Generic Options

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.

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

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 redefines 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 initialization, most OpenVPN options that reference files will operate in a pre-chroot context.

In many cases, the dir parameter can point to an empty directory, 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 getentropy() 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 specified. 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 OpenVPN 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 recommended values and should be avoided if possible.

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

• 2.5.x or lower: --allow-compression asym is automatically added to the configuration if no other compression options are present.
• 2.4.x or lower: The cipher in --cipher is appended to --data-ciphers.
• 2.3.x or lower: --data-ciphers-fallback is automatically added with the same cipher as --cipher.
• 2.3.6 or lower: --tls-version-min 1.0 is added to the configuration 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 corresponds to one command line option, but with the leading :code:-- 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 comments.

Note that OpenVPN 2.0 and higher performs backslash-based shell escaping for characters not in single quotations, so the following 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 completed.

Valid syntaxes::

daemon
daemon progname

This option will cause all message and error output to be sent to the syslog file (such as :code:/var/log/messages), except for the output of scripts and ifconfig commands, which will go to :code:/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 useful in linking OpenVPN messages in the syslog file with specific tunnels. When unspecified, progname defaults to :code:openvpn.

When OpenVPN is run with the --daemon option, it will try to delay daemonization until the majority of initialization functions which are capable of generating fatal errors are complete. This means that initialization scripts can test the return status 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 initialization are non-fatal.

Note: as soon as OpenVPN has daemonized, it can not ask for usernames, passwords, or key pass phrases anymore. This has certain consequences, namely that using a password-protected private key will fail unless the --askpass option is used to tell OpenVPN to ask for the pass phrase.

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

--disable-occ DEPRECATED Disable "options consistency check" (OCC) in configurations 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 temporary 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.

--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-option opt1 opt2 opt3 ... optN

When one of options opt1 ... optN is encountered in the configuration 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 security 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 encountered 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 unprivileged environment.

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

--mlock Disable paging by calling the POSIX mlockall function. Requires that OpenVPN be initially run as root (though OpenVPN can subsequently 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 discarded.

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 mlockall(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).

--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 :code: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 :code:SIGUSR1 signals are remapped to :code:SIGHUP (restart without persisting state) or :code:SIGTERM (exit).

signal can be set to :code:SIGHUP or :code: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:

:code:0 Strictly no calling of external programs.

:code:1 (Default) Only call built-in executables such as ifconfig, ip, route, or netsh.

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

:code: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 :code:execve or :code:system. As of OpenVPN 2.3, this flag is no longer accepted.

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.

On Windoes it is a strict requirement to have the 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::

 --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 backslashes (\\) and the space character.

--setcon context Apply SELinux context after initialization. This essentially provides the ability to restrict OpenVPN's rights to only network I/O operations, thanks to SELinux. This goes further than --user and --chroot in that those two, while being great security 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 initialization, 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 initialization.

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-tun option.

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

Valid syntaxes: ::

status file
status file n

Status can also be written to the syslog by sending a :code: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:

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

:code:2 A more reliable format for external processing. Compared to version :code: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.

:code:3 Identical to :code: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

or

::

 openvpn --test-crypto --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 compiler, OpenSSL crypto library, or OpenVPN's crypto code. Since it is a self-test mode, problems with encryption and authentication can be debugged independently of network and tunnel issues.

Older versions of OpenVPN used the --secret argument to specify a static key for this test. Newer version generate a random key for the test.

--tmp-dir dir Specify a directory dir for temporary files instead of the default :code:TMPDIR (or "/tmp" if unset). Note that it must be writable by the main process after it has dropped root privileges.

This directory will be used to communicate with scripts and plugins:

• --client-connect scripts and :code:OPENVPN_PLUGIN_CLIENT_CONNECT plug-in hook to dynamically generate client-specific configuration :code:client_connect_config_file and return success/failure via :code:client_connect_deferred_file when using deferred client connect method

• :code:OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plug-in hooks returns success/failure via :code:auth_control_file when using deferred auth method and pending authentication via :code:auth_pending_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 initialization, dropping privileges in the process. This option is useful 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 openvpn, 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 :code: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 :code: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.