linux routing to multiple providers

From: Rob Hill (
Date: Wed May 24 2000 - 12:58:49 EST

I have a client that has dedicated Internet access via a T1 link. There is a
provider in their building as well connectivity at a much cheaper rate. I'll
refer to the current ISP as 'ISP1' and the up and coming competitor as 'ISP2
'. My client would like to save money on Internet connectivity but only if
the service is comparable. What they would like to do now is allow some of
the users to stay with ISP1 and some of the users to stay with ISP2. To make
my job more interesting, they would like this to be handled by my firewall.

My firewall is Slackware 7 Linux running kernel 2.2.14 with the latest
version of ipchains. All of the internal machines have non-routable ip
addresses so I am using ip masquerading to give them internet access. This
is working perfectly.

I have a friend who is very good with Linux and we were discussing this
scenario several months ago and he claims to have done it using kernel
features - which he probably has. My friend has since disappeared and so I
am presenting this to the Linux community for assistance. Now let's say I
put another network card in my firewall and I get my connection for ISP2 and
run it into that NIC. How would I make both interfaces accessible from the
Internet? Normally I only have one default gateway, but here I would have
two. Also, how could I specify what machines internally would use ISP1 and
which ones would use ISP2?

To see this post with some diagrams, go to

Rob Hill

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