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Monthly Archives / April 2014

  • Apr 03 / 2014
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Linux

Setup multiple default routes on Linux

The problem ?

Your system is running on Linux with multiple physical network interface cards (NIC) and each card has its own default gateway. By default, you can only set up a single default gateway on a system.

In our example, we will consider 2 NICs (eth0 and eth1) enabled with default gateway configured on eth0 interface.

rt1

On this scheme, we can imagine two different cases:

  1. The flow coming into eth0 will be returned through eth0 (default gateway)
  2. The flow coming into eth1 will be returned through eth0 (default gateway)

In the first case, there is no problem, each NIC will work independently but in the second case, if you configured default gateway on eth0, whatever the network card receiving the request, it will answer through eth0 which is the default gateway.

Solution

The solution will be to use a program called iproute2, which is included and installed in all current Linux distributions. The expected result is:

  1. The flow coming into eth0 is returned through eth0
  2. The flow coming into eth1 is returned through eth1

It should so look like the following scheme:

rt2

First of all, you will have to define new routing tables in the file /etc/iproute2/rt_tables by defining your own tables. Here, we are defining two new tables called rt0 and rt1:

Afterwards, you will have to configure your network cards (under /etc/network/interfaces if you’re running on a Debian-like system) as below:

And now, restart your networking service to set this new configuration up. This will be permanent.

You can now connect to your server using both interfaces, and it will automatically answer correctly through the good gateway !

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