RE: What could this icmp mean?

["David Gillett" <gillettdavid () fhda edu>]

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?