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Linux Advanced Routing And Traffic Control (LARTC)


Networking in Linux is one of essential part for the success of this operating system. The flexibility and robustness are the key point for the success. However, the user-friendliness, at the very early stage, was not good which resembles the old Unix. Nowadays, many modern Linux distros come with good interface on setting up many aspects of networking stuff and many things can be configured automatically when the hardwares detected.

Many organizations need to have an advanced routing for their network infrastructure. Basic network infra cannot cope with certain conditions. This is when the advanced routing comes into play. In Linux, we have iproute2 package to work hand-in-hand with iptables and recent kernel for advanced routing. This topic is thoroughly covered on LARTC home page at My article here just covers basic things.

Make it work

Let's take a look at this scenario :

Scenario 1

We want to route packets that come from local network(s) to two different or two same ISPs. Say the two ISPs are tm1 and tm2 with the associated IP respectively (see above diagram --deleted. Will update soon! - 16/11/2005).

Our work is in the router box. Login as root and set two tables:

echo 1 tm1 >> /etc/iproute2/rt_tables echo 2 tm2 >> /etc/iproute2/rt_tables

These commands will put 2 new table entries in file rt_tables. The content of the file after previous commands :

255 local
254 main
253 default
0 unspec
1 tm1
2 tm2

Now we have 3 routing tables :

The next step is to populate the routing rules to the tables:

tm1 table

~# ip route add default via dev eth1 table tm1
~#ip rule add from table tm1

The packets that come from will go to tm1 routing table and then will be passed to the tm1 gateway (default route) which is on device eth1

tm2 table

~#ip route add default via dev eth2 table tm2
~#ip rule add from table tm2

The packets that come from will go to tm2 routing table and then will be passed to the tm2 gateway (default route) which is on device eth2

To see the routing tables after previous commands :

To see tm1 table:

~#ip route show table tm1

To see tm2 table:

~#ip route show table tm2

To see main table:

~#ip route show table main

Please note that this kind of routing can't be done without iproute2 package.Please make sure that this package is installed first with your distro utility.On Mandrake, this can be done with urmpi as simple as urpmi -v iproute2.

let say that you want to route packets based on their destination ports. You can do this with the help of iptables. To mark the packets that have the 22 and 80 as destination port, we will use the mangle table as below :

~#iptables -t mangle -A PREROUTING -i eth0 -p tcp --dport 80 -j MARK --set-mark 1
~#iptables -t mangle -A PREROUTING -i eth0 -p tcp --dport 22 -j MARK --set-mark 2

Let say you want to separate their route based on the destination port:

All packets with destination port 80 will go out via table tm1

~#ip route add default via dev eth1 table tm1
~#ip rule add from all fwmark 1 table tm1

All packets with destination port 22 will go out via table tm2

~#ip route add default via dev eth2 table tm2
~#ip rule add from all fwmark 2 table tm2

All packets with destination port 22 will go out via table tm2

Happy experimenting!


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