Ethernet Bridging
Ethernet bridging combines an ethernet interface with one or more virtual TAP interfaces under a single bridge interface.
Ethernet bridging essentially involves combining an ethernet interface with one or more virtual TAP interfaces and bridging them under the umbrella of a single bridge interface. Ethernet bridges represent the software analog to a physical ethernet switch. The ethernet bridge can be considered a kind of software switch that can be used to connect multiple ethernet interfaces (physical or virtual) on a single machine while sharing a single IP subnet.
By bridging a physical ethernet NIC with an OpenVPN-driven TAP interface at two separate locations, it is possible to logically merge both ethernet networks as if they were a single ethernet subnet.
Bridging setup
This example will guide you in configuring an OpenVPN server-side ethernet bridge. Multiple clients can connect to the bridge, and each client's TAP interface will be assigned an IP address that is part of the server's LAN.
There are two methods for handling client IP address allocation:
Let OpenVPN manage its own client IP address pool using the server-bridge directive.
Configure the DHCP server on the LAN to also grant IP address leases to VPN clients.
In this example, we will use the first method, where the OpenVPN server manages its own IP address pool on the LAN subnet, separate from the pool used by the DHCP server (if one exists).
For our example, we will use these bridge settings:
Setting | bridge-start parameter | Value |
|---|---|---|
Ethernet Interface | eth | eth0 |
Local IP Address | ip | 192.168.8.4 |
Local Netmask | eth_netmask | 255.255.255.0 |
Local Broadcast Address | eth_broadcast | 192.168.8.255 |
VPN client address pool | 192.168.8.128 to 192.168.8.254 | |
Virtual Bridge Interface | br | br0 |
Virtual TAP Interface | tap | tap0 |
The first step is to follow the How To up to the "Starting up the VPN and testing for initial connectivity" section. Next, proceed below according to whether you are setting up the bridge on Linux or Windows.
Bridge server on Linux
First, make sure you have the bridge-utils package installed.
Edit the bridge-start script below. Set the br, tap, eth, eth_ip, eth_netmask, and eth_broadcast parameters according to the physical ethernet interface you want to bridge. Make sure to use a private interface connected to a LAN protected from the internet by a firewall. You can use the Linux ifconfig command to get the necessary information about your network interfaces to fill in the bridge-start parameters.
Now run the bridge-start script. It will create a persistent tap0 interface and bridge it with the active ethernet interface.
Next, we will edit the OpenVPN server configuration file to enable a bridging configuration.
Comment out the line that says dev tun and replace it instead with:
dev tap0
Comment out the line that begins with server and replace it with:
server-bridge 192.168.8.4 255.255.255.0 192.168.8.128 192.168.8.254
Now set up the Linux firewall to permit packets to flow freely over the newly created tap0 and br0 interfaces:
iptables -A INPUT -i tap0 -j ACCEPT iptables -A INPUT -i br0 -j ACCEPT iptables -A FORWARD -i br0 -j ACCEPT
The OpenVPN bridge can now be started and stopped using this sequence:
Run bridge-start.
Run openvpn.
Stop openvpn.
Run bridge-stop.
At this point, the bridging-specific aspects of the configuration are complete, and you can continue where you left off in the HOWTO.
Bridge server on Windows XP
This configuration requires Windows XP or higher on the bridge side. To my knowledge, Windows 2000 does not support bridging. However, a Windows 2000 machine can be a client on a bridged network, where the other end of the OpenVPN connection where the bridging is occurring is a Linux or Windows XP machine.
When OpenVPN is installed on Windows, it automatically creates a single TAP-Win32 adapter, which will be assigned a name like "Local Area Connection 2". Go to the Network Connections control panel and rename it "tap-bridge".
Next, select tap-bridge and your ethernet adapter with the mouse, right-click, and select Bridge Connections. This will create a new bridge adapter icon in the control panel.
Set the TCP/IP properties on the bridge adapter to an IP of 192.168.8.4 and a subnet mask of 255.255.255.0.
Next, edit the OpenVPN server configuration file to enable a bridging configuration.
Comment out the line which says dev tun and replace it instead with:
dev tap dev-node tap-bridge
Comment out the line that begins with server and replace it with:
server-bridge 192.168.8.4 255.255.255.0 192.168.8.128 192.168.8.254
If you're running XP SP2, go to the firewall control panel and disable firewall filtering on the bridge and TAP adapters.
At this point, the bridging-specific aspects of the configuration are complete, and you can continue where you left off in the HOWTO.
Bridge client configuration
Use the sample OpenVPN client configuration as a starting point. Comment out the line that says dev tun and replace it instead with:
dev tap
Finally, ensure that the client configuration file is consistent with the directives used in the server configuration. The major thing to check for is that the proto (udp or tcp) directives are consistent. Also, make sure that comp-lzo and fragment, if used, are present in both client and server config files.
Ethernet bridging notes
When using an ethernet bridging configuration, the first step is to construct the ethernet bridge — a virtual network interface container for other ethernet interfaces, either real as in physical NICs or virtual as in TAP interfaces. The ethernet bridge interface must be set up before OpenVPN is actually started.
There is no portable method for generating an ethernet bridge interface — each OS has its own method (see below for examples).
OpenVPN may be started once the bridge interface has been created and appropriate ethernet interfaces have been added to it.
A bridge interface is a virtual network interface formed by combining one or more ethernet interfaces, each of which may be a physical NIC or a virtual TAP interface used for VPN tunneling.
When you set up an ethernet bridge, you should manually set the IP address and subnet of the bridge interface and not use an ifconfig directive in the OpenVPN config. This is because, unlike a TUN/TAP interface, OpenVPN cannot programmatically set a bridge interface's IP address and netmask.
The OpenVPN config should specify the TAP interface component of the bridge interface in its dev directive, not the name of the bridge interface itself.
On Windows, use the dev-node directive to name the TAP-Win32 adapter added to the bridge (the dev-node name refers to the adapter name shown in the Network Connections panel).
On Linux/BSD/Unix, for the dev tap directive, use the explicit TUN/TAP unit number that you added to the bridge, such as dev tap0.
If you are running OpenVPN in point-to-point mode, omit an ifconfig directive, and if you are using client/server mode, use the server-bridge directive on the server.
You must manually set the TCP/IP settings on the bridge interface when bridging. For example, this can be done on Linux with an ifconfig command. In contrast, it can be done on Windows XP by setting the TCP/IP properties of the bridge interface in the Network Connections panel (the Network Connections panel on Windows XP and higher allows for point-and-click bridging).
Ensure that only TAP interfaces are bridged with private ethernet interfaces protected behind a firewall. Never bridge a TAP interface with the same ethernet interface you use to connect to the internet, as that would create a potential security hole.
The addresses used for local and remote should not be part of the bridged subnet — otherwise, you will end up with a routing loop.
An important point to understand with Ethernet bridging is that each network interface added to the bridge will lose its individual identity in terms of specific settings such as IP address and netmask. Only the TCP/IP settings of the bridge interface itself will be relevant.
A common mistake people make when manually configuring an Ethernet bridge is adding their primary ethernet adapter to the bridge before setting the IP and netmask of the bridge interface. The result is that the primary ethernet interface "loses" its settings. Still, the equivalent bridge interface settings have not yet been defined, so the net effect is a loss of connectivity on the ethernet interface.
In most cases, it is possible to set up a usable bridge configuration with the ethernet bridge only configured on the server side, not the client side. If this is done, the client machines will become multi-homed when they connect to the server, i.e., they will still have their regular ethernet interface, but upon connection to the OpenVPN server, they will now have a new TAP interface bridged with the server's ethernet interface (and possibly all of the TAP interfaces of other connecting clients as well if the client-to-client directive is used on the server).
Notes — Ethernet bridging on Windows
The Windows Notes page has additional information on ethernet bridging.
Notes — Ethernet bridging on Linux
The scripts below will handle bridge setup and shutdown on Linux. They are available in the sample-scripts subdirectory of the OpenVPN tarball.
Sample scripts
#!/bin/bash
#################################
# Set up Ethernet bridge on Linux
# Requires: bridge-utils
#################################
# Define Bridge Interface
br="br0"
# Define list of TAP interfaces to be bridged,
# for example tap="tap0 tap1 tap2".
tap="tap0"
# Define physical ethernet interface to be bridged
# with TAP interface(s) above.
eth="eth0"
eth_ip="192.168.8.4"
eth_netmask="255.255.255.0"
eth_broadcast="192.168.8.255"
for t in $tap; do
openvpn --mktun --dev $t
done
brctl addbr $br
brctl addif $br $eth
for t in $tap; do
brctl addif $br $t
done
for t in $tap; do
ifconfig $t 0.0.0.0 promisc up
done
ifconfig $eth 0.0.0.0 promisc up
ifconfig $br $eth_ip netmask $eth_netmask broadcast $eth_broadcast#!/bin/bash
####################################
# Tear Down Ethernet bridge on Linux
####################################
# Define Bridge Interface
br="br0"
# Define list of TAP interfaces to be bridged together
tap="tap0"
ifconfig $br down
brctl delbr $br
for t in $tap; do
openvpn --rmtun --dev $t
done