

Valid syntax:

service exit-event [0|1]

In general, end-users should never need to explicitly use this
option, as it is automatically added by the OpenVPN service
wrapper when a given OpenVPN configuration is being run as a
service.

exit-event is the name of a Windows global event object, and
OpenVPN will continuously monitor the state of this event object
and exit when it becomes signaled.

The second parameter indicates the initial state of exit-event
and normally defaults to 0.

Multiple OpenVPN processes can be simultaneously executed with
the same exit-event parameter. In any case, the controlling
process can signal exit-event, causing all such OpenVPN pro-
cesses to exit.

When executing an OpenVPN process using the --service directive,
OpenVPN will probably not have a console window to output sta-
tus/error messages, therefore it is useful to use --log or
--log-append to write these messages to a file.

--show-adapters
(Standalone) Show available TAP-Win32 adapters which can be
selected using the --dev-node option. On non-Windows systems,
the ifconfig(8) command provides similar functionality.

--show-net
(Standalone) Show OpenVPN's view of the system routing table and
network adapter list.

--show-net-up
Output OpenVPN's view of the system routing table and network
adapter list to the syslog or log file after the TUN/TAP adapter
has been brought up and any routes have been added.

--show-valid-subnets
(Standalone) Show valid subnets for --dev tun emulation. Since
the TAP-Win32 driver exports an ethernet interface to Windows,
and since TUN devices are point-to-point in nature, it is neces-
sary for the TAP-Win32 driver to impose certain constraints on
TUN endpoint address selection.

Namely, the point-to-point endpoints used in TUN device emula-
tion must be the middle two addresses of a /30 subnet (netmask
255.255.255.252).

--tap-sleep n
Cause OpenVPN to sleep for n seconds immediately after the
TAP-Win32 adapter state is set to "connected".

This option is intended to be used to troubleshoot problems with
the --ifconfig and --ip-win32 options, and is used to give the
TAP-Win32 adapter time to come up before Windows IP Helper API
operations are applied to it.

--win-sys path
Set the Windows system directory pathname to use when looking
for system executables such as route.exe and netsh.exe. By
default, if this directive is not specified, OpenVPN will use
the SystemRoot environment variable.

This option has changed behaviour since OpenVPN 2.3. Earlier you
had to define --win-sys env to use the SystemRoot environment
variable, otherwise it defaulted to C:\\WINDOWS. It is not
needed to use the env keyword any more, and it will just be
ignored. A warning is logged when this is found in the configu-
ration file.

--windows-driver drv
Specifies which tun driver to use. Values are ovpn-dco
(default), tap-windows6 and wintun. ovpn-dco and wintun require
--dev tun. wintun also requires OpenVPN process to run elevated,
or be invoked using the Interactive Service.

Standalone Debug Options
--show-gateway args
(Standalone) Show current IPv4 and IPv6 default gateway and
interface towards the gateway (if the protocol in question is
enabled).

Valid syntax:

--show-gateway
--show-gateway IPv6-target

For IPv6 this queries the route towards ::/128, or the specified
IPv6 target address if passed as argument. For IPv4 on Linux,
Windows, MacOS and BSD it looks for a 0.0.0.0/0 route. If there
are more specific routes, the result will not always be matching
the route of the IPv4 packets to the VPN gateway.

Advanced Expert Options
These are options only required when special tweaking is needed, often
used when debugging or testing out special usage scenarios.

--hash-size args
Set the size of the real address hash table to r and the virtual
address table to v.

Valid syntax:

hash-size r v

By default, both tables are sized at 256 buckets.

--bcast-buffers n
Allocate n buffers for broadcast datagrams (default 256).

--persist-local-ip
Preserve initially resolved local IP address and port number
across SIGUSR1 or --ping-restart restarts.

--persist-remote-ip
Preserve most recently authenticated remote IP address and port
number across SIGUSR1 or --ping-restart restarts.

--rcvbuf size
Set the TCP/UDP socket receive buffer size. Defaults to operat-
ing system default.

--shaper n
Limit bandwidth of outgoing tunnel data to n bytes per second on
the TCP/UDP port. Note that this will only work if mode is set
to p2p. If you want to limit the bandwidth in both directions,
use this option on both peers.

OpenVPN uses the following algorithm to implement traffic shap-
ing: Given a shaper rate of n bytes per second, after a datagram
write of b bytes is queued on the TCP/UDP port, wait a minimum
of (b / n) seconds before queuing the next write.

It should be noted that OpenVPN supports multiple tunnels
between the same two peers, allowing you to construct full-speed
and reduced bandwidth tunnels at the same time, routing low-pri-
ority data such as off-site backups over the reduced bandwidth
tunnel, and other data over the full-speed tunnel.

Also note that for low bandwidth tunnels (under 1000 bytes per
second), you should probably use lower MTU values as well (see
above), otherwise the packet latency will grow so large as to
trigger timeouts in the TLS layer and TCP connections running
over the tunnel.

OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.

--sndbuf size
Set the TCP/UDP socket send buffer size. Defaults to operating
system default.

--tcp-queue-limit n
Maximum number of output packets queued before TCP (default 64).

When OpenVPN is tunneling data from a TUN/TAP device to a remote
client over a TCP connection, it is possible that the TUN/TAP
device might produce data at a faster rate than the TCP connec-
tion can support. When the number of output packets queued
before sending to the TCP socket reaches this limit for a given
client connection, OpenVPN will start to drop outgoing packets
directed at this client.

--txqueuelen n
(Linux only) Set the TX queue length on the TUN/TAP interface.
Currently defaults to operating system default.

--disable-dco
Disables the opportunistic use of data channel offloading if
available. Without this option, OpenVPN will opportunistically
use DCO mode if the config options and the running kernel sup-
ports using DCO.

Data channel offload currently requires data-ciphers to only
contain AEAD ciphers (AES-GCM and Chacha20-Poly1305) and Linux
with the ovpn-dco module.

Note that some options have no effect or cannot be used when DCO
mode is enabled.

On platforms that do not support DCO disable-dco has no effect.

UNSUPPORTED OPTIONS
Options listed in this section have been removed from OpenVPN and are
no longer supported

--client-cert-not-required
Removed in OpenVPN 2.5. This should be replaxed with --ver-
ify-client-cert none.

--ifconfig-pool-linear
Removed in OpenVPN 2.5. This should be replaced with --topology
p2p.

--key-method
Removed in OpenVPN 2.5. This option should not be used, as
using the old key-method weakens the VPN tunnel security. The
old key-method was also only needed when the remote side was
older than OpenVPN 2.0.

--management-client-pf
Removed in OpenVPN 2.6. The built-in packet filtering (pf)
functionality has been removed.

--ncp-disable
Removed in OpenVPN 2.6. This option mainly served a role as
debug option when NCP was first introduced. It should no longer
be necessary.

--no-iv
Removed in OpenVPN 2.5. This option should not be used as it
weakens the VPN tunnel security. This has been a NOOP option
since OpenVPN 2.4.

--no-replay
Removed in OpenVPN 2.5. This option should not be used as it
weakens the VPN tunnel security.

--ns-cert-type
Removed in OpenVPN 2.5. The nsCertType field is no longer sup-
ported in recent SSL/TLS libraries. If your certificates does
not include key usage and extended key usage fields, they must
be upgraded and the --remote-cert-tls option should be used
instead.

--prng Removed in OpenVPN 2.6. We now always use the PRNG of the SSL
library.

CONNECTION PROFILES
Client configuration files may contain multiple remote servers which it
will attempt to connect against. But there are some configuration
options which are related to specific --remote options. For these use
cases, connection profiles are the solution.

By enacpulating the --remote option and related options within <connec-
tion> and </connection>, these options are handled as a group.

An OpenVPN client will try each connection profile sequentially until
it achieves a successful connection.

--remote-random can be used to initially "scramble" the connection
list.

Here is an example of connection profile usage:

client
dev tun

<connection>
remote 198.19.34.56 1194 udp
</connection>

<connection>
remote 198.19.34.56 443 tcp
</connection>

<connection>
remote 198.19.34.56 443 tcp
http-proxy 192.168.0.8 8080
</connection>

<connection>
remote 198.19.36.99 443 tcp
http-proxy 192.168.0.8 8080
</connection>

persist-key
persist-tun
pkcs12 client.p12
remote-cert-tls server
verb 3

First we try to connect to a server at 198.19.34.56:1194 using UDP. If
that fails, we then try to connect to 198.19.34.56:443 using TCP. If
that also fails, then try connecting through an HTTP proxy at
192.168.0.8:8080 to 198.19.34.56:443 using TCP. Finally, try to connect
through the same proxy to a server at 198.19.36.99:443 using TCP.

The following OpenVPN options may be used inside of a <connection>
block:

bind, connect-retry, connect-retry-max, connect-timeout,
explicit-exit-notify, float, fragment, http-proxy, http-proxy-option,
key-direction, link-mtu, local, lport, mssfix, mtu-disc, nobind, port,
proto, remote, rport, socks-proxy, tls-auth, tls-crypt, tun-mtu and,
tun-mtu-extra.

A defaulting mechanism exists for specifying options to apply to all
<connection> profiles. If any of the above options (with the exception
of remote ) appear outside of a <connection> block, but in a configura-
tion file which has one or more <connection> blocks, the option setting
will be used as a default for <connection> blocks which follow it in
the configuration file.

For example, suppose the nobind option were placed in the sample con-
figuration file above, near the top of the file, before the first <con-
nection> block. The effect would be as if nobind were declared in all
<connection> blocks below it.

INLINE FILE SUPPORT
OpenVPN allows including files in the main configuration for the --ca,
--cert, --dh, --extra-certs, --key, --pkcs12, --secret, --crl-verify,
--http-proxy-user-pass, --tls-auth, --auth-gen-token-secret,
--peer-fingerprint, --tls-crypt, --tls-crypt-v2 and --verify-hash
options.

Each inline file started by the line <option> and ended by the line
</option>

Here is an example of an inline file usage

<cert>
-----BEGIN CERTIFICATE-----
[...]
-----END CERTIFICATE-----
</cert>

When using the inline file feature with --pkcs12 the inline file has to
be base64 encoded. Encoding of a .p12 file into base64 can be done for
example with OpenSSL by running openssl base64 -in input.p12

SIGNALS
SIGHUP Cause OpenVPN to close all TUN/TAP and network connections,
restart, re-read the configuration file (if any), and reopen
TUN/TAP and network connections.

SIGUSR1
Like SIGHUP`, except don't re-read configuration file, and pos-
sibly don't close and reopen TUN/TAP device, re-read key files,
preserve local IP address/port, or preserve most recently
authenticated remote IP address/port based on --persist-tun,
--persist-key, --persist-local-ip and --persist-remote-ip
options respectively (see above).

This signal may also be internally generated by a timeout condi-
tion, governed by the --ping-restart option.

This signal, when combined with --persist-remote-ip, may be sent
when the underlying parameters of the host's network interface
change such as when the host is a DHCP client and is assigned a
new IP address. See --ipchange for more information.

SIGUSR2
Causes OpenVPN to display its current statistics (to the syslog
file if --daemon is used, or stdout otherwise).

SIGINT, SIGTERM
Causes OpenVPN to exit gracefully.

FAQ
https://community.openvpn.net/openvpn/wiki/FAQ

HOWTO
The manual openvpn-examples(5) gives some examples, especially for
small setups.

For a more comprehensive guide to setting up OpenVPN in a production
setting, see the OpenVPN HOWTO at
https://openvpn.net/community-resources/how-to/

PROTOCOL
An ongoing effort to document the OpenVPN protocol can be found under
https://github.com/openvpn/openvpn-rfc

WEB
OpenVPN's web site is at https://community.openvpn.net/

Go here to download the latest version of OpenVPN, subscribe to the
mailing lists, read the mailing list archives, or browse the Git repos-
itory.

BUGS
Report all bugs to the OpenVPN team info@openvpn.net

SEE ALSO
openvpn-examples(5), dhcpcd(8), ifconfig(8), openssl(1), route(8),
scp(1) ssh(1)

NOTES
This product includes software developed by the OpenSSL Project (-
https://www.openssl.org/)

For more information on the TLS protocol, see
http://www.ietf.org/rfc/rfc2246.txt

For more information on the LZO real-time compression library see
https://www.oberhumer.com/opensource/lzo/

COPYRIGHT
Copyright (C) 2002-2020 OpenVPN Inc This program is free software; you
can redistribute it and/or modify it under the terms of the GNU General
Public License version 2 as published by the Free Software Foundation.

AUTHORS
James Yonan james@openvpn.net

OPENVPN(8)

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