Sending packets out of incoming interface

It is desirable, in some circumstances on a server with multiple NIC’s to send outgoing traffic on incoming interface. In order to do so the server has to have policy routing implemented.

Let’s assume that the server is running CentOS 6 and that there are two network interfaces as follows:

Here is a simple example. First create routing table /etc/iproute2/rt_tables:

50 internal1
60 internal2

Names internal1 and internal2 are just random names. Now, create /etc/sysconfig/network-scripts/rule-eth0 which just defines under which circumstances routing table should be used:

iif eth0 table internal1
from table internal1

Next, define a rule for the second network interface /etc/sysconfig/network-scripts/rule-eth1as follows:

iif eth1 table internal2
from table internal2

Next, define route to using eth0 interface. The following lines go into /etc/sysconfig/network-scripts/route-eth0: dev eth0 table internal1
default via dev eth0 table internal1

And the same thing needs to be done for the second interface – eth1. The following lines need to be placed in /etc/sysconfig/network-scripts/route-eth1: dev eth1 table internal2
default via dev eth0 table internal2

Finally, remove GATEWAY stanza from /etc/sysconfig/network. That’s it. This will persist reboots as well.

Posted on September 3, 2017 at 10:39 by somedude · Permalink
In: centos, linux, linux tips, networking, redhat, Uncategorized

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