1. Your devices on the 10.30.1.x network have 10.30.1.254 as their
default gateway, but traffic to 10.30.0.x needs to go by 10.30.1.1
instead. 10.30.1.254 is attempting to inform 10.30.1.16 that if
it wants to send traffic to 10.30.0.4, it could save a hop by
sending those packets directly to 10.30.1.1 instead of relying
on 10.30.1.254 to forward them.
Since most clients don't know what to do with ICMP redirects, and
will just ignore them, it's common to turn them off at the router
interface.
2. ICMP packets carry, as payload, a portion of the packet that triggered
the ICMP. It's no surprise that the checksum contained within this
partial quotation is a checksum for the full packet, and not just the
quoted portion. It would be a near-miracle if these computed to the
same value.
David Gillett
-----Original Message-----
From: Tomas [mailto:s.tomas@gmail.com]
Sent: Tuesday, February 22, 2005 5:11 AM
To: security-basics@securityfocus.com
Subject: What could this icmp mean?
Hello list,
We have networks (10.30.0.0/24 and 10.30.1.0/24) connected
trough VPN and
one internet line. The gateways for VPN are 10.30.0.1 from
one side and
10.30.1.1 from the other, and 10.30.1.254 for internet (for
both networks).
I've launched tcpdump today on my internet firewall's
internal interface
(10.30.1.254) and I found this:
10.30.1.254 > 10.30.1.16: icmp: redirect 10.30.0.4 to host
10.30.1.1 for
10.30.1.16.445 > 10.30.0.4.1959: [|tcp] (DF) (ttl 127, id
7691, bad cksum
c76d! differs by 100) (ttl 255, id 23807)
I'm a bit confused, what could this icmp mean? First of all,
I'm sure that
neither of these hosts (10.30.1.254, 10.30.1.16, 10.30.0.4)
are sending any
icmp requests (I'm not sure about 10.30.1.1; it's not in my
control). And
the second of all, why the checksum is bad?
