ICMP redirect message on console

Paul Civati paul at xciv.org
Sat Feb 19 15:08:41 GMT 2005


Henrik Morsing <henrik at morsing.cc> wrote:

> It has for a long time been a problem for me that if my ADSL modem
> disconnects or similar network problem, the serial console gets flooded
> with "ICMP redirect..." messages.

What are the exact messages?

> Because I often can't connect over the
> network when this happens it means that it very difficult to work on the
> box to correct the problem.
> I initially thought it was ipmon logging but killing it doesn't help. The
> only thing that works is removing the default route but I would like to
> know if there is a way to prevent to messages being sent to the console in
> the first place.

ICMP redirects occur at the IP/kernel level.  Normally this happens,
for example, when you send traffic via default to a router, but the
router has a more specific route pointing at another host on the
local LAN.  The router sends an ICMP redirect to alter the sending
hosts routing table, to make it send traffic (for the relevant 
network) to the correct local host.

I'm guessing that when your ADSL goes down the router might have
a secondary default route that points somewhere else on your local
LAN.

Anyhow, you should be able to stop it listening to the ICMP directs
with a sysctl tunable (net.inet.icmp.drop_redirect looks likely).

-Paul-

-- 
Paul Civati <paul(at)racksense.com> 0870 321 2855
Rack Sense Ltd - Managed/Business hosting - www.racksense.com   
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