Let's take a real example once again, I have 2 (actually 3, about time I returned them) cable modems, connected to a Linux NAT ('masquerading') router. People living here pay me to use the Internet. Suppose one of my house mates only visits hotmail and wants to pay less. This is fine with me, but they'll end up using the low-end cable modem.
The 'fast' cable modem is known as 22.214.171.124 and is a PPP link to 126.96.36.199. The 'slow' cable modem is known by various ip addresses, 188.8.131.52 in this example and is a link to 184.108.40.206.
The local table:
[ahu@home ahu]$ ip route list table local broadcast 127.255.255.255 dev lo proto kernel scope link src 127.0.0.1 local 10.0.0.1 dev eth0 proto kernel scope host src 10.0.0.1 broadcast 10.0.0.0 dev eth0 proto kernel scope link src 10.0.0.1 local 220.127.116.11 dev ppp0 proto kernel scope host src 18.104.22.168 broadcast 10.255.255.255 dev eth0 proto kernel scope link src 10.0.0.1 broadcast 127.0.0.0 dev lo proto kernel scope link src 127.0.0.1 local 22.214.171.124 dev ppp2 proto kernel scope host src 126.96.36.199 local 127.0.0.1 dev lo proto kernel scope host src 127.0.0.1 local 127.0.0.0/8 dev lo proto kernel scope host src 127.0.0.1
Lots of obvious things, but things that need to be specified somewhere. Well, here they are. The default table is empty.
Let's view the 'main' table:
[ahu@home ahu]$ ip route list table main 188.8.131.52 dev ppp2 proto kernel scope link src 184.108.40.206 220.127.116.11 dev ppp0 proto kernel scope link src 18.104.22.168 10.0.0.0/8 dev eth0 proto kernel scope link src 10.0.0.1 127.0.0.0/8 dev lo scope link default via 22.214.171.124 dev ppp0
We now generate a new rule which we call 'John', for our hypothetical house mate. Although we can work with pure numbers, it's far easier if we add our tables to /etc/iproute2/rt_tables.
# echo 200 John >> /etc/iproute2/rt_tables # ip rule add from 10.0.0.10 table John # ip rule ls 0: from all lookup local 32765: from 10.0.0.10 lookup John 32766: from all lookup main 32767: from all lookup default
Now all that is left is to generate John's table, and flush the route cache:
# ip route add default via 126.96.36.199 dev ppp2 table John # ip route flush cache
And we are done. It is left as an exercise for the reader to implement this in ip-up.