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 10.60.0.1 via 10.20.0.1

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 10.46.0.1 table newrtable

Here default gateway for this route table is 10.46.0.1. 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