Bridge

2014-04-30T09:39:43Z

Pierrep: adding info specific to dom0 bridging

[[Category:Networking]]
This document describes how to configure a [http://en.wikipedia.org/wiki/Bridging_%28networking%29 network bridge] interface in Alpine Linux.

== Using brctl ==
Bridges are manually managed with the '''brctl''' command.
<pre>
Usage: brctl COMMAND [BRIDGE [INTERFACE]]

Manage ethernet bridges

Commands:
show Show a list of bridges
addbr BRIDGE Create BRIDGE
delbr BRIDGE Delete BRIDGE
addif BRIDGE IFACE Add IFACE to BRIDGE
delif BRIDGE IFACE Delete IFACE from BRIDGE
setageing BRIDGE TIME Set ageing time
setfd BRIDGE TIME Set bridge forward delay
sethello BRIDGE TIME Set hello time
setmaxage BRIDGE TIME Set max message age
setpathcost BRIDGE COST Set path cost
setportprio BRIDGE PRIO Set port priority
setbridgeprio BRIDGE PRIO Set bridge priority
stp BRIDGE [1|0] STP on/off
</pre>

To manually create a bridge interface br0:
{{Cmd|brctl addbr br0}}

To add interface eth0 and eth1 to the bridge br0:
{{Cmd|brctl addif br0 eth0
brctl addif br0 eth1}}

Note that you need to set the link status to ''up'' on the added interfaces.
{{Cmd|ip link set dev eth0 up
ip link set dev eth1 up}}

[[Category:Networking]]

== Configuration file ==
{{Note|Alpine Linux v2.4 or newer is required for this}}
Install the scripts that configures the bridge.
{{Cmd|apk add bridge}}

Bridging is then configured in ''/etc/network/interfaces'' with the ''bridge-ports'' keyword.
Note that you normally don't assign ip addresses to the bridged interfaces (eth0 and eth1 in our example) but to the bridge itself (br0).

In this example the address 192.168.0.1/24 is used.

<pre>
auto br0
iface br0 inet static
bridge-ports eth0 eth1
bridge-stp 0
address 192.168.0.1
netmask 255.255.255.0
</pre>

You can set the various options with those keywords:
; bridge-aging
: Set ageing time
; bridge-fd
: Set bridge forward delay
; bridge-hello
: Set hello time
; bridge-maxage
: Set bridge max message age
; bridge-pathcost
: Set path cost
; bridge-portprio
: Set port priority
; bridge-bridgeprio
: Set bridge priority
; bridge-stp
: STP on/off

== Using pre-up/post-down ==
For older versions of Alpine Linux, or if you want be able to control the bridge interfaces individually, you need to use pre-up/post-down hooks.

Example ''/etc/network/interfaces'':

<pre>
auto br0
iface br0 inet static
pre-up brctl addbr br0
pre-up echo 0 > /proc/sys/net/bridge/bridge-nf-call-arptables
pre-up echo 0 > /proc/sys/net/bridge/bridge-nf-call-iptables
pre-up echo 0 > /proc/sys/net/bridge/bridge-nf-call-ip6tables
address 192.168.0.253
netmask 255.255.255.0
gateway 192.168.0.254
post-down brctl delbr br0

auto eth0
iface eth0 inet manual
up ip link set $IFACE up
up brctl addif br0 $IFACE
down brctl delif br0 $IFACE || true
down ip link set $IFACE down

auto eth1
iface eth1 inet manual
up ip link set $IFACE up
up brctl addif br0 $IFACE
down brctl delif br0 $IFACE || true
down ip link set $IFACE down
</pre>

That way you create br0 with: ifup br0, and you can add/remove
individual interfaces to the bridge with ifup eth0, ifdown eth0.

== Bridging for a Xen dom0 ==
Bridging in a dom0 is a bit specific as it consists in bridging a real interface (i.e. ethX) with a virtual interface (i.e. vifX.Y).
At bridge creation time, the virtual interface does not exist and will be added by the Xen toolstack when a domU is booting (see Xen documentation on how to link the virtual interface to the correct bridge).

;Particularities :
- the bridge consists of a single physical interface <br/>
- the physical interface does not have an IP and is configured as manual <br/>
- the bridge will have the IP and will be auto, resulting in bringing up the physical interface <br/>

This translates to this sample config :

Example ''/etc/network/interfaces'':

<pre>
auto eth0
iface eth0 inet manual

auto br0
iface br0 inet static
address 192.168.0.253
netmask 255.255.255.0
gateway 192.168.0.254
bridge_port eth0
bridge_stp 0
</pre>

After the domU OS is started, the virtual interface wil be added and the working bridge can be checked with
<pre>
brctl show

ifconfig -a
</pre>

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Bridge