Hackery

Simple dual upstream gateways in CentOS

Fri 23 May 2008

Update 2019-05-05: see this revised post for a simpler implementation method and a gotcha to watch out for. HT to Jim MacLeod for suggested improvements in his comments below.

Had to setup some simple policy-based routing on CentOS again recently, and had forgotten the exact steps. So here's the simplest recipe for CentOS that seems to work. This assumes you have two upstream gateways (gw1 and gw2), and that your default route is gw1, so all you're trying to do is have packets that come in on gw2 go back out gw2.

1) Define an extra routing table e.g.

$ cat /etc/iproute2/rt_tables
#
# reserved values
#
255     local
254     main
253     default
0       unspec
#
# local tables
#
102     gw2
#

2) Add a default route via gw2 (here 172.16.2.254) to table gw2 on the appropriate interface (here eth1) e.g.

$ cat /etc/sysconfig/network-scripts/route-eth1
default table gw2 via 172.16.2.254

3) Add an ifup-local script to add a rule to use table gw2 for eth1 packets e.g.

$ cat /etc/sysconfig/network-scripts/ifup-local
#!/bin/bash
#
# Script to add/delete routing rules for gw2 devices
#

GW2_DEVICE=eth1
GW2_LOCAL_ADDR=172.16.2.1

if [ $(basename $0) = ifdown-local ]; then
  OP=del
else
  OP=add
fi

if [ "$1" = "$GW2_DEVICE" ]; then
  ip rule $OP from $GW2_LOCAL_ADDR table gw2
fi

4) Use the ifup-local script also as ifdown-local, to remove that rule

$ cd /etc/sysconfig/network-scripts
$ ln -s ifup-local ifdown-local

5) Restart networking, and you're done!

# service network restart

For more, see: