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If you have a large router, you may well cater for the needs of different people, who should be served differently. The routing policy database allows you to do this by having multiple sets of routing tables.
If you want to use this feature, make sure that your kernel is compiled with the "IP: advanced router" and "IP: policy routing" features.
When the kernel needs to make a routing decision, it finds out which table needs to be consulted. By default, there are three tables. The old 'route' tool modifies the main and local tables, as does the ip tool (by default).
The default rules:
This lists the priority of all rules. We see that all rules apply to all
packets ('from all'). We've seen the 'main' table before, it is output by
If we want to do fancy things, we generate rules which point to different tables which allow us to override system wide routing rules.
For the exact semantics on what the kernel does when there are more matching rules, see Alexey's ip-cref documentation.
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 you'll end up using the low-end cable modem.
The 'fast' cable modem is known as 184.108.40.206 and is a PPP link to 220.127.116.11. The 'slow' cable modem is known by various ip addresses, 18.104.22.168 in this example and is a link to 22.214.171.124.
The local table:
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:
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.
Now all that is left is to generate Johns table, and flush the route cache:
And we are done. It is left as an exercise for the reader to implement this in ip-up.
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