Thursday, 2 August 2012

Routing table in linux

Routing table in linux
Linux find route for a particular host or network by checking the routing table. Whenever we enter a route entry by default it move into main routing table.
root# ip route show
root# ip route add via
All above command will apply on main routing table.

We can create custom routing tables and set rule to forward certain traffic to new created routing table.
To create a new routing table, edit the file /etc/iproute2/rt_tables and add entry for new table

root# vi /etc/iproute2/rt_tables

and add following line
100 newrtable
100 is id and newrtable is name of routing table.

To check current entries in newly created routing table

root# ip route show table newrtable

To add route entry in routing table

root# ip route add default via table newrtable

Here default gateway for this route table is check table again

root# ip route show table newrtable

To forward traffic to this newly created route table , iptables command can be used along with ip rule command

root# iptables -t mangle -A OUTPUT -p tcp --dport 22 -j MARK --set-mark 100
Here ssh traffic marked with lablel 100
and then

root# ip rule add fwmark 100 lookup newrtable
Here the traffic marked with lablel 100 routed to table newrtable

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