[LARTC] Redundant internet connections.

From: Grant Taylor <gtaylor@riverviewtech.net>
To: lartc@vger.kernel.org
Subject: [LARTC] Redundant internet connections.
Date: Thu, 21 Jun 2007 07:05:56 +0000	[thread overview]
Message-ID: <467A2354.1070805@riverviewtech.net> (raw)

(I know that what I'm wanting to do can be done, but for some reason I 
can not get it to work for the life of me.  I think I have been staring 
at it too long and too closely.)

I have two different internet connections from two cooperating ISPs.  I 
also have a small 8 block of IPs that are globally routable that both 
ISPs will route to me via my world facing globally routable IPs that I 
have with them.  I.e. ISP A has a route to 75.19.28.7/29 via 12.34.56.78 
and ISP B has a route to 75.19.28.7/29 via 87.65.43.21.

I want to use one ISP as the primary default gateway and the other ISP 
as a backup default gateway.  That is to say I want to *NOT* use load 
balancing rather just redundancy in this situation.

I do *NOT* need to use NAT because I do have the globally routable IP 
address on *ALL* interfaces.

I.e.
eth0:  75.19.28.6 (DMZ)
eth1:  12.34.56.78 (ISP A)
eth2:  87.65.43.21 (ISP B)

I want this router to use the default gateway for ISP A of 12.34.56.254 
and only use the default gateway of ISP B 78.65.43.1 if the default 
gateway of ISP A can not be reached.

If I set up the interfaces with their IPs and subnets and set up 
multiple default routes with varying metrics (for priority) and test by 
taking an interface down, things work.  However, this is not a realistic 
test because the interface will never physically go down.

For the sake of discussion, let one link be a DSL modem and the other 
link be a cable modem.  Each of the links is an external modem that uses 
an ethernet cable to connect in to the router.  Thus no matter what the 
state of the link coming in to my facility is, the link on the Linux 
router will always be up b/c the ethernet between the router and the 
modems sitting on the next shelf down will always be up.

I need a way for the Linux kernel to try to use a default gateway and 
switch to another one if it does not see any traffic.

Any help that any one could offer will be greatly appreciated.

Thanks in advance,

Grant. . . .
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