Setting up bridged networking for Xen on CentOS 7

WARNING: Before changing networking, be sure that you have non-network access to the target system in case something goes wrong.

First, find your main network device:

ip route show | grep 'default' | awk '{print $5}'

This will normally be eth0. The rest of this guide will assume you want to make a bridge named xenbr0 and set eth0 as its slave.

First, create xenbr0, giving it reasonable defaults:

nmcli con add type bridge con-name xenbr0 ifname xenbr0
nmcli con modify xenbr0 bridge.stp no
nmcli con modify xenbr0 bridge.hello-time 0

Then find out the connection name for eth0. This will normally be "System eth0":

# nmcli con show
NAME         UUID                                  TYPE            DEVICE 
System eth0  1619a4a0-518e-4547-9ade-31dff5ef8f5e  802-3-ethernet  eth0   

Finally, set "System eth0" as a slave to xenbr0:

nmcli con modify "System eth0" connection.master xenbr0 connection.slave-type bridge

And restart networking:

systemctl restart network

Check to see if it worked:

# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast master xenbr0 state UP qlen 1000
    link/ether 30:5b:d6:f1:d6:a2 brd ff:ff:ff:ff:ff:ff
4: xenbr0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP 
    link/ether 30:5b:d6:f1:d6:a2 brd ff:ff:ff:ff:ff:ff
    inet 10.80.237.209/20 brd 10.80.239.255 scope global dynamic xenbr0
       valid_lft 1798sec preferred_lft 1798sec
    inet6 fe80::325b:d6ff:fef1:d6a2/64 scope link 
       valid_lft forever preferred_lft forever