Network bridge
A bridge is a piece of software used to unite two or more network segments. A bridge behaves like a virtual network switch, working transparently (the other machines do not need to know about its existence). Any real devices (e.g. eth0) and virtual devices (e.g. tap0) can be connected to it.
This article explains how to create a bridge that contains at least an ethernet device. This is useful for things like the bridge mode of QEMU, setting a software based access point, etc.
Creating a bridge
There are a number of ways to create a bridge.
With iproute2
This section describes the management of a network bridge using the ip tool from the iproute2 package, which is required by the base meta package.
Create a new bridge and change its state to up:
# ip link add name bridge_name type bridge # ip link set bridge_name up
To add an interface (e.g. eth0) into the bridge, its state must be up:
# ip link set eth0 up
Adding the interface into the bridge is done by setting its master to bridge_name:
# ip link set eth0 master bridge_name
To show the existing bridges and associated interfaces, use the bridge utility (also part of iproute2). See bridge(8) for details.
# bridge link
This is how to remove an interface from a bridge:
# ip link set eth0 nomaster
The interface will still be up, so you may also want to bring it down:
# ip link set eth0 down
To delete a bridge issue the following command:
# ip link delete bridge_name type bridge
This will automatically remove all interfaces from the bridge. The slave interfaces will still be up, though, so you may also want to bring them down after.
With bridge-utils
This section describes the management of a network bridge using the legacy brctl tool from the bridge-utils package, which is available in the official repositories. See brctl(8) for full listing of options.
Create a new bridge:
# brctl addbr bridge_name
Add a device to a bridge, for example eth0:
# brctl addif bridge_name eth0
Show current bridges and what interfaces they are connected to:
$ brctl show
Set the bridge device up:
# ip link set dev bridge_name up
Delete a bridge, you need to first set it to down:
# ip link set dev bridge_name down # brctl delbr bridge_name
br_netfilter module:
# modprobe br_netfilter
See also Kernel modules#Automatic module loading with systemd.
With netctl
See Bridge with netctl.
With systemd-networkd
See systemd-networkd#Bridge interface.
With NetworkManager
GNOME's Network settings can create bridges, but currently will not auto-connect to them or slave/attached interfaces. Open Network Settings, add a new interface of type Bridge, add a new bridged connection, and select the MAC address of the device to attach to the bridge.
KDE's plasma-nm can create bridges. In order to view, create and modify bridge interfaces open the Connections window either by right clicking the Networks applet in the system tray and selecting Configure Network Connections... or from System Settings > Connections. Click the Configuration button in the lower left corner of the module and enable "Show virtual connections". A session restart will be necessary to use the enabled functionality.
nm-connection-editor can create bridges in the same manner as GNOME's Network settings.
nmcli from networkmanager can create bridges. Creating a bridge with STP disabled (to avoid the bridge being advertised on the network):
$ nmcli connection add type bridge ifname br0 stp no
Making interface enp30s0 a slave to the bridge:
$ nmcli connection add type bridge-slave ifname enp30s0 master br0
Setting the existing connection as down (you can get it with nmcli connection show --active):
$ nmcli connection down Connection
Setting the new bridge as up:
$ nmcli connection up bridge-br0
If NetworkManager's default interface for the device you added to the bridge connects automatically, you may want to disable that by clicking the gear next to it in Network Settings, and unchecking "Connect automatically" under "Identity."
Assigning an IP address
When the bridge is fully set up, it can be assigned an IP address:
With iproute2
# ip address add dev bridge_name 192.168.66.66/24
With NetworkManager
Give it the desired address:
# nmcli connection modify interface ipv4.addresses desired_IP
Set up a DNS server (this will also avoid not being able to load any pages after you apply the changes):
# nmcli connection modify interface ipv4.dns DNS_server
Set the IP address to static:
# nmcli connection modify interface ipv4.method manual
Apply the changes:
# nmcli connection up interface
Tips and tricks
Wireless interface on a bridge
To add a wireless interface to a bridge, you first have to assign the wireless interface to an access point or start an access point with hostapd. Otherwise the wireless interface will not be added to the bridge.
See also Debian:BridgeNetworkConnections#Bridging with a wireless NIC.
Speeding up traffic destinated to the bridge itself
In some situations the bridge not only serves as a bridge box, but also talks to other hosts. Packets that arrive on a bridge port and that are destinated to the bridge box itself will by default enter the iptables INPUT chain with the logical bridge port as input device. These packets will be queued twice by the network code, the first time they are queued after they are received by the network device. The second time after the bridge code examined the destination MAC address and determined it was a locally destinated packet and therefore decided to pass the frame up to the higher protocol stack.[1]
The way to let locally destinated packets be queued only once is by brouting them in the BROUTING chain of the broute table. Suppose br0 has an IP address and that br0's bridge ports do not have an IP address. Using the following rule should make all locally directed traffic be queued only once:
# ebtables -t broute -A BROUTING -d $MAC_OF_BR0 -p ipv4 -j redirect --redirect-target DROP
The replies from the bridge will be sent out through the br0 device (assuming your routing table is correct and sends all traffic through br0), so everything keeps working neatly, without the performance loss caused by the packet being queued twice.
The redirect target is needed because the MAC address of the bridge port is not necessarily equal to the MAC address of the bridge device. The packets destinated to the bridge box will have a destination MAC address equal to that of the bridge br0, so that destination address must be changed to that of the bridge port.
Troubleshooting
No networking after bridge configuration
It may help to remove all IP addresses and routes from the interface (e.g. eth0) that was added to the bridge and configure these parameters for the bridge instead.
First of all, make sure there is no dhcpcd instance running for eth0, otherwise the deleted addresses may be reassigned.
Remove address and route from the eth0 interface:
# ip addr del address dev eth0 # ip route del address dev eth0
Now IP address and route for the earlier configured bridge must be set. This is usually done by starting a DHCP client for this interface. Otherwise, consult Network configuration for manual configuration.
No networking on hosted servers after bridge configuration
As the MAC address of the bridge is not necessarily equal to the MAC address of the networking card usually used by the server, the server provider might drop traffic coming out from the bridge, resulting in a loss of connectivity when bridging e.g. the server ethernet interface. Configuring the bridge to clone the mac address of the ethernet interface might therefore be needed for hosted servers.