This article is  published at [[http://www.debian-administration.org/articles/377|Debian Administration]]

====== Introduction ======
Debian suits perfectly for use as a gateway for computers on your LAN. However once bandwidth usage grows it could be handy to just add another internet uplink to your gateway. Debian does not cater for this out of the box so this document describes how to setup your debian gateway for multiple uplinks. 

The whole setup is based on the [[http://lartc.org/howto/|Linux Advanced Routing & Traffic Control HOWTO]]

====== Required packages ======

The most important and only package we need is `iproute`
<file>
apt-get install iproute
</file>

The iproute package is a tool to talk to more advanced routing capabilities of the linux kernel. I suggest you read the man page and the howto mentioned above. Some basic theory of iproute is that it does it routing through tables just like iptables does. It determines what table to route through based on rules you define.

====== Setup ======

As an example I have 2 uplinks to the internet. The first uplink device eth1 has ipaddress 1.0.0.1/24 and gateway 1.0.0.2. The second uplink device eth2 has ip 2.0.0.1/24 and gateway 2.0.0.2.

First we will define the tables. We do this by editing /etc/iproute2/rt_tables which looks like this:
<file>
#
# reserved values
#
255     local
254     main
253     default
0       unspec
#
# local
#
#1      inr.ruhep
</file>
In this file we append a new table name with an unique number for every uplink.
<file>
#
# reserved values
#
255     local
254     main
253     default
0       unspec
#
# local
#
#1      inr.ruhep
200     uplink1
201     uplink2
</file>

We can now focus on setting up the interfaces file. Setup /etc/network/interfaces like this.
<file>
auto lo
iface lo inet loopback
auto eth0
#LAN interface
iface eth0 inet static
        address 10.0.0.1
        netmask 255.255.255.0
#Uplink1
auto eth1
iface eth1 inet static
        address 1.0.0.1
        netmask 255.255.255.0
        post-up ip route add 1.0.0.2/32 dev eth1 src 1.0.0.1 table uplink1
        post-up ip route add default via 1.0.0.2 table uplink1
        post-up ip rule add from 1.0.0.1 table uplink1
        post-down ip rule del from 1.0.0.1 table uplink1
#Uplink2
auto eth2
iface eth2 inet static
        address 2.0.0.1
        netmask 255.255.255.0
        post-up ip route add 2.0.0.2/32 dev eth2 src 2.0.0.1 table uplink2
        post-up ip route add default via 2.0.0.2 table uplink2
        post-up ip rule add from 2.0.0.1 table uplink2
        post-down ip rule del from 2.0.0.1 table uplink2
</file>
You are now ready to bring the interfaces up. 
<file>
ifup -a
</file>
There is no default gateway at this moment. We want to balance traffic over both uplinks. The following command will set this up.
<file>
ip route add default scope global nexthop via 1.0.0.2 dev eth1 weight 1 nexthop via 2.0.0.2 eth2 weight 1
</file>
As stated in the howto mentioned above "The weight parameters can be tweaked to favor one provider over the other".

We have now setup our gateway for multiple uplinks.
An other benefit of this setup is that the gateway responds through the same uplink as external traffic originated from. 

====== Final thoughts ======
Remember the balancing of the uplinks is route based and routes are cached. You will not get double bandwidth when downloading a file. You can flush the route cache with 'ip route flush cache'.  The uplink balancing is not perfect but to get a better solution you need to patch and recompile the kernel. Have a look at the patches provided by Julian Anastatov. [[http://www.ssi.bg/~ja/#routes]]