Why would certain packets not reach nat PREROUTING chain?

Sandro Dentella sandro at e-den.it
Thu Nov 10 11:15:39 CET 2005


On Wed, Nov 09, 2005 at 08:57:45PM -0500, Adam Rosi-Kessel wrote:
> I'm troubeshooting an issue of accessing a VPN through NAT. Right now the
> problem can be reduced to the following question:
> 
> Under what conditions would inbound packets not be routing through the nat
> PREROUTING chain?

That's a problem that puzzles me too. Do you have fancy routing tables?
(several different tables setup w/ iproute2).

I also have a setup in which icmp packets will not get to PREROUTING. My
understanding is that the kernel does not understand they are destined for
that box: could that be your situation?

In my stup, ifconfig eth3:
eth3      Link encap:Ethernet  HWaddr 00:0A:5E:59:EF:09
          inet addr:192.168.111.1  Bcast:192.168.111.255  Mask:255.255.255.0

but these packets 

10:18:07.676131 IP 192.168.111.1 > 217.27.90.70: icmp 64: echo request seq 213
10:18:07.726977 IP 217.27.90.70 > 192.168.111.1: icmp 64: echo reply seq 213

do not enter PREROUTING and are just discarded 

In my case this is due to a peculiar routing tables setup,
http://mailman.ds9a.nl/pipermail/lartc/2005q4/017168.html that I have not
been able to debug: I didn't receice any hint on this list, lartc and
netdev. Is it such an obscure matter?


sandro



-- 
Sandro Dentella  *:-)
e-mail: sandro at e-den.it 
http://www.tksql.org                    TkSQL Home page - My GPL work



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