Why would certain packets not reach nat PREROUTING chain?

Jozsef Kadlecsik kadlec at blackhole.kfki.hu
Thu Nov 10 17:44:16 CET 2005

On Thu, 10 Nov 2005, Sandro Dentella wrote:

> 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.

Packets which cannot be associated with any existing connection
known by the conntrack subsystem will traverse the NAT table.

If a packet is related to any connection, which can mean:

- the packet belongs to a connection
- it is an ICMP error packet about a connection
- it is a packet of a channel (like FTP data), which can be
  associated to a connection by an appropriate helper module

then that packet won't enter the NAT table.

Best regards,
E-mail  : kadlec at blackhole.kfki.hu, kadlec at sunserv.kfki.hu
PGP key : http://www.kfki.hu/~kadlec/pgp_public_key.txt
Address : KFKI Research Institute for Particle and Nuclear Physics
          H-1525 Budapest 114, POB. 49, Hungary

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