Difference between revisions of "OpenVPN Bridge"

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[[Category:Networking (English)]]
+
[[Category:Virtual Private Network]]
[[Category:HOWTOs (English)]]
+
This page describes how to create a network bridge on Arch Linux and host an OpenVPN server using a IP layer-2 based Ethernet bridge (TAP) rather than a IP layer-3 based IP tunnel (TUN). The general [[OpenVPN]] page describes setting up PAM authentication or OpenSSL security certificates in more detail.
'''TODO:''' check this now updated article for accuracy, readability and completeness
+
 
+
This page describes multiple ways to create a network bridge on Arch Linux and host an OpenVPN server using a IP layer-2 based Ethernet bridge (TAP) rather than a IP layer-3 based IP tunnel (TUN). The general [[OpenVPN]] page describes setting up PAM authentication or OpenSSL security certificates in more detail.
+
  
 
==Introduction==
 
==Introduction==
  
The [http://openvpn.net/index.php/open-source/documentation.html OpenVPN documentation pages] give a full overview of server-side and client-side options that OpenVPN supports. It is easier to set up OpenVPN in tunneling mode and control routing the traffic and it is generally advised to do so if it serves your purpose. However, some applications, such as Windows file sharing or Samba, rely on network broadcasts at the Ethernet level and benefit from believing they are physically located on the same subnet, and software bridging serves this purpose.
+
The [http://openvpn.net/index.php/open-source/documentation.html OpenVPN documentation] page gives a full overview of server-side and client-side options that OpenVPN supports. It is easier to set up OpenVPN in tunneling mode and control routing the traffic and it is generally advised to do so if it serves your purpose. However, some network applications, such as Windows file sharing, rely on network broadcasts at the Ethernet level and benefit from believing they are physically located on the same subnet, and software bridging serves this purpose.
 +
 
 +
There are multiple ways to set bridging up. The dynamic method is where OpenVPN will be managing its own bridge on the system and will start, stop and configure it itself. This is the quickest way to set bridging up, although it interrupts other network services when OpenVPN starts and stops. If the system is going to manage its own bridge, maybe because other virtual network adapters connect to the bridge besides just that of OpenVPN, then it is preferable to use the static method.
  
==Installation==
+
==Dynamic Bridge Installation==
  
The first thing you want to do is install OpenVPN and Linux bridging utilities.
+
You will need to install OpenVPN and Linux bridging utilities
 
<pre>
 
<pre>
 
pacman -S openvpn bridge-utils
 
pacman -S openvpn bridge-utils
 
</pre>
 
</pre>
  
==Configuration==
+
==Dynamic Bridge Configuration==
  
Earlier versions of guides for OpenVPN provided by the OpenVPN team or various Linux packagers give example scripts for constructing a bridge when starting OpenVPN and destroying it when shutting OpenVPN down.  
+
OpenVPN will create/destroy the TAP device automatically for the name specified in the config file. OpenVPN settings common to TUN or TAP are not shown in the example config file below, only settings that affect TAP mode. Make sure the 'up' and 'down' scripts are executable with 'chmod +x' after you write them.
  
However, this is a somewhat deprecated approach, since OpenVPN as of 2.1.1 defaults to not allowing itself to call external scripts or programs unless explicitly enabled to, for security reasons.
+
/etc/openvpn/server.conf (sections common to TUN and TAP omitted)
 
+
Also, constructing the bridge is relatively slow compared to all other parts of the network initialization process. (In fact, so slow that dhcpcd will time out before the bridge is ready. See [[#Troubleshooting]].) Also, when restarting OpenVPN after configuration changes, there is no reason to rebuild a working bridge, interrupting all your other network applications. So, setting up a static bridge configuration as follows is one recommended method.
+
 
+
===Bridge===
+
 
+
Add a tap interface for OpenVPN to use in '''/etc/conf.d/openvpn-tapdev'''
+
 
<pre>
 
<pre>
#
+
# this uses a dhcp server, server-side
# /etc/conf.d/openvpn-tapdev
+
# clients must support binding their dhcp client to their tap adapter
#
+
# do not append 'nogw' if using dhcp
# Place openvpn-tapdev before network into your DAEMONS array
+
server-bridge
# This will create permanent tap devices which you can use for bridging
+
# can specify interface, like tap0 or tap1
#
+
#  or use up/down routing scripts to handle
# Example:
+
# more than one, if needed
# TAPDEVS="work home"
+
dev tap0
# Will create two tap devices "work" and "home"
+
# needed to call scripts like up/down
#
+
# which call external programs within the scripts
TAPDEVS="tap0"
+
script-security 2
 +
# user defined scripts for adding/removing tap to bridge
 +
#  'dev mtu link_mtu ifconfig_local_ip ifconfig_remote_ip' are appended if set
 +
# make sure 'user' has permission to run 'down' ('up' will be root)
 +
up "up br0 eth0"
 +
down "down br0 eth0"
 +
# call 'down' before TUN/TAP close
 +
down-pre
 +
# drop root priveledges once connected
 +
#  good idea, for servers running on linux
 +
# 'up' script not affected, 'down' script is
 +
;user nobody
 +
;group nobody
 
</pre>
 
</pre>
  
Add a rule for bridging this tap interface, and your other interfaces, to a bridge interface in '''/etc/conf.d/bridges'''
+
/etc/openvpn/up
 
<pre>
 
<pre>
#
+
#!/bin/bash
# Settings for layer-2 bridges
+
br=$1
#
+
eth=$2
# For each bridge interface declared in INTERFACES (in rc.conf), declare
+
dev=$3
# a bridge_${IF} variable that contains the real ethernet interfaces that
+
mtu=$4
# should be bridged together.
+
cd /usr/sbin/
#
+
# Then list the bridge interface name in the BRIDGE_INTERFACES array.
+
#
+
# example:
+
#
+
# in /etc/rc.conf:
+
#    eth0="eth0 up"
+
#    eth1="eth1 up"
+
#    br0="br0 192.168.0.2 netmask 255.255.255.0 up"
+
#    INTERFACES=(lo eth0 eth1 br0)
+
#
+
# in /etc/conf.d/bridges
+
#    bridge_br0="eth0 eth1"
+
#    BRIDGE_INTERFACES=(br0)
+
#
+
bridge_br0="tap0 eth0"
+
BRIDGE_INTERFACES=(br0)
+
</pre>
+
  
Now, if using the default network daemon, invoke your bridge in '''/etc/rc.conf'''
+
# only if you start dhcpcd and leave it
 +
#  running for eth
 +
#dhcpcd -k $eth
  
'''a)''' With DHCP:
+
# needed if script is run independently
<pre>
+
# but when run through openvpn
#...
+
# openvpn will do this automatically
tap0="tap0 up"
+
# could also use 'ip tuntap ..'
eth0="eth0 up"
+
#openvpn --mktun --dev $dev
br0="dhcp"
+
#alias example
+
br0_0="br0:0 192.168.3.252 netmask 255.255.255.0 up"
+
INTERFACES=(tap0 eth0 br0 br0_0)
+
#...
+
</pre>
+
  
'''b)''' With a fixed IP:
+
brctl addbr $br
<pre>
+
# set forwarding delay to 0
#...
+
# otherwise dhcp called below would timeout
eth0="eth0 0.0.0.0 promisc up"
+
brctl setfd $br 0
tap0="tap0 0.0.0.0 promisc up"
+
brctl addif $br $eth
br0="br0 <server IP> netmask <netmask> broadcast <broadcast IP>"
+
# order matters here.. right now there is only
 +
#  one mac in the bridge's table
 +
# if there were two.. there is no guarantee
 +
#  which would be passed to the dhcp server
 +
dhcpcd $br
 +
brctl addif $br $dev
  
INTERFACES=(eth0 tap0 br0)
+
ip link set $eth up promisc on mtu $mtu
 
+
ip link set $dev up promisc on mtu $mtu
gateway="default gw <gateway IP>"
+
#...
+
 
</pre>
 
</pre>
  
===Server===
+
/etc/openvpn/down
 
+
There are a few example server configurations located in '''/usr/share/openvpn/examples''' to look at.
+
 
+
Here is a server configuration that uses dhcp, and some features only available since 2.1.1, saved as '''/etc/openvpn/server.conf'''
+
 
+
{{Note|Setting '''multihome''' allows OpenVPN to listen on multiple interfaces with UDP but respond only on the one it first received a request from. Otherwise, if listening on multiple interfaces, OpenVPN may switch from one to the other during communication with a client, and clients would not accept packets originating from something other than the original endpoint. Lowering the value for '''script-security''' is necessary to write and invoke scripts that call external programs during OpenVPN operation}}
+
 
<pre>
 
<pre>
# /etc/openvpn/server.conf
+
#!/bin/bash
# 2009.12.31
+
br=$1
#
+
eth=$2
# address to bind to, instead of all available
+
cd /usr/sbin/
;local 192.168.3.252
+
# new features, as of v2.1.1
+
#can listen on multiple ips over udp
+
multihome
+
# needed to allow internally called scripts like up/down
+
#  to call external programs like ifup, etc
+
;script-security 2
+
  
# tcp might work better on certain "dev tun" setups
+
dhcpcd -k $br
#  but not for wrapping more tcp or further encrypted
+
#  streams, as that would be redundant, and very slow
+
# "port 1194" and "proto udp" are defaults
+
port 1194
+
proto udp
+
  
# could specify interface, like tap0 or tap1
+
ip link set $br down
#  or use up/down routing scripts to handle
+
brctl delbr $br
#  more than one, if needed
+
dev tap0
+
  
# simple scripts
+
# needed if script is run independently
# for adding/removing  to tap
+
# but when run through openvpn
;up "up.sh br0:0"
+
# openvpn will do this automatically
;down "down.sh br0:0"
+
#  could also use 'ip tuntap ..'
 +
#openvpn --rmtun --dev $dev
  
# identical certificate on server & client
+
# only if you start dhcpcd and leave it
ca config/keys/ca.crt
+
#  running for eth
 +
#dhcpcd $eth
 +
</pre>
  
# server's own cert/key
+
These examples are for using dhcp. If you are going to use static IP addresses, you will need to adjust accordingly.
cert config/keys/server.crt
+
key config/keys/server.key  # keep secret
+
  
# for certificate handshake
+
==Using Systemd==
dh config/keys/dh1024.pem
+
  
# no arguments will use this subnet's dhcp server
+
The OpenVPN systemd script looks for <name>.conf files in the /etc/openvpn folder by default. So assuming you have a file named server.conf:
#  not openvpn dynamic/static assigment
+
<pre>
# either way is good, but if you know you're not conflicting
+
systemctl enable openvpn@server
#  with any other IP addressing schemes on your subnet,
+
systemctl start openvpn@server
#  this is much faster
+
</pre>
# this directive expands to include "mode server" and "tls-server"
+
#  so including them elsewhere is redundant
+
;server-bridge 192.168.3.252 255.255.255.0 192.168.3.1 192.168.3.16
+
# like what dhcp does, reuses IPs
+
;ifconfig-pool-persist ipp.txt
+
  
# this one uses a dhcp server, server-side
+
Be careful about having dhcpcd enabled separately (ie. dhcpcd@eth0.service) at the same time. It is possible, though unlikely, for it to complete after OpenVPN and ruin your dhcp setup for OpenVPN. You could probably disable dhcpcd@eth0.service since you know openvpn@server.service will be resetting dhcp anyway.
#  potentially better for controlling ip addresses from one location
+
#  clients must support binding their dhcp client to their tap adapter
+
server-bridge nogw # 'nogw' is optional
+
  
# openvpn server routes client packets to each other itself
+
{{Warning| The Static Bridge section does not describe a method using systemd at all. In addition, it may contain outdated information. It should be revised at some point.}}
#  should happen anyway in 'dev tap' mode, but this saves time
+
client-to-client
+
  
# ping clients to auto close server side connection
+
==Static Bridge Installation==
keepalive 10 60
+
  
# 0 for server, 1 for client
+
The first thing you want to do is install OpenVPN, the Linux bridging utilities and [[netcfg]].
tls-auth config/keys/ta.key 0 # This file is secret
+
<pre>
 +
pacman -S openvpn bridge-utils netcfg
 +
</pre>
  
# cryptographic cipher.
+
==Static Bridge Configuration==
;cipher BF-CBC        # Blowfish (default)
+
cipher AES-128-CBC  # AES
+
  
# compression is useful for xfer of
+
Earlier versions of guides for OpenVPN provided by the OpenVPN team or various Linux packagers give example scripts for constructing a bridge when starting OpenVPN and destroying it when shutting OpenVPN down.
#  not already compressed files, like database
+
#  files, otherwise add needless overhead
+
# comp-lzo [mode] ; yes|no|adaptive, adaptive default
+
;comp-lzo
+
  
# not needed yet
+
However, this is a somewhat deprecated approach, since OpenVPN as of 2.1.1 defaults to not allowing itself to call external scripts or programs unless explicitly enabled to, for security reasons.
;max-clients 100
+
  
# drop root priveledges once connected
+
Also, constructing the bridge is relatively slow compared to all other parts of the network initialization process. (In fact, so slow that dhcpcd will time out before the bridge is ready. See [[#Troubleshooting]].) Also, when restarting OpenVPN after configuration changes, there is no reason to rebuild a working bridge, interrupting all your other network applications. So, setting up a static bridge configuration as follows is the recommended method.
#  good idea, for servers running on linux
+
user nobody
+
group nobody
+
  
# avoid accessing things you no longer can
+
To create an OpenVPN bridge for your server, you are going to have to use [[netcfg]] and create two network profiles - one for the tap interface and one for the bridge.
persist-key
+
persist-tun
+
 
+
# short status file showing current connections
+
#  rewritten every minute.
+
status openvpn-status.log
+
 
+
# use one or the other, useful for managing multiple
+
#  concurrent servers on a system
+
;log        openvpn.log
+
;log-append  openvpn.log
+
 
+
# 0 is silent, except for fatal errors
+
# 4 is reasonable for general usage
+
# 5 and 6 can help to debug connection problems
+
# 9 is extremely verbose
+
verb 3
+
 
+
# silence repeating messages past certain number, in log
+
;mute 20
+
</pre>
+
  
The following modules will be automatically loaded, but you could specify them by editing '''/etc/rc.conf'''
+
Go to /etc/network.d/. Then copy the tuntap example file to the directory.
 
<pre>
 
<pre>
#...
+
cd /etc/network.d/
MODULES=(... tun bridge ...)
+
cp examples/tuntap openvpn_tap
#...
+
 
</pre>
 
</pre>
  
Now, add the following daemons to '''/etc/rc.conf'''
+
Now edit openvpn_tap to create a tap interface. It may look like this.
{{Note|'''openvpn-tapdev''' must come before '''network'''}}
+
 
<pre>
 
<pre>
#...
+
INTERFACE='tap0'
DAEMONS=(... openvpn-tapdev network openvpn ...)
+
CONNECTION='tuntap'
#...
+
MODE='tap'
 +
USER='nobody'
 +
GROUP='nobody'
 
</pre>
 
</pre>
  
===Client===
+
Do not configure the IP address here, this is going to be done for the bridge interface!
  
The following is a matching '''client.ovpn''' for the options used in '''server.conf''' above, tested in Windows.
+
To create the bridge profile, copy the example file:
{{Note|Windows supports authenticating via a dhcp server located on the OpenVPN server's side automatically because of how the TCP stack works on Windows; a Linux client may take more steps to get dhcp to work}}
+
<pre>
+
# /%openvpn%/config/client.conf
+
# 2009.12.31
+
  
# defines order of certificate authentification
 
# this directive expands to "pull" "tls-client"
 
#  so including them elsewhere is redundant
 
client
 
 
# type of server
 
dev tap
 
 
# windows needs tap name, if more than one
 
;dev-node OpenVPN Bridge Connection
 
 
# remote <hostname> [port] [proto]
 
remote remote 1194 udp
 
 
# only works for peers using the "remote" option
 
# ok if the ip address for remote changes during session
 
float
 
# uses a random port client-side
 
nobind
 
 
# this is for laptops or internet conditions
 
#  where openvpn server hostname cannot be resolved easily,
 
#  or changes often, etc
 
# infinte is the default, or value for seconds
 
resolv-retry infinite
 
 
# public
 
ca keys/ca.crt
 
cert keys/satellite.crt
 
# private
 
key keys/satellite.key
 
# needed when specified in server
 
# 0 = server, 1 = client
 
tls-auth keys/ta.key 1
 
 
# verify that the server has certificate field "server"
 
# protects against certain attacks
 
ns-cert-type server
 
 
;cipher BF-CBC
 
cipher AES-128-CBC
 
 
# comp-lzo [mode] ; yes|no|adaptive, adaptive default
 
;comp-lzo
 
 
# try to preserve some states across restarts
 
persist-key
 
persist-tun
 
 
verb 3
 
</pre>
 
 
==Tips and Tricks==
 
 
{{Warning|This script doesn't always correctly report [FAIL] or [DONE] so output can't be relied upon and may be confusing. Also, it is very old and should probably be revisited and revised}}
 
 
You may not always want a static bridge, as there may be cases that you don't always want OpenVPN on, and when off you would prefer not having the bridge in place. In that case you have multiple options to achieve this. You could do this by replacing '''/etc/rc.d/openvpn''' with
 
 
<pre>
 
<pre>
#!/bin/bash
+
cp examples/bridge openvpn_bridge
 
+
# /etc/rc.d/openvpn
+
#
+
# An init script to start and stop OpenVPN daemons
+
 
+
. /etc/rc.conf
+
. /etc/rc.d/functions
+
 
+
openvpn_config_dir=/etc/openvpn
+
 
+
make_bridge ()
+
{
+
#echo "# mkbr $1"
+
# for example $1 = "br0" and
+
# $br0 = ("br0 192.168.2.1 netmask 255.255.255.0 broadcast 192.168.2.255" eth1)
+
eval brvar="(\"\${${1}[@]}\")"
+
brdev=$1
+
 
+
brctl addbr $brdev
+
add_to_bridge ${brvar[1]} $brdev
+
 
+
        ifconfig ${brvar[0]}
+
return $?
+
}
+
 
+
add_to_bridge ()
+
{
+
#echo "# addbr $1 $2"
+
# for example $1=tap0 and $2=br0
+
ifconfig $1 down >/dev/null 2>&1
+
brctl addif $2 $1
+
ifconfig $1 0.0.0.0 promisc up
+
}
+
 
+
destroy_bridge ()
+
{
+
eval brvar="(\"\${${1}[@]}\")"
+
brdev=$1
+
+
ifconfig $brdev down
+
brctl delbr $brdev
+
}
+
 
+
make_vpn ()
+
{
+
#echo  "# mkvpn $1"
+
# for example $1 = vpn0 and
+
# $vpn0 = ("default.conf" tap0 br0)
+
eval vpnvar="(\"\${${1}[@]}\")"
+
+
openvpn --mktun --dev ${vpnvar[1]} > /dev/null
+
if [ "${vpnvar[2]}" != "" ]; then
+
add_to_bridge ${vpnvar[1]} ${vpnvar[2]}
+
fi
+
 
+
openvpn --cd $openvpn_config_dir --daemon --config ${vpnvar[0]}
+
return $?
+
}
+
 
+
destroy_vpn ()
+
{
+
eval vpnvar="(\"\${${1}[@]}\")"
+
openvpn --rmtun --dev ${vpnvar[1]} > /dev/null
+
return $?
+
}
+
 
+
case "$1" in
+
        start)
+
        stat_busy "Starting OpenVPN daemons"
+
 
+
        # enable IP forwarding
+
        echo 1 > /proc/sys/net/ipv4/ip_forward
+
 
+
# create bridge(s)
+
error=0
+
      for brconf in ${BRIDGES[@]}; do
+
              if echo $brconf | grep '^[^\!]' >/dev/null 2>&1; then
+
                      make_bridge $brconf || error=1
+
              fi
+
      done
+
 
+
# create vpn(s)
+
      for vpnconf in ${VPNS[@]}; do
+
              if echo $vpnconf | grep '^[^\!]' >/dev/null 2>&1; then
+
                      make_vpn $vpnconf || error=1
+
              fi
+
      done
+
 
+
if [ $error -eq 0 ]; then
+
stat_done
+
else
+
stat_fail
+
fi
+
;;
+
        stop)
+
        stat_busy "Stopping OpenVPN daemons"
+
 
+
        killall `which openvpn` 2> /dev/null
+
 
+
# destroy bridge(s)
+
error=0
+
      for brconf in ${BRIDGES[@]}; do
+
              if echo $brconf | grep '^[^\!]' >/dev/null 2>&1; then
+
                      destroy_bridge $brconf || error=1
+
              fi
+
      done
+
 
+
# destroy vpn(s)
+
      for vpnconf in ${VPNS[@]}; do
+
              if echo $vpnconf | grep '^[^\!]' >/dev/null 2>&1; then
+
                      destroy_vpn $vpnconf || error=1
+
              fi
+
      done
+
 
+
if [ $error -eq 0 ]; then
+
stat_done
+
else
+
stat_fail
+
fi
+
;;
+
        restart)
+
                $0 stop
+
sleep 1
+
                $0 start
+
                ;;
+
        *)
+
                echo $"Usage: $0 {start|stop|restart}"
+
                RETVAL=1
+
esac
+
 
</pre>
 
</pre>
  
and then make the script executable.
+
Now edit openvpn_bridge. It may look like this:
 
<pre>
 
<pre>
chmod 755 /etc/rc.d/openvpn
+
INTERFACE="br0"
 +
CONNECTION="bridge"
 +
DESCRIPTION="OpenVPN Bridge"
 +
BRIDGE_INTERFACES="eth0 tap0"
 +
IP='static'
 +
ADDR='192.168.11.1'
 +
GATEWAY='192.168.11.254'
 +
DNS=('192.168.11.254')
 
</pre>
 
</pre>
 +
For more information, for example how to use DHCP instead, check the [[netcfg]] article.
  
Or, you can create scripts OpenVPN can use with '''up''' and '''down''' options to create and destroy tap interfaces dynamically, rather than using the provided openvpn-tapdev daemon, to add more options when adding it to a bridge. Here are some generic ones; make sure they are executable.
+
Now set the NETWORKS array in /etc/conf.d/netcfg (order is important!):
  
'''up.sh'''
 
 
<pre>
 
<pre>
#!/bin/bash -e
+
NETWORKS=(openvpn_tap openvpn_bridge)
 
+
BR=$1
+
DEV=$2
+
MTU=$3
+
 
+
/sbin/ifconfig $DEV mtu $MTU promisc up
+
/usr/sbin/brctl addif $BR $DEV
+
 
+
exit 0
+
 
</pre>
 
</pre>
  
'''down.sh'''
+
Then add net-profiles to your [[Rc.conf#Daemons|DAEMONS array]] (net-profiles must be before openvpn!):
 
<pre>
 
<pre>
#!/bin/bash -e
+
DAEMONS=(... net-profiles openvpn ...)
 
+
BR=$1
+
DEV=$2
+
 
+
/usr/sbin/brctl delif $BR $DEV
+
/sbin/ifconfig $DEV down
+
 
+
exit 0
+
 
</pre>
 
</pre>
  
==Troubleshooting==
+
==Static Bridge Troubleshooting==
  
 
Q: Why does starting the network [FAIL] ?
 
Q: Why does starting the network [FAIL] ?
  
A: If you followed the server.conf example above, it is because you are using dhcp on the bridge and setting up the bridge takes longer than dhcpcd is willing to wait. You can fix this by dropping the forwarding delay when adding the bridge to a lower number by adding the following line to '''/etc/rc.d/network'''
+
A:This is probably because you are using DHCP on the bridge and setting up the bridge takes longer than dhcpcd is willing to wait. You can fix this by setting the FWD_DELAY parameter in your bridge network profile (openvpn_bridge). Start with a value of 5 and decrease it until it works.
<pre>
+
.. /usr/sbin/brctl addbr $br
+
+  /usr/sbin/brctl setfd $br 5
+
.. eval brifs="\$bridge_${br}"
+
</pre>
+
  
 
==More Resources==
 
==More Resources==

Revision as of 12:32, 27 October 2012

This page describes how to create a network bridge on Arch Linux and host an OpenVPN server using a IP layer-2 based Ethernet bridge (TAP) rather than a IP layer-3 based IP tunnel (TUN). The general OpenVPN page describes setting up PAM authentication or OpenSSL security certificates in more detail.

Introduction

The OpenVPN documentation page gives a full overview of server-side and client-side options that OpenVPN supports. It is easier to set up OpenVPN in tunneling mode and control routing the traffic and it is generally advised to do so if it serves your purpose. However, some network applications, such as Windows file sharing, rely on network broadcasts at the Ethernet level and benefit from believing they are physically located on the same subnet, and software bridging serves this purpose.

There are multiple ways to set bridging up. The dynamic method is where OpenVPN will be managing its own bridge on the system and will start, stop and configure it itself. This is the quickest way to set bridging up, although it interrupts other network services when OpenVPN starts and stops. If the system is going to manage its own bridge, maybe because other virtual network adapters connect to the bridge besides just that of OpenVPN, then it is preferable to use the static method.

Dynamic Bridge Installation

You will need to install OpenVPN and Linux bridging utilities

pacman -S openvpn bridge-utils

Dynamic Bridge Configuration

OpenVPN will create/destroy the TAP device automatically for the name specified in the config file. OpenVPN settings common to TUN or TAP are not shown in the example config file below, only settings that affect TAP mode. Make sure the 'up' and 'down' scripts are executable with 'chmod +x' after you write them.

/etc/openvpn/server.conf (sections common to TUN and TAP omitted)

# this uses a dhcp server, server-side
#  clients must support binding their dhcp client to their tap adapter
# do not append 'nogw' if using dhcp
server-bridge
# can specify interface, like tap0 or tap1
#  or use up/down routing scripts to handle
#  more than one, if needed
dev tap0
# needed to call scripts like up/down
#  which call external programs within the scripts
script-security 2
# user defined scripts for adding/removing tap to bridge
#  'dev mtu link_mtu ifconfig_local_ip ifconfig_remote_ip' are appended if set
# make sure 'user' has permission to run 'down' ('up' will be root)
up "up br0 eth0"
down "down br0 eth0"
# call 'down' before TUN/TAP close
down-pre
# drop root priveledges once connected
#  good idea, for servers running on linux
# 'up' script not affected, 'down' script is
;user nobody
;group nobody

/etc/openvpn/up

#!/bin/bash
br=$1
eth=$2
dev=$3
mtu=$4
cd /usr/sbin/

# only if you start dhcpcd and leave it
#  running for eth
#dhcpcd -k $eth

# needed if script is run independently
# but when run through openvpn
# openvpn will do this automatically
#  could also use 'ip tuntap ..'
#openvpn --mktun --dev $dev

brctl addbr $br
# set forwarding delay to 0
#  otherwise dhcp called below would timeout
brctl setfd $br 0
brctl addif $br $eth
# order matters here.. right now there is only
#  one mac in the bridge's table
# if there were two.. there is no guarantee
#  which would be passed to the dhcp server
dhcpcd $br
brctl addif $br $dev

ip link set $eth up promisc on mtu $mtu
ip link set $dev up promisc on mtu $mtu

/etc/openvpn/down

#!/bin/bash
br=$1
eth=$2
cd /usr/sbin/

dhcpcd -k $br

ip link set $br down
brctl delbr $br

# needed if script is run independently
# but when run through openvpn
# openvpn will do this automatically
#  could also use 'ip tuntap ..'
#openvpn --rmtun --dev $dev

# only if you start dhcpcd and leave it
#  running for eth
#dhcpcd $eth

These examples are for using dhcp. If you are going to use static IP addresses, you will need to adjust accordingly.

Using Systemd

The OpenVPN systemd script looks for <name>.conf files in the /etc/openvpn folder by default. So assuming you have a file named server.conf:

systemctl enable openvpn@server
systemctl start openvpn@server

Be careful about having dhcpcd enabled separately (ie. dhcpcd@eth0.service) at the same time. It is possible, though unlikely, for it to complete after OpenVPN and ruin your dhcp setup for OpenVPN. You could probably disable dhcpcd@eth0.service since you know openvpn@server.service will be resetting dhcp anyway.

Warning: The Static Bridge section does not describe a method using systemd at all. In addition, it may contain outdated information. It should be revised at some point.

Static Bridge Installation

The first thing you want to do is install OpenVPN, the Linux bridging utilities and netcfg.

pacman -S openvpn bridge-utils netcfg

Static Bridge Configuration

Earlier versions of guides for OpenVPN provided by the OpenVPN team or various Linux packagers give example scripts for constructing a bridge when starting OpenVPN and destroying it when shutting OpenVPN down.

However, this is a somewhat deprecated approach, since OpenVPN as of 2.1.1 defaults to not allowing itself to call external scripts or programs unless explicitly enabled to, for security reasons.

Also, constructing the bridge is relatively slow compared to all other parts of the network initialization process. (In fact, so slow that dhcpcd will time out before the bridge is ready. See #Troubleshooting.) Also, when restarting OpenVPN after configuration changes, there is no reason to rebuild a working bridge, interrupting all your other network applications. So, setting up a static bridge configuration as follows is the recommended method.

To create an OpenVPN bridge for your server, you are going to have to use netcfg and create two network profiles - one for the tap interface and one for the bridge.

Go to /etc/network.d/. Then copy the tuntap example file to the directory.

cd /etc/network.d/
cp examples/tuntap openvpn_tap

Now edit openvpn_tap to create a tap interface. It may look like this.

INTERFACE='tap0'
CONNECTION='tuntap'
MODE='tap'
USER='nobody'
GROUP='nobody'

Do not configure the IP address here, this is going to be done for the bridge interface!

To create the bridge profile, copy the example file:

cp examples/bridge openvpn_bridge

Now edit openvpn_bridge. It may look like this:

INTERFACE="br0"
CONNECTION="bridge"
DESCRIPTION="OpenVPN Bridge"
BRIDGE_INTERFACES="eth0 tap0"
IP='static'
ADDR='192.168.11.1'
GATEWAY='192.168.11.254'
DNS=('192.168.11.254')

For more information, for example how to use DHCP instead, check the netcfg article.

Now set the NETWORKS array in /etc/conf.d/netcfg (order is important!):

NETWORKS=(openvpn_tap openvpn_bridge)

Then add net-profiles to your DAEMONS array (net-profiles must be before openvpn!):

DAEMONS=(... net-profiles openvpn ...)

Static Bridge Troubleshooting

Q: Why does starting the network [FAIL] ?

A:This is probably because you are using DHCP on the bridge and setting up the bridge takes longer than dhcpcd is willing to wait. You can fix this by setting the FWD_DELAY parameter in your bridge network profile (openvpn_bridge). Start with a value of 5 and decrease it until it works.

More Resources

OpenVPN | General page on configuring OpenVPN, including setting up authentication methods.


Any additions, clarifications, reorganizations, feedback etc. etc. are more than appreciated.