[2607] in linux-net channel archive
ICMP redirects and IP forwarding
daemon@ATHENA.MIT.EDU (Carlos Carvalho)
Sat Apr 20 15:38:13 1996
Date: Sat, 20 Apr 96 16:23 EST
From: Carlos Carvalho <carlos@fisica.ufpr.br>
To: linux-net@vger.rutgers.edu
Alan has said several times that when you configure the kernel for IP
forwarding it ignores ICMP packets. This is causing a problem in our
campus.
We have a central router and several local linux routers in the
departments. Only these are connected to the optical fiber backbone.
When a machine in one department wants to talk to another in another
department. The packets follow this way:
machine A -> local router -> central router -> local router -> machine B
The problem is that the central router sends an ICMP to the first
local router to tell it that it can send directly to the second local
router (obviously, since they're both in the optical fiber), BUT THE
LOCAL ROUTERS DON'T LEARN WITH THE ICMP BECAUSE THEY'RE CONFIGURED
WITH IP FORWARDING.
All the ICMP's are generating useless traffic and congesting the
backbone, as well as producing overhead on the central router. I see
two alternatives: a) build static routes to all the local routers
(there are about 25); b) use gated. This would be unnecessary if linux
didn't ignore the ICMP.
So the question is what's the reason for this behaviour? Is it in some
RFC or is it a decision of the linux developers?
Carlos
