NAME¶
openvpn - secure IP tunnel daemon.
SYNOPSIS¶
openvpn [ options ... ]
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 OpenVPN, 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:
http://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
tunnels.
OpenVPN is designed to work with the
TUN/TAP virtual networking interface
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.
- --help
- Show options.
- --config file
- Load additional config options from file where each line
corresponds 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
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
http://openvpn.net/examples.html
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
Tunnel Options:¶
- --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.
- --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.
- --remote host [port] [proto]
- Remote host name or IP address. On the client, multiple --remote
options may be specified for redundancy, each referring to a different
OpenVPN server. Specifying multiple --remote options for this
purpose is a special case of the more general connection-profile feature.
See the <connection> documentation below.
The OpenVPN client will try to connect to a server at host:port in
the order specified by the list of --remote options.
proto indicates the protocol to use when connecting with the remote,
and may be "tcp" or "udp".
For forcing IPv4 or IPv6 connection suffix tcp or udp with 4/6 like
udp4/udp6/tcp4/tcp6.
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.
Note that since UDP is connectionless, connection failure is defined by the
--ping and --ping-restart options.
Note the following corner case: 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 settings, 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 authentication 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 protocol specification (4/6 suffix),
OpenVPN will try both IPv4 and IPv6 addresses, in the order getaddrinfo()
returns them.
- --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
modified to "<random-chars>.foo.bar.gov".
- <connection>
- Define a client connection profile. Client connection profiles are groups
of OpenVPN options that describe how to connect to a given OpenVPN server.
Client connection profiles are specified within an OpenVPN configuration
file, and each profile is bracketed by <connection> and
</connection>.
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
ns-cert-type 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,
link-mtu, local, lport, mssfix,
mtu-disc, nobind, port, proto, remote,
rport, socks-proxy, 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 configuration 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
configuration file above, near the top of the file, before the first
<connection> block. The effect would be as if nobind
were declared in all <connection> blocks below it.
- --proto-force p
- When iterating through connection profiles, only consider profiles using
protocol p ('tcp'|'udp').
- --remote-random
- When multiple --remote address/ports are specified, or if
connection profiles are being used, initially randomize the order of the
list as a kind of basic load-balancing measure.
- --proto p
- Use protocol p for communicating with remote host. p can be
udp, tcp-client, or tcp-server.
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 --connect-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 capability 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 advantageous from a
security and robustness perspective, such as tunneling non-IP or
application-level UDP protocols, or tunneling protocols which don't
possess a built-in reliability layer.
- --connect-retry n [max]
- Wait n seconds between connection attempts (default=5). Repeated
reconnection attempts are slowed down after 5 retries per remote by
doubling the wait time after each unsuccessful attempt. The optional
argument max specifies the maximum value of wait time in seconds at
which it gets capped (default=300).
- --connect-retry-max n
- n specifies the number of times each --remote or
<connection> entry is tried. Specifying n as one would
try each entry exactly once. A successful connection resets the counter.
(default=unlimited).
- --show-proxy-settings
- Show sensed HTTP or SOCKS proxy settings. Currently, only Windows clients
support this option.
- --http-proxy server port [authfile|'auto'|'auto-nct']
[auth-method]
- Connect to remote host through an HTTP proxy at address server and
port port. If HTTP Proxy-Authenticate is required, authfile
is a file containing a username and password on 2 lines, 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)
auth-method 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).
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.
- --http-proxy-option type [parm]
- Set extended HTTP proxy options. 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.
- --socks-proxy server [port] [authfile]
- Connect to remote host through a Socks5 proxy at address server and
port port (default=1080). authfile (optional) is a file
containing a username and password on 2 lines, or "stdin" to
prompt from console.
- --resolv-retry n
- If hostname resolve fails for --remote, retry resolve for n
seconds before failing.
Set n to "infinite" to retry indefinitely.
By default, --resolv-retry infinite is enabled. You can disable by
setting n=0.
- --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, however if
packets arrive from a new address and pass all authentication 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 packets
from any address, not only the address which was specified in the
--remote option.
- --ipchange cmd
- Run command cmd when our remote ip-address is initially
authenticated or changes.
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 spaces.
When cmd is executed two arguments are appended after any arguments
specified in cmd , as follows:
cmd ip_address port_number
Don't use --ipchange in --mode server mode. Use a
--client-connect 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 configure
our IP address change script (see man page for dhcpcd(8) ) to
deliver a SIGHUP or SIGUSR1 signal to OpenVPN. OpenVPN will
then reestablish a connection with its most recently authenticated peer on
its new IP address.
- --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 assignment for
OpenVPN and has been used since version 2.0-beta17. Previous versions used
port 5000 as the default.
- --lport port
- Set local TCP/UDP port number or name. Cannot be used together with
--nobind option.
- --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.
- --bind [ipv6only]
- 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 ipv6only keyword is present OpenVPN will bind only to IPv6 (as
oposed to IPv6 and IPv4) when a IPv6 socket is opened.
- --nobind
- Do not bind to local address and port. The IP stack will allocate 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 connections by using the --remote
option.
- --dev tunX | tapX | null
- TUN/TAP virtual network device ( 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 represent different
underlying network layers.
tun devices encapsulate IPv4 or IPv6 (OSI Layer 3) while tap
devices encapsulate Ethernet 802.3 (OSI Layer 2).
- --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.
- --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 subnet
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 on OpenVPN 2.0.
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 allocates a single IP address per connecting
client. Only use when none of the connecting clients are Windows systems.
This mode is functionally equivalent to the --ifconfig-pool-linear
directive which is available in OpenVPN 2.0 and is now deprecated.
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. Only available when server and clients are
OpenVPN 2.1 or higher, or OpenVPN 2.0.x which has been manually patched
with the --topology directive code. When used on Windows, requires
version 8.2 or higher of the TAP-Win32 driver. When used on *nix, requires
that the tun driver supports an ifconfig(8) command which sets a
subnet instead of a remote endpoint IP address.
This option exists in OpenVPN 2.1 or higher.
Note: Using --topology subnet changes the interpretation of the
arguments of --ifconfig to mean "address netmask", no
longer "local remote".
- --dev-node node
- Explicitly set the device node rather than using /dev/net/tun, /dev/tun,
/dev/tap, etc. 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.
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.
- --lladdr address
- Specify the link layer address, more commonly known as the MAC address.
Only applied to TAP devices.
- --iproute cmd
- Set alternate command to execute instead of default iproute2 command. May
be used in order to execute OpenVPN in unprivileged environment.
- --ifconfig l rn
- Set TUN/TAP adapter parameters. l is the IP address of the local
VPN endpoint. For TUN devices in point-to-point mode, rn is the IP
address of the remote VPN endpoint. For TAP devices, or TUN devices used
with --topology subnet, rn 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 connections
(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) command, is
designed to simplify TUN/TAP tunnel configuration by providing a standard
interface to the different ifconfig implementations on different
platforms.
--ifconfig parameters which are IP addresses can also be specified as
a DNS or /etc/hosts file resolvable name.
For TAP devices, --ifconfig should not be used if the TAP interface
will be getting an IP address lease from a DHCP server.
- --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 component of the check.
For example, if you have a configuration where the local host uses
--ifconfig but the remote host does not, use
--ifconfig-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.
- --route network/IP [netmask] [gateway] [metric]
- Add route to routing table after connection is established. Multiple
routes can be specified. Routes will be automatically torn down in reverse
order prior to TUN/TAP device close.
This option is intended as a convenience proxy for the route(8) shell
command, while at the same time providing portable semantics across
OpenVPN's platform space.
netmask default -- 255.255.255.255
gateway default -- taken from --route-gateway or the second
parameter to --ifconfig when --dev tun is specified.
metric default -- taken from --route-metric otherwise 0.
The default can be specified by leaving an option blank or setting 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 special
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
routing 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-gateway gw|'dhcp'
- Specify a default gateway gw 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.
- --route-metric m
- Specify a default metric m for use with --route.
- --route-delay [n] [w]
- Delay n seconds (default=0) after connection establishment, before
adding routes. If n is 0, routes will be added immediately upon
connection establishment. If --route-delay is omitted, routes will
be added immediately after TUN/TAP device open and --up script
execution, before any --user or --group privilege 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 waiting
w seconds (w=30 by default) for the TAP-Win32 adapter to come up
before adding routes.
- --route-up cmd
- Run command cmd after routes are added, subject to
--route-delay.
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 spaces.
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.
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 spaces.
See the "Environmental Variables" section below for additional
parameters passed as environmental variables.
- --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 properties of the client's TUN/TAP
interface.
- --allow-pull-fqdn
- Allow client to pull DNS names from server (rather than being limited to
IP address) for --ifconfig, --route, and
--route-gateway.
- --client-nat snat|dnat network netmask alias
- 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.
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.
- --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 servers 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 Windows 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 Windows 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 unicast space.
!ipv4 -- Do not redirect IPv4 traffic - typically used in the flag
pair ipv6 !ipv4 to redirect IPv6-only.
- --link-mtu n
- Sets an upper bound on the size of UDP packets which are sent between
OpenVPN peers. It's best not to set this parameter unless you know what
you're doing.
- --redirect-private [flags]
- Like --redirect-gateway, but omit actually changing the default gateway.
Useful when pushing private subnets.
- --tun-mtu n
- Take the TUN device MTU to be n and derive the link MTU from it
(default=1500). In most cases, you will probably want to leave this
parameter set to its default value.
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 or 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.
- --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.
- --mtu-disc type
- Should we do Path MTU discovery on TCP/UDP channel? Only supported on OSes
such as Linux that supports the necessary system call to set.
'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.
- --fragment max
- Enable internal datagram fragmentation so that no UDP datagrams are sent
which are larger than max bytes.
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 fragmentation.
- --mssfix max
- 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 1450.
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 IPv4 packets to be transmitted over a link with MTU
1473 or higher without IP level fragmentation.
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.
Both --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
- --sndbuf size
- Set the TCP/UDP socket send buffer size. Defaults to operation system
default.
- --rcvbuf size
- Set the TCP/UDP socket receive buffer size. Defaults to operation system
default.
- --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.
- --socket-flags flags...
- Apply the given flags to the OpenVPN transport socket. Currently, 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 connection 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.
- --txqueuelen n
- (Linux only) Set the TX queue length on the TUN/TAP interface. Currently
defaults to 100.
- --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 shaping: 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-priority 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.
- --inactive n [bytes]
- 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.
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 "activity", nor are they
counted as traffic, as they are used internally by OpenVPN and are not an
indication of actual user activity.
- --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 OpenVPN 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
cryptographically 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.
- --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 http://dyndns.org/ + 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 seconds by
default. This default will hold until the client pulls a replacement value
from the server, based on the --keepalive setting 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
--persist-tun, --persist-key, --persist-local-ip, and
--persist-remote-ip options.
Also note that --ping-exit and --ping-restart are mutually
exclusive and cannot be used together.
- --keepalive interval timeout
- A helper directive designed to simplify the expression of --ping
and --ping-restart.
This option can be used on both client and server side, but it is in enough
to add this on the server side as it will push appropriate --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
- --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 connects.
- --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.
- --persist-key
- Don't re-read key files across SIGUSR1 or --ping-restart.
This option can be combined with --user nobody to allow restarts
triggered by the SIGUSR1 signal. Normally if you drop root
privileges 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.
- --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.
- --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.
- --up cmd
- Run command cmd after successful TUN/TAP device open (pre
--user UID change).
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 spaces.
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 link_mtu ifconfig_local_ip ifconfig_remote_ip [ init
| restart ]
For --dev tap execute as:
cmd tap_dev tap_mtu link_mtu ifconfig_local_ip ifconfig_netmask [ init |
restart ]
See the "Environmental Variables" section below for additional
parameters passed as environmental variables.
Note that if cmd includes arguments, all OpenVPN-generated arguments
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 OpenVPN where the TUN/TAP instance is
preserved (the --persist-tun option will enable such preservation).
A restart can be generated by a SIGUSR1 signal, a --ping-restart
timeout, or a connection reset when the TCP protocol is enabled with the
--proto option. If a restart occurs, and --up-restart has
been specified, the up script will be called with restart as the
last parameter.
NOTE: on restart, OpenVPN will not pass the full set of environment
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 configured).
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
automatically 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" protocol.
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.
- --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 spaces.
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.
- --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.
- --setenv name value
- Set a custom environmental variable name=value to pass to
script.
- --setenv FORWARD_COMPATIBLE 1
- Relax config file syntax checking 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.
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.
It is also possible to tag a single directive so as not to trigger 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 name value
- Set a custom environmental variable OPENVPN_name=value to pass to
script.
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.
- --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.
- --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 ifconfig, 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 v2.3, 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 v2.3 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 backslashes (\)
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.
- --disable-occ
- 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.
- --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 nobody or somebody similarly unprivileged,
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).
- --group group
- Similar to the --user option, this option changes the group ID of
the OpenVPN process to group after initialization.
- --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 "." 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
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 random. Newer linux kernels and some
BSDs implement a getrandom() or getentropy() syscall that removes the need
for /dev/urandom to be available.
- --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-key and --persist-tun
options.
- --daemon [progname]
- Become a daemon after all initialization functions are completed. 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 supercede 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 "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
(this requirement is new in 2.3.7, and is a consequence of calling
daemon() before initializing the crypto layer).
Further, using --daemon together with --auth-user-pass
(entered on console) and --auth-nocache will fail as soon as key
renegotiation (and reauthentication) occurs.
- --syslog [progname]
- Direct log output to system logger, but do not become a daemon. See
--daemon directive above for description of progname
parameter.
- --errors-to-stderr
- Output errors to stderr instead of stdout unless log output is redirected
by one of the --log options.
- --passtos
- Set the TOS field of the tunnel packet to what the payload's TOS is.
- --inetd [wait|nowait] [progname]
- Use this option when OpenVPN is being run from the inetd or
xinetd(8) server.
The wait/nowait option must match what is specified in the
inetd/xinetd config file. The nowait mode can only be used with
--proto tcp-server. The default is wait. The nowait
mode can be used to instantiate the OpenVPN daemon as a classic TCP
server, where client connection requests are serviced on a single port
number. For additional information on this kind of configuration, see the
OpenVPN FAQ: http://openvpn.net/faq.html#oneport
This option precludes the use of --daemon, --local, or
--remote. Note that this option causes message and error output to
be handled in the same way as the --daemon option. The optional
progname parameter is also handled exactly as in --daemon.
Also note that in wait mode, each OpenVPN tunnel requires a separate
TCP/UDP port and a separate inetd or xinetd entry. See the OpenVPN 1.x
HOWTO for an example on using OpenVPN with xinetd:
http://openvpn.net/1xhowto.html
- --log file
- Output logging messages to file, including output to stdout/stderr
which is generated by called scripts. If file already exists it
will be truncated. This option takes effect immediately when it is parsed
in the command line and will supercede syslog output if --daemon or
--inetd is also specified. This option is persistent 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, logging 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.
- --suppress-timestamps
- Avoid writing timestamps to log messages, even when they otherwise would
be prepended. In particular, this applies to log messages sent to
stdout.
- --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.
- --writepid file
- Write OpenVPN's main process ID to file.
- --nice n
- Change process priority after initialization ( n greater than 0 is
lower priority, n less than zero is higher priority).
- --fast-io
- (Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding a call to
poll/epoll/select prior to the write operation. The purpose of such a call
would normally be to block until the device or socket is ready to accept
the write. Such blocking is unnecessary 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.
- --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 interfaces, 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.
Note: this option is only relevant for UDP servers.
Note 2: 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).
- --echo [parms...]
- Echo parms to log output.
Designed to be used to send messages to a controlling application which is
receiving the OpenVPN log output.
- --remap-usr1 signal
- Control whether internally or externally generated SIGUSR1 signals are
remapped to SIGHUP (restart without persisting state) or SIGTERM (exit).
signal can be set to "SIGHUP" or "SIGTERM". By
default, no remapping occurs.
- --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 output.
0 -- No output except fatal errors.
1 to 4 -- Normal usage range.
5 -- Output 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 for additional
information on debug levels).
- --status file [n]
- Write operational status to file every n seconds.
Status can also be written to the syslog by sending a SIGUSR2
signal.
- --status-version [n]
- Choose the status file format version number. Currently n can be 1,
2, or 3 and defaults to 1.
- --mute n
- Log at most n consecutive messages in the same category. This is
useful to limit repetitive logging of similar message types.
- --compress [algorithm]
- Enable a compression algorithm.
The algorithm parameter may be "lzo", "lz4", or
empty. LZO and LZ4 are different compression algorithms, with LZ4
generally offering the best performance with least CPU usage. For
backwards compatibility with OpenVPN versions before 2.4, use
"lzo" (which is identical to the older option "--comp-lzo
yes").
If the algorithm parameter is empty, compression will be turned off,
but the packet framing for compression will still be enabled, allowing a
different setting to be pushed later.
- --comp-lzo [mode]
- Use LZO compression -- may add up to 1 byte per packet for incompressible
data. mode may be "yes", "no", or
"adaptive" (default).
This option is deprecated in favor of the newer --compress option.
In a server mode setup, it is possible to selectively turn compression 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 setting
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
- 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 incompressible (or
pre-compressed) packets over the tunnel, such as an FTP or rsync transfer
of a large, compressed file. With adaptive compression, OpenVPN will
periodically sample the compression process to measure its efficiency. If
the data being sent over the tunnel is already compressed, the compression
efficiency will be very low, triggering openvpn to disable compression for
a period of time until the next re-sample test.
- --management IP port [pw-file]
- Enable a TCP server on IP:port to handle daemon management
functions. pw-file, if specified, is a password file (password on
first line) or "stdin" to prompt from standard input. The
password provided will set the password which TCP clients will need to
provide in order to access management functions.
The management interface can also listen on a unix domain socket, for those
platforms that support it. To use a unix domain socket, specify the unix
socket pathname in place of IP and set port to 'unix'. While
the default behavior is to create a unix domain socket that may be
connected to by any process, the --management-client-user and
--management-client-group directives can be used 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 = "tunnel". Tunnel mode will cause the management
interface to listen for a TCP connection on the local VPN address of the
TUN/TAP interface.
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 OpenVPN
source distribution.
It is strongly recommended that IP be set to 127.0.0.1 (localhost) to
restrict accessibility of the management server to local clients.
- --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-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 specific
--remote (client-only).
- --management-query-remote
- Allow management interface to override --remote directives
(client-only).
- --management-external-key
- Allows usage for external private key file instead of --key option
(client-only).
- --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-external-key.
- --management-forget-disconnect
- Make OpenVPN forget passwords when management session disconnects.
This directive does not affect the --http-proxy username/password. It
is always cached.
- --management-hold
- Start OpenVPN in a hibernating state, until a client of the management
interface explicitly starts it with the hold release command.
- --management-signal
- Send SIGUSR1 signal to OpenVPN if management session disconnects. 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-log-cache n
- Cache the most recent n lines of log file history for usage by the
management channel.
- --management-up-down
- Report tunnel up/down events to management interface.
- --management-client-auth
- Gives management interface client the responsibility to authenticate
clients after their client certificate has been verified. See
management-notes.txt in OpenVPN distribution for detailed notes.
- --management-client-pf
- Management interface clients must specify a packet filter file for each
connecting client. See management-notes.txt in OpenVPN distribution for
detailed notes.
- --management-client-user u
- When the management interface is listening on a unix domain socket, only
allow connections from user u.
- --management-client-group g
- When the management interface is listening on a unix domain socket, only
allow connections from group g.
- --plugin module-pathname [init-string]
- Load plug-in module from the file module-pathname, passing
init-string as an argument to the module initialization function.
Multiple plugin modules may be loaded into one OpenVPN process.
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, see /usr/share/openvpn/plugin.
The documentation is in doc and the actual plugin modules are in
lib.
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 module/script controls an authentication function (such
as tls-verify, auth-user-pass-verify, or client-connect), then every
module and script must return success (0) in order for the connection to
be authenticated.
- --keying-material-exporter label len
- 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_PLUGIN_TLS_FINAL callback.
Note that exporter labels have the potential to collide with existing PRF
labels. In order to prevent this, labels MUST begin with
"EXPORTER".
This option requires OpenSSL 1.0.1 or newer.
Server Mode¶
Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is supported, and
can be enabled with the
--mode server option. In server mode, OpenVPN
will listen on a single port for incoming client connections. 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.
- --server network netmask ['nopool']
- 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.
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 gateway netmask pool-start-IP pool-end-IP
- --server-bridge ['nogw']
-
A helper directive similar to --server which is designed to simplify
the configuration of OpenVPN's server mode in ethernet bridging
configurations.
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 ethernet NIC interface.
For example, on Linux this is done with the brctl tool, and with
Windows XP it is done in the Network Connections 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 gateway/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
connecting 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
- --push option
- Push a config file option back to the client for remote execution. 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, --inactive, --ping, --ping-exit, --ping-restart,
--setenv, --auth-token, --persist-key, --persist-tun,
--echo, --comp-lzo, --socket-flags, --sndbuf,
--rcvbuf
- --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.
- --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.
- --push-peer-info
- Push additional information about the client to server. The following data
is always pushed to the server:
IV_VER=<version> -- the client OpenVPN version
IV_PLAT=[linux|solaris|openbsd|mac|netbsd|freebsd|win] -- the client OS
platform
IV_LZO_STUB=1 -- if client was built with LZO stub capability
IV_LZ4=1 -- if the client supports LZ4 compressions.
IV_PROTO=2 -- if the client supports peer-id floating mechansim
IV_NCP=2 -- negotiable ciphers, client supports --cipher pushed by
the server, a value of 2 or greater indicates client supports AES-GCM-128
and AES-GCM-256.
IV_UI_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.
When --push-peer-info is enabled the additional information consists
of the following data:
IV_HWADDR=<mac address> -- the MAC address of clients default gateway
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_"
- --disable
- Disable a particular client (based on the common name) from connecting.
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.
- --ifconfig-pool start-IP end-IP [netmask]
- Set aside a pool of subnets to be dynamically allocated to connecting
clients, similar to a DHCP server. For tun-style tunnels, each client will
be given a /30 subnet (for interoperability with Windows clients). For
tap-style tunnels, individual addresses will be allocated, and the
optional netmask parameter will also be pushed to clients.
- --ifconfig-pool-persist file [seconds]
- Persist/unpersist ifconfig-pool data to file, at seconds
intervals (default=600), as well as on program startup and shutdown.
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
<Common-Name>,<IP-address>.
If seconds = 0, file will be treated as read-only. This is
useful 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 common name will always receive the
given IP address. If you want guaranteed assignment, use
--ifconfig-push
- --ifconfig-pool-linear
- Modifies the --ifconfig-pool directive to allocate individual TUN
interface addresses for clients rather than /30 subnets. NOTE: This option
is incompatible with Windows clients.
This option is deprecated, and should be replaced with --topology p2p
which is functionally equivalent.
- --ifconfig-push local remote-netmask [alias]
- Push virtual IP endpoints for client tunnel, overriding the
--ifconfig-pool dynamic allocation.
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.
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.
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).
- --iroute network [netmask]
- Generate an internal route to a specific client. The netmask
parameter, if omitted, defaults to 255.255.255.255.
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.
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 subnet 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.
- --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-originating
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.
- --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.
- --client-connect cmd
- Run command cmd on client connection.
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 spaces.
The command is passed the common name and IP address of the
just-authenticated client as environmental variables (see environmental
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.
- --client-disconnect cmd
- Like --client-connect but called on client instance shutdown. Will
not be called unless the --client-connect script and plugins (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 function 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 passed the same pathname as the
corresponding --client-connect command as its last argument. (after
any arguments specified in cmd ).
- --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 readable by the OpenVPN process after it
has dropped it's root privileges.
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, and
--config.
- --ccd-exclusive
- Require, as a condition of authentication, that a connecting client has a
--client-config-dir file.
- --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:
* --client-connect scripts to dynamically generate client-specific
configuration files.
* OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin hook to return
success/failure via auth_control_file when using deferred auth method
* OPENVPN_PLUGIN_ENABLE_PF plugin hook to pass filtering rules via
pf_file
- --hash-size r v
- Set the size of the real address hash table to r and the virtual
address table to v. By default, both tables are sized at 256
buckets.
- --bcast-buffers n
- Allocate n buffers for broadcast datagrams (default=256).
- --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 connection 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.
- --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-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 authenticated 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-generated by a
--client-connect script to override the global value for a
particular client.
Note that this directive affects OpenVPN's internal routing table, not the
kernel routing table.
- --stale-routes-check n [t]
- Remove routes haven't had activity for n seconds (i.e. the ageing
time).
This check is ran every t seconds (i.e. check interval).
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
- --connect-freq n sec
- Allow a maximum of n new connections per sec seconds from
clients. This is designed to contain DoS attacks which flood the server
with connection requests using certificates which will ultimately fail to
authenticate.
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.
- --learn-address cmd
- Run command cmd to validate client virtual addresses or routes.
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 spaces.
Three arguments will be appended to any arguments in cmd as follows:
[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 interface. 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 policies with regard to the client's high-level
common name, rather than the low level client virtual addresses.
- --auth-user-pass-verify cmd method
- Require the client to provide a username/password (possibly in addition to
a client certificate) for authentication.
OpenVPN will run command cmd to validate the username/password
provided by the client.
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 spaces.
If method is set to "via-env", OpenVPN will call
script with the environmental variables username and
password set to the username/password strings provided by the
client. Be aware 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 script, 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 prevent 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, or
a failure code (1) to reject the client.
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 username 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 handled. 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
sample-scripts/auth-pam.pl in the OpenVPN source distribution.
- --auth-gen-token [lifetime]
- After successful user/password authentication, the OpenVPN server will
with this option generate a temporary authentication token and push that
to client. On the following renegotiations, 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 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.
This feature is useful for environments which is 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.
- --opt-verify
- Clients that connect with options that are incompatible 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, fragment, keydir, cipher, auth,
keysize, secret, no-replay, no-iv, tls-auth, key-method, tls-server, and
tls-client.
This option requires that --disable-occ NOT be used.
- --auth-user-pass-optional
- Allow connections by clients that do not specify a username/password.
Normally, when --auth-user-pass-verify or
--management-client-auth is 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, passing the responsibility to the
user-defined authentication module/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
username/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.
- --client-cert-not-required (DEPRECATED)
- Don't require client certificate, client will authenticate using
username/password only. Be aware that using this directive is less secure
than requiring certificates from all clients.
Please note: This option is now deprecated and will be removed in
OpenVPN v2.5. It is replaced by --verify-client-cert which allows
for more flexibility. The option --verify-client-cert none is
functionally equivalent to --client-cert-not-required
- --verify-client-cert none|optional|require
- Specify whether the client is required to supply a valid certificate.
Possible options are
none : a client certificate is not required. the client need to
authenticate using username/password only. Be aware that using this
directive is less secure than requiring certificates 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 compromise 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 certificate
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.
- --username-as-common-name
- For --auth-user-pass-verify authentication, use the authenticated
username as the common name, rather than the common name from the client
cert.
- --compat-names [no-remapping] (DEPRECATED)
- Until OpenVPN v2.3 the format of the X.509 Subject fields was formatted
like this:
- /C=US/L=Somewhere/CN=John Doe/emailAddress=john@example.com
- In addition the old behaviour was to remap any character other than
alphanumeric, underscore ('_'), dash ('-'), dot ('.'), and slash ('/') to
underscore ('_'). The X.509 Subject string as returned by the
tls_id environmental variable, could additionally contain colon
(':') or equal ('=').
- When using the --compat-names option, this old formatting and
remapping will be re-enabled again. This is purely implemented for
compatibility reasons when using older plug-ins or scripts which does not
handle the new formatting or UTF-8 characters.
- In OpenVPN v2.3 the formatting of these fields changed into a more
standardised format. It now looks like:
- C=US, L=Somewhere, CN=John Doe, emailAddress=john@example.com
- The new default format in OpenVPN v2.3 also does not do the character
remapping which happened earlier. This new format enables proper support
for UTF-8 characters in the usernames, X.509 Subject fields and Common
Name variables and it complies to the RFC 2253, UTF-8 String
Representation of Distinguished Names.
The no-remapping mode flag can be used with the --compat-names
option to be compatible with the now deprecated --no-name-remapping
option. It is only available at the server. When this mode flag is used,
the Common Name, Subject, and username strings are allowed to include any
printable character including space, but excluding control characters such
as tab, newline, and carriage-return. no-remapping is only available on
the server side.
Please note: This option is immediately deprecated. It is only
implemented to make the transition to the new formatting less intrusive.
It will be removed in OpenVPN v2.5. So please update your scripts/plug-ins
where necessary.
- --no-name-remapping (DEPRECATED)
- The --no-name-remapping option is an alias for
--compat-names no-remapping. It ensures compatibility with
server configurations using the --no-name-remapping option.
Please note: This option is now deprecated. It will be removed in
OpenVPN v2.5. So please make sure you support the new X.509 name
formatting described with the --compat-names option as soon as
possible.
- --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 connection 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 theoretically 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 dictionary by the proxy receiver to determine
the origin of the connection. Each generated file will be automatically
deleted when the proxied connection is torn down.
Not implemented on Windows.
Client Mode¶
Use client mode when connecting to an OpenVPN server which has
--server,
--server-bridge, or
--mode server in it's configuration.
- --client
- A helper directive designed to simplify the configuration of OpenVPN's
client mode. This directive is equivalent to:
pull
tls-client
- --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 accept|ignore|reject text
- Filter options received from the server if the option starts with
text. Runs on client. 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.
This option may be used only on clients. 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.
- --auth-user-pass [up]
- Authenticate with server using username/password. up is 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 message 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
username/password and/or private key password before attempting a
reconnection.
Note that while this option cannot be pushed, it can be controlled from the
management interface.
- --static-challenge t e
- Enable static challenge/response protocol using challenge text t,
with echo flag given by e (0|1).
The echo flag indicates whether or not the user's response to the challenge
should be echoed.
See management-notes.txt in the OpenVPN distribution for a description of
the OpenVPN challenge/response protocol.
- --server-poll-timeout n
- --connect-timeout n when connecting to a remote server do not wait
for more than n seconds waiting for a response before trying the
next server. The default value is 120s. This timeout includes proxy and
TCP connect timeouts.
- --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. The n parameter (default=1) controls the maximum number of
attempts that the client will try to resend the exit notification message.
In UDP server mode, send RESTART control channel command to connected
clients. The n parameter (default=1) controls client behavior. With
n = 1 client will attempt to reconnect 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.
- --allow-recursive-routing
- When this option is set, OpenVPN will not drop incoming tun packets with
same destination as host.
Data Channel Encryption Options:¶
These options are meaningful for both Static & TLS-negotiated key modes
(must be compatible between peers).
- --secret file [direction]
- Enable Static Key encryption mode (non-TLS). Use pre-shared secret
file which was generated with --genkey.
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
bidirectionally, one for HMAC and the other for encryption/decryption.
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 complicated
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, everything 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 generated by OpenVPN. Anyone eavesdropping on
the wire would see nothing but random-looking data.
- --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).
- --auth alg
- Authenticate data channel packets and (if enabled) tls-auth control
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, the specified --auth
algorithm is ignored for the data channel, and the authentication 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 control 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
- Encrypt data channel packets with cipher algorithm alg. The default
is BF-CBC, an abbreviation for Blowfish in Cipher Block Chaining
mode.
Using BF-CBC is no longer recommended, because of it's 64-bit block size.
This small block size allows attacks based on collisions, as demonstrated
by SWEET32. See https://community.openvpn.net/openvpn/wiki/SWEET32 for
details.
To see other ciphers that are available with OpenVPN, use the
--show-ciphers option.
Set alg=none to disable encryption.
As of OpenVPN 2.4, cipher negotiation (NCP) can override the cipher
specified by --cipher. See --ncp-ciphers and
--ncp-disable for more on NCP.
- --ncp-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".
For servers, the first cipher from cipher_list will be pushed to
clients that support cipher negotiation.
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 both peers support and do not disable NCP, the negotiated cipher will
override the cipher specified by --cipher.
Additionally, to allow for more smooth transition, if NCP is enabled,
OpenVPN will inherit the cipher of the peer if that cipher is different
from the local --cipher setting, but the peer cipher is one of the
ciphers specified in --ncp-ciphers. E.g. a non-NCP client
(<=2.3, or with --ncp-disabled set) connecting to a NCP server (2.4+)
with "--cipher BF-CBC" and "--ncp-ciphers
AES-256-GCM:AES-256-CBC" set can either specify "--cipher
BF-CBC" or "--cipher AES-256-CBC" and both will work.
- --ncp-disable
- Disable "negotiable crypto parameters". This completely disables
cipher negotiation.
- --keysize n
- Size of cipher key in bits (optional). If unspecified, defaults to
cipher-specific default. The --show-ciphers option (see below)
shows all available OpenSSL ciphers, their default key sizes, and whether
the key size can be changed. Use care in changing a cipher's default key
size. Many ciphers have not been extensively cryptanalyzed with
non-standard key lengths, and a larger key may offer no real guarantee of
greater security, or may even reduce security.
- --prng alg [nsl]
- (Advanced) For PRNG (Pseudo-random number generator), use digest algorithm
alg (default=sha1), and set nsl (default=16) to the size in
bytes of the nonce secret length (between 16 and 64).
Set alg=none to disable the PRNG and use the OpenSSL RAND_bytes
function instead for all of OpenVPN's pseudo-random number needs.
- --engine [engine-name]
- Enable OpenSSL hardware-based crypto engine functionality.
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.
- --no-replay
- (Advanced) Disable OpenVPN's protection against replay attacks. Don't use
this option unless you are prepared to make a tradeoff of greater
efficiency in exchange for less security.
OpenVPN provides datagram replay protection by default.
Replay protection is accomplished by tagging each outgoing datagram with an
identifier that is guaranteed to be unique for the key being used. The
peer that receives the datagram will check for the uniqueness of the
identifier. If the identifier was already received in a previous datagram,
OpenVPN will drop the packet. Replay protection is important to defeat
attacks such as a SYN flood attack, where the attacker listens in the
wire, intercepts a TCP SYN packet (identifying it by the context in which
it occurs in relation to other packets), then floods the receiving peer
with copies of this packet.
OpenVPN's replay protection is implemented in slightly different ways,
depending on the key management mode you have selected.
In Static Key mode or when using an CFB or OFB mode cipher, OpenVPN uses a
64 bit unique identifier that combines a time stamp with an incrementing
sequence number.
When using TLS mode for key exchange and a CBC cipher mode, OpenVPN uses
only a 32 bit sequence number without a time stamp, since OpenVPN can
guarantee the uniqueness of this value for each key. As in IPSec, if the
sequence number is close to wrapping back to zero, OpenVPN will trigger a
new key exchange.
To check for replays, OpenVPN uses the sliding window algorithm used
by IPSec.
- --replay-window n [t]
- Use a replay protection sliding-window of size n and a time window
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 possibility 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 constraints.
(a) The packet cannot be a replay (unless --no-replay is
specified, 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 containing 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
"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 networks?
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.
- --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.
- --replay-persist file
- Persist replay-protection state across sessions using file to save
and reload the state.
This option will strengthen protection against replay attacks, especially
when you are using OpenVPN in a dynamic context (such as with
--inetd) when OpenVPN sessions are frequently started and stopped.
This option will keep a disk copy of the current replay protection 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.
- --no-iv
-
DEPRECATED This option will be removed in OpenVPN 2.5.
(Advanced) Disable OpenVPN's use of IV (cipher initialization vector). Don't
use this option unless you are prepared to make a tradeoff of greater
efficiency in exchange for less security.
OpenVPN uses an IV by default, and requires it for CFB and OFB cipher modes
(which are totally insecure without it). Using an IV is important for
security when multiple messages are being encrypted/decrypted with the
same key.
IV is implemented differently depending on the cipher mode used.
In CBC mode, OpenVPN uses a pseudo-random IV for each packet.
In CFB/OFB mode, OpenVPN uses a unique sequence number and time stamp as the
IV. In fact, in CFB/OFB mode, OpenVPN uses a datagram space-saving
optimization that uses the unique identifier for datagram replay
protection as the IV.
- --use-prediction-resistance
- Enable prediction resistance on PolarSSL'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.
Note that this option only works with PolarSSL versions greater than
1.1.
- --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
or
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 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.
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 channels which
are multiplexed over a single TCP/UDP port. OpenVPN initiates 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 channel 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 specified 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 certificates
& keys:
https://github.com/OpenVPN/easy-rsa
- --tls-server
- Enable TLS and assume server role during TLS handshake. Note that OpenVPN
is designed as a peer-to-peer application. The designation of client or
server is only for the purpose of negotiating the TLS control
channel.
- --tls-client
- Enable TLS and assume client role during TLS handshake.
- --ca file
- Certificate authority (CA) file in .pem format, also referred to as the
root certificate. This file can have multiple certificates 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 variable 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 PolarSSL.
When using the --capath option, you are required to supply valid CRLs
for the CAs too. 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 and
openssl crl for more information.
- --dh file
- File containing Diffie Hellman parameters in .pem format (required for
--tls-server only).
Set file=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 PolarSSL 1.3+).
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.
- --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 certificate should have been signed by
the key of a certificate authority 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 ).
- --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 certificate chain
without trusting them to verify the peer-submitted certificate, as would
be the case if the certs were placed in the ca file.
- --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).
- --tls-version-min version ['or-highest']
- Sets the minimum TLS version we will accept from the peer (default is
"1.0"). 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".
- --pkcs12 file
- Specify a PKCS #12 file containing local private key, local certificate,
and root CA certificate. This option can be used instead of --ca,
--cert, and --key. Not available with PolarSSL.
- --verify-hash hash
- Specify SHA1 fingerprint for level-1 cert. 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 certificate 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
- --pkcs11-cert-private [0|1]...
- Set if access to certificate object should be performed after login. Every
provider has its own setting.
- --pkcs11-id name
- Specify the serialized certificate id to be used. The id can be gotten by
the standalone --show-pkcs11-ids 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 triggered, 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.
- --pkcs11-pin-cache seconds
- Specify how many seconds the PIN can be cached, the default is until the
token is removed.
- --pkcs11-protected-authentication [0|1]...
- Use PKCS#11 protected authentication path, useful for biometric and
external keypad devices. Every provider has its own setting.
- --pkcs11-providers provider...
- Specify a RSA Security Inc. PKCS #11 Cryptographic Token Interface
(Cryptoki) providers to load. This option can be used instead of
--cert, --key, and --pkcs12.
If p11-kit is present on the system, its p11-kit-proxy.so module will
be loaded by default if either the --pkcs11-id or
--pkcs11-id-management options are specified without
--pkcs11-provider being given.
- --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.
- --cryptoapicert select-string
- Load the certificate and private key from the Windows Certificate System
Store (Windows/OpenSSL Only).
Use this option instead of --cert and --key.
This makes it possible to use any smart card, supported by Windows, 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 certificate's
subject:
cryptoapicert "SUBJ:Peter Runestig"
To select a certificate, based on certificate's thumbprint:
cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."
The thumbprint hex string can easily be copy-and-pasted from the Windows
Certificate Store GUI.
- --key-method m
- Use data channel key negotiation method m. The key method must
match on both sides of the connection.
After OpenVPN negotiates a TLS session, a new set of keys for protecting the
tunnel data channel is generated and exchanged over the TLS session.
In method 1 (the default for OpenVPN 1.x), both sides generate random
encrypt and HMAC-send keys which are forwarded to the other host over the
TLS channel. Method 1 is deprecated in OpenVPN 2.4 , and will be
removed in OpenVPN 2.5.
In method 2, (the default for OpenVPN 2.0) the client generates a random
key. Both client and server also generate some random seed material. All
key source material is exchanged over the TLS channel. The actual keys are
generated using the TLS PRF function, taking source entropy from both
client and server. Method 2 is designed to closely parallel the key
generation process used by TLS 1.0.
Note that in TLS mode, two separate levels of keying occur:
(1) The TLS connection is initially negotiated, with both sides of the
connection producing certificates and verifying the certificate (or other
authentication info provided) of the other side. The --key-method
parameter has no effect on this process.
(2) After the TLS connection is established, the tunnel session keys are
separately negotiated over the existing secure TLS channel. Here,
--key-method determines the derivation of the tunnel session
keys.
- --tls-cipher l
- A list l of allowable TLS ciphers delimited by a colon
(":").
This 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 PolarSSL documentation for details on the cipher list
interpretation.
Use --show-tls to see a list of TLS ciphers supported by your crypto
library.
Warning! --tls-cipher is an expert feature, which - if used correcly
- can improve the security of your VPN connection. But it is also easy to
unwittingly use it to carefully align a gun with your foot, or just break
your connection. Use with care!
The default for --tls-cipher is to use PolarSSL's default cipher list when
using PolarSSL or
"DEFAULT:!EXP:!LOW:!MEDIUM:!kDH:!kECDH:!DSS:!PSK:!SRP:!kRSA"
when using OpenSSL.
- --tls-timeout n
- Packet retransmit timeout on TLS control channel if no acknowledgment 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.
- --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 DISCOURAGED 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 n
- Renegotiate data channel key after n seconds (default=3600).
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.
- --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.
- --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 critical path of tunnel data
forwarding.
- --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.
- --tls-exit
- Exit on TLS negotiation failure.
- --tls-auth file [direction]
- Add an additional layer of HMAC authentication on top of the TLS control
channel to mitigate DoS attacks and attacks on the TLS stack.
In a nutshell, --tls-auth enables a kind of "HMAC firewall"
on OpenVPN's TCP/UDP port, where TLS control channel packets bearing an
incorrect HMAC signature can be dropped immediately without response.
file (required) is a file in OpenVPN static key format which can be
generated by --genkey
Older versions (up to 2.3) supported a freeform passphrase file. This is no
longer supported in newer versions (2.4+).
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 signature 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
protection 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-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:
- •
- provides more privacy by hiding the certificate used for the TLS
connection,
- •
- makes it harder to identify OpenVPN traffic as such,
- •
- 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.
- --askpass [file]
- Get certificate password from console or file before we daemonize.
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 command 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.
- --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/password. It
is always cached.
- --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_PLUGIN_CLIENT_CONNECT or
OPENVPN_PLUGIN_CLIENT_CONNECT_V2 calls. This option provides a possibility
to replace the clients password 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 have been set. If the
OpenVPN server side rejects the authentication token, the client will
receive an AUTH_FAIL and disconnect.
The purpose of this is to enable two factor authentication methods, such as
HOTP or TOTP, to be used without needing to retrieve a new OTP code each
time the connection is renegotiated. Another use case is to cache
authentication data on the client without needing to have the users
password cached in memory 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.
- --tls-verify cmd
- Run command cmd to verify the X509 name of a pending TLS connection
that has otherwise passed all other tests of certification (except for
revocation via --crl-verify directive; the revocation test occurs
after the --tls-verify test).
cmd should return 0 to allow the TLS handshake to proceed, or 1 to
fail.
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 spaces.
When cmd is executed two arguments are appended after any arguments
specified in cmd , as follows:
cmd certificate_depth subject
These arguments are, respectively, the current certificate depth and the
X509 common name (cn) of the peer.
This feature is useful if the peer you want to trust has a certificate 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.
- --tls-export-cert directory
- Store the certificates the clients uses 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 certificate is available via the peer_cert
environment variable.
- --x509-username-field [ext:]fieldname
- Field in the X.509 certificate subject to be used as the username
(default=CN). Typically, this option is specified with fieldname as
either of the following:
--x509-username-field emailAddress
--x509-username-field ext:subjectAltName
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
multiple email addresses in ext:fieldname, the last occurrence is
chosen.
When this option is used, the --verify-x509-name option will match
against the chosen fieldname instead of the Common Name.
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.
- --verify-x509-name name type
- 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.
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' and
--verify-x509-name Server-1 name or you could use
--verify-x509-name Server -name-prefix 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 OpenVPN with
a custom CA certificate that is under your control. 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 values
from full cert chain. Values will be encoded as
X509_<depth>_<attribute>=<value>. Multiple
--x509-track options can be defined to track multiple
attributes.
- --ns-cert-type client|server
- Require that peer certificate was signed with an explicit
nsCertType designation of "client" or "server".
This is a useful security option for clients, to ensure that the host they
connect with is a designated server.
See the easy-rsa/build-key-server script for an example of how to generate a
certificate with the nsCertType field set to "server".
If the server certificate's nsCertType field is set to "server",
then the clients can verify this with --ns-cert-type server.
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
--ns-cert-type, --verify-x509-name, or --tls-verify.
- --remote-cert-ku v...
- Require that peer certificate was signed with an explicit key
usage.
This is a useful security option for clients, to ensure that the host they
connect to is a designated server.
The key usage should be encoded in hex, more than one key usage can be
specified.
- --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-tls client|server
- Require that peer certificate was signed with an explicit key usage
and extended key usage based on RFC3280 TLS rules.
This is a useful security option for clients, to ensure that the host they
connect to is a designated server.
The --remote-cert-tls client option is equivalent to
--remote-cert-ku 80 08 88 --remote-cert-eku "TLS Web Client
Authentication"
The key usage is digitalSignature and/or keyAgreement.
The --remote-cert-tls server option is equivalent to
--remote-cert-ku a0 88 --remote-cert-eku "TLS Web Server
Authentication"
The key usage is digitalSignature and ( keyEncipherment or keyAgreement ).
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, --verify-x509-name, or --tls-verify.
- --crl-verify crl ['dir']
- Check peer certificate against the file crl in PEM format.
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 containing a client key and
certificate was stolen. By adding the stolen certificate to the CRL file,
you could reject any connection 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.
If the optional dir flag is specified, enable a different mode where
crl is 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 privileges to read the file.
- --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 preference (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-curves
- (Standalone) Show all available elliptic curves to use with the
--ecdh-curve option.
Generate a random key:¶
Used only for non-TLS static key encryption mode.
- --genkey
- (Standalone) Generate a random key to be used as a shared secret, for use
with the --secret option. This file must be shared with the peer
over a pre-existing secure channel such as scp(1)
- --secret file
- Write key to file.
TUN/TAP persistent tunnel config mode:¶
Available with linux 2.4.7+. These options comprise a standalone mode of OpenVPN
which can be used to create and delete persistent tunnels.
- --mktun
- (Standalone) Create a persistent tunnel on platforms which support 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 eliminate 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 terminates 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 connectivity 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 persistent by
default.
- --rmtun
- (Standalone) Remove a persistent tunnel.
- --dev tunX | tapX
- TUN/TAP device
- --user user
- Optional user to be owner of this tunnel.
- --group group
- Optional group to be owner of this tunnel.
Windows-Specific Options:¶
- --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 have changed behaviour in 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 configuration file.
- --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 protocol. 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 normally 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 -1. If offset is positive, 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 broadcast
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 instantiations, 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
automatically."
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 negotiation used by the TAP-Win32 adapter. Note that if the
netsh failover occurs, the TAP-Win32 adapter TCP/IP properties 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 configuration.
- --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.
- --dhcp-option type [parm]
- Set extended TAP-Win32 TCP/IP properties, must be used with --ip-win32
dynamic or --ip-win32 adaptive. This option can be used to set
additional TCP/IP properties on the TAP-Win32 adapter, and is particularly
useful for configuring an OpenVPN client to access a Samba server across
the VPN.
DOMAIN name -- Set Connection-specific DNS Suffix.
DNS addr -- Set primary domain name server IPv4 address. Repeat this
option to set secondary DNS server addresses.
DNS6 addr -- Set primary domain name server IPv6 address. Repeat this
option to set secondary DNS server IPv6 addresses.
Note: currently this is handled using netsh (the existing DHCP code can only
do IPv4 DHCP, and that protocol only permits IPv4 addresses anywhere). The
option will be put into the environment, so an --up script could
act upon it if needed.
WINS addr -- Set primary WINS server address (NetBIOS over TCP/IP
Name Server). Repeat this option to set secondary WINS server addresses.
NBDD addr -- Set primary NBDD server address (NetBIOS over TCP/IP
Datagram Distribution Server) Repeat this option to set secondary NBDD
server addresses.
NTP addr -- 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), and 8 = h-node (query name server, then broadcast).
NBS scope-id -- Set NetBIOS over TCP/IP Scope. A NetBIOS Scope ID
provides 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 single 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 computers 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 NetBIOS scopes courtesy of
NeonSurge@abyss.com)
DISABLE-NBT -- Disable Netbios-over-TCP/IP.
Note that if --dhcp-option is pushed via --push to a
non-windows client, the option will be saved in the client's environment
before the up script is called, under the name
"foreign_option_{n}".
- --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.
- --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.
- --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 suitable for Windows XP) to
ignore said error. Note that pushing unknown options from server does not
trigger fatal errors.
- --dhcp-renew
- Ask Windows to renew the TAP adapter lease on startup. This option is
normally unnecessary, as Windows automatically triggers 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.
- --dhcp-release
- Ask Windows to release the TAP adapter lease on shutdown. This option has
the same caveats as --dhcp-renew above.
- --register-dns
- Run ipconfig /flushdns and ipconfig /registerdns on connection initiation.
This is known to kick Windows into recognizing pushed DNS servers.
- --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.
- --service exit-event [0|1]
- 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. 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 processes to exit.
When executing an OpenVPN process using the --service directive,
OpenVPN will probably not have a console window to output status/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.
- --allow-nonadmin [TAP-adapter]
- (Standalone) Set TAP-adapter to allow access from
non-administrative 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.
- --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 necessary for the TAP-Win32
driver to impose certain constraints on TUN endpoint address selection.
Namely, the point-to-point endpoints used in TUN device emulation must be
the middle two addresses of a /30 subnet (netmask 255.255.255.252).
- --show-net
- (Standalone) Show OpenVPN's view of the system routing table and network
adapter list.
PKCS#11 Standalone Options:¶
- --show-pkcs11-ids [provider] [cert_private]
- (Standalone) Show PKCS#11 token object list. 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 debugging
information.
Standalone Debug Options:¶
- --show-gateway [v6target]
- (Standalone) Show current IPv4 and IPv6 default gateway and interface
towards the gateway (if the protocol in question is enabled). If an IPv6
address is passed as argument, the IPv6 route for this host is
reported.
The following options exist to support IPv6 tunneling in peer-to-peer and
client-server mode. All options are modeled after their IPv4 counterparts, so
more detailed explanations given there apply here as well (except for
--topology , which has no effect on IPv6).
- --ifconfig-ipv6 ipv6addr/bits ipv6remote
- configure IPv6 address ipv6addr/bits on the ``tun'' device. The
second parameter is used as route target for --route-ipv6 if no
gateway is specified.
- --route-ipv6 ipv6addr/bits [gateway] [metric]
- setup IPv6 routing in the system to send the specified IPv6 network into
OpenVPN's ``tun''. The gateway parameter is only used for IPv6 routes
across ``tap'' devices, and if missing, the ``ipv6remote'' field from
--ifconfig-ipv6 is used.
- --server-ipv6 ipv6addr/bits
- convenience-function to enable a number of IPv6 related options at once,
namely --ifconfig-ipv6, --ifconfig-ipv6-pool and --push
tun-ipv6 Is only accepted if ``--mode server'' or ``--server'' is set.
Pushing of the --tun-ipv6 directive is done for older clients which
require an explicit ``--tun-ipv6'' in their configuration.
- --ifconfig-ipv6-pool ipv6addr/bits
- Specify an IPv6 address pool for dynamic assignment to clients. The pool
starts at ipv6addr and matches the offset determined from the start
of the IPv4 pool.
- --ifconfig-ipv6-push ipv6addr/bits ipv6remote
- for ccd/ per-client static IPv6 interface configuration, see
--client-config-dir and --ifconfig-push for more
details.
- --iroute-ipv6 ipv6addr/bits
- for ccd/ per-client static IPv6 route configuration, see --iroute
for more details how to setup and use this, and how --iroute and
--route interact.
SCRIPTING AND ENVIRONMENTAL VARIABLES¶
OpenVPN exports a series of environmental variables for use by user-defined
scripts.
Script Order of Execution¶
- --up
- Executed after TCP/UDP socket bind and TUN/TAP open.
- --tls-verify
- Executed when we have a still untrusted remote peer.
- --ipchange
- Executed after connection authentication, or remote IP address
change.
- --client-connect
- Executed in --mode server mode immediately after client
authentication.
- --route-up
- Executed after connection authentication, either immediately after, or
some number of seconds after as defined by the --route-delay
option.
- --route-pre-down
- Executed right before the routes are removed.
- --client-disconnect
- Executed in --mode server mode on client instance shutdown.
- --down
- Executed after TCP/UDP and TUN/TAP close.
- --learn-address
- Executed in --mode server mode whenever an IPv4 address/route or
MAC address is added to OpenVPN's internal routing table.
- --auth-user-pass-verify
- Executed in --mode server mode on new client connections, when the
client is still untrusted.
String Types and Remapping¶
In certain cases, OpenVPN will perform remapping of characters in strings.
Essentially, any characters outside the set of permitted characters for each
string type will be converted to underbar ('_').
Q: Why is string remapping necessary?
A: It's an important security feature to prevent the malicious coding 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?
A: 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 permitted
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_PLUGIN_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() function 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 2.0.1-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 associated 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.
- 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" otherwise. Set on program initiation and reset on
SIGHUP.
- daemon_log_redirect
- Set to "1" if the --log or --log-append directives
are specified, 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.
- 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
--ifconfig-ipv6 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_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) commands which normally
occurs prior to --up script execution.
- ifconfig_ipv6_remote
- The remote VPN endpoint IPv6 address specified in the
--ifconfig-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
ifconfig) commands which normally occurs prior to --up script
execution.
- ifconfig_netmask
- The subnet mask of the virtual ethernet segment that is specified 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
- The maximum packet size (not including the IP header) of tunnel data in
UDP tunnel transport mode. Set prior to --up or --down
script execution.
- 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" or "gateway"
("netmask" is contained as "/nnn" in the
route_ipv6_network_{n}, 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 connection.
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 (controlled 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 (or 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 (or 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 properly. 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 similar 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 example, 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
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 and
--tls-crypt 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 possibly
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 condition,
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 above 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.
TUN/TAP DRIVER SETUP¶
If you are running Linux 2.4.7 or higher, you probably have the TUN/TAP driver
already installed. If so, there are still a few things you need to do:
Make device:
mknod /dev/net/tun c 10 200
Load driver:
modprobe tun
EXAMPLES¶
Prior to running these examples, you should have OpenVPN installed on two
machines with network connectivity between them. If you have not yet installed
OpenVPN, consult the INSTALL file included in the OpenVPN distribution.
TUN/TAP Setup:¶
If you are using Linux 2.4 or higher, make the tun device node and load the tun
module:
- mknod /dev/net/tun c 10 200
- modprobe tun
If you installed from RPM, the
mknod step may be omitted, because the RPM
install does that for you.
Only Linux 2.4 and newer are supported.
For other platforms, consult the INSTALL file at
http://openvpn.net/install.html for more information.
Firewall Setup:¶
If firewalls exist between the two machines, they should be set to forward UDP
port 1194 in both directions. If you do not have control over the firewalls
between the two machines, you may still be able to use OpenVPN by adding
--ping 15 to each of the
openvpn commands used below in the
examples (this will cause each peer to send out a UDP ping to its remote peer
once every 15 seconds which will cause many stateful firewalls to forward
packets in both directions without an explicit firewall rule).
If you are using a Linux iptables-based firewall, you may need to enter the
following command to allow incoming packets on the TUN device:
- iptables -A INPUT -i tun+ -j ACCEPT
See the firewalls section below for more information on configuring firewalls
for use with OpenVPN.
VPN Address Setup:¶
For purposes of our example, our two machines will be called
bob.example.com and
alice.example.com. If you are constructing a
VPN over the internet, then replace
bob.example.com and
alice.example.com with the internet hostname or IP address that each
machine will use to contact the other over the internet.
Now we will choose the tunnel endpoints. Tunnel endpoints are private IP
addresses that only have meaning in the context of the VPN. Each machine will
use the tunnel endpoint of the other machine to access it over the VPN. In our
example, the tunnel endpoint for bob.example.com will be 10.4.0.1 and for
alice.example.com, 10.4.0.2.
Once the VPN is established, you have essentially created a secure alternate
path between the two hosts which is addressed by using the tunnel endpoints.
You can control which network traffic passes between the hosts (a) over the
VPN or (b) independently of the VPN, by choosing whether to use (a) the VPN
endpoint address or (b) the public internet address, to access the remote
host. For example if you are on bob.example.com and you wish to connect to
alice.example.com via
ssh without using the VPN (since
ssh has
its own built-in security) you would use the command
ssh
alice.example.com. However in the same scenario, you could also use the
command
telnet 10.4.0.2 to create a telnet session with
alice.example.com over the VPN, that would use the VPN to secure the session
rather than
ssh.
You can use any address you wish for the tunnel endpoints but make sure that
they are private addresses (such as those that begin with 10 or 192.168) and
that they are not part of any existing subnet on the networks of either peer,
unless you are bridging. If you use an address that is part of your local
subnet for either of the tunnel endpoints, you will get a weird feedback loop.
Example 1: A simple tunnel without security¶
On bob:
- openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1
10.4.0.2 --verb 9
On alice:
- openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2
10.4.0.1 --verb 9
Now verify the tunnel is working by pinging across the tunnel.
On bob:
- ping 10.4.0.2
On alice:
- ping 10.4.0.1
The
--verb 9 option will produce verbose output, similar to the
tcpdump(8) program. Omit the
--verb 9 option to have OpenVPN run
quietly.
Example 2: A tunnel with static-key security (i.e. using a pre-shared secret)¶
First build a static key on bob.
- openvpn --genkey --secret key
This command will build a random key file called
key (in ascii format).
Now copy
key to alice over a secure medium such as by using the
scp(1) program.
On bob:
- openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1
10.4.0.2 --verb 5 --secret key
On alice:
- openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2
10.4.0.1 --verb 5 --secret key
Now verify the tunnel is working by pinging across the tunnel.
On bob:
- ping 10.4.0.2
On alice:
- ping 10.4.0.1
Example 3: A tunnel with full TLS-based security¶
For this test, we will designate
bob as the TLS client and
alice
as the TLS server.
Note that client or server designation only has meaning
for the TLS subsystem. It has no bearing on OpenVPN's peer-to-peer, UDP-based
communication model.
First, build a separate certificate/key pair for both bob and alice (see above
where
--cert is discussed for more info). Then construct Diffie Hellman
parameters (see above where
--dh is discussed for more info). You can
also use the included test files client.crt, client.key, server.crt,
server.key and ca.crt. The .crt files are certificates/public-keys, the .key
files are private keys, and ca.crt is a certification authority who has signed
both client.crt and server.crt. For Diffie Hellman parameters you can use the
included file dh1024.pem.
Note that all client, server, and certificate
authority certificates and keys included in the OpenVPN distribution are
totally insecure and should be used for testing only.
On bob:
- openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1
10.4.0.2 --tls-client --ca ca.crt --cert client.crt --key client.key
--reneg-sec 60 --verb 5
On alice:
- openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2
10.4.0.1 --tls-server --dh dh1024.pem --ca ca.crt --cert server.crt --key
server.key --reneg-sec 60 --verb 5
Now verify the tunnel is working by pinging across the tunnel.
On bob:
- ping 10.4.0.2
On alice:
- ping 10.4.0.1
Notice the
--reneg-sec 60 option we used above. That tells OpenVPN to
renegotiate the data channel keys every minute. Since we used
--verb 5
above, you will see status information on each new key negotiation.
For production operations, a key renegotiation interval of 60 seconds is
probably too frequent. Omit the
--reneg-sec 60 option to use OpenVPN's
default key renegotiation interval of one hour.
Routing:¶
Assuming you can ping across the tunnel, the next step is to route a real subnet
over the secure tunnel. Suppose that bob and alice have two network interfaces
each, one connected to the internet, and the other to a private network. Our
goal is to securely connect both private networks. We will assume that bob's
private subnet is 10.0.0.0/24 and alice's is 10.0.1.0/24.
First, ensure that IP forwarding is enabled on both peers. On Linux, enable
routing:
- echo 1 > /proc/sys/net/ipv4/ip_forward
and enable TUN packet forwarding through the firewall:
- iptables -A FORWARD -i tun+ -j ACCEPT
On bob:
- route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2
On alice:
- route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1
Now any machine on the 10.0.0.0/24 subnet can access any machine on the
10.0.1.0/24 subnet over the secure tunnel (or vice versa).
In a production environment, you could put the route command(s) in a script and
execute with the
--up option.
FIREWALLS¶
OpenVPN's usage of a single UDP port makes it fairly firewall-friendly. You
should add an entry to your firewall rules to allow incoming OpenVPN packets.
On Linux 2.4+:
- iptables -A INPUT -p udp -s 1.2.3.4 --dport 1194 -j ACCEPT
This will allow incoming packets on UDP port 1194 (OpenVPN's default UDP port)
from an OpenVPN peer at 1.2.3.4.
If you are using HMAC-based packet authentication (the default in any of
OpenVPN's secure modes), having the firewall filter on source address can be
considered optional, since HMAC packet authentication is a much more secure
method of verifying the authenticity of a packet source. In that case:
- iptables -A INPUT -p udp --dport 1194 -j ACCEPT
would be adequate and would not render the host inflexible with respect to its
peer having a dynamic IP address.
OpenVPN also works well on stateful firewalls. In some cases, you may not need
to add any static rules to the firewall list if you are using a stateful
firewall that knows how to track UDP connections. If you specify
--ping
n, OpenVPN will be guaranteed to send a packet to its peer at least once
every
n seconds. If
n is less than the stateful firewall
connection timeout, you can maintain an OpenVPN connection indefinitely
without explicit firewall rules.
You should also add firewall rules to allow incoming IP traffic on TUN or TAP
devices such as:
- iptables -A INPUT -i tun+ -j ACCEPT
to allow input packets from tun devices,
- iptables -A FORWARD -i tun+ -j ACCEPT
to allow input packets from tun devices to be forwarded to other hosts on the
local network,
- iptables -A INPUT -i tap+ -j ACCEPT
to allow input packets from tap devices, and
- iptables -A FORWARD -i tap+ -j ACCEPT
to allow input packets from tap devices to be forwarded to other hosts on the
local network.
These rules are secure if you use packet authentication, since no incoming
packets will arrive on a TUN or TAP virtual device unless they first pass an
HMAC authentication test.
FAQ¶
http://openvpn.net/faq.html
HOWTO¶
For a more comprehensive guide to setting up OpenVPN in a production setting,
see the OpenVPN HOWTO at
http://openvpn.net/howto.html
PROTOCOL¶
For a description of OpenVPN's underlying protocol, see
http://openvpn.net/security.html
WEB¶
OpenVPN's web site is at
http://openvpn.net/
Go here to download the latest version of OpenVPN, subscribe to the mailing
lists, read the mailing list archives, or browse the SVN repository.
BUGS¶
Report all bugs to the OpenVPN team <info@openvpn.net>.
SEE ALSO¶
dhcpcd(8),
ifconfig(8),
openssl(1),
route(8),
scp(1)
ssh(1)
NOTES¶
This product includes software developed by the OpenSSL Project (
http://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
http://www.oberhumer.com/opensource/lzo/
COPYRIGHT¶
Copyright (C) 2002-2010 OpenVPN Technologies, 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 <jim@yonan.net>