ipsec connection problems though netfilter.

[Martin Cheatle <marche@systems-pro.net>]

My Network

               internet
                  |
      +----------------------+
      |                      |
     eth2                    |
      |                      |
  Firewall-eth1-----netB----ISA
      |
     eth0
      |
     netA

Firewall Rules
iptables -t nat -A POSTROUTING -o eth2 -j SNAT --to-source 'external ip 
address'
iptables -A FORWARD -i eth0 -o eth2 -m state --state 
NEW,RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i eth2 -o eth0 -m state --state RELATED,ESTABLISHED 
-j ACCEPT
iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -j ACCEPT


MS/ISA Server listening for ipsec/l2tp on both adapters

My Problem

If I connect a MS/XP client to the MS/ISA server via ipsec/l2tp directly 
from netB or the internet every thing works fine.
If I try to connect from netA through to netB the client fails to connect.
If try to connect from netA to the MS/ISA server via the internet 
address all works ok.

The only difference in my rules is that the traffic gets NAT'ed on the 
way out to the internet.
I have tried NATing the traffic leaving the firewall on eth1 but this 
did not resolve the issue.
I have added -m state rules from eth0 to eth1 and back, this did not help.

The only difference I have seen in the traffic between the connections 
through the firewall is a ip-fragment between netA and netB during the 
isakmp stage of the connection both then procced to start ESP traffic.

Has anyone seen this type of problem before?

Eventually I do want to remove the ISA server of the internet and route 
the ipsec/l2tp traffic through the firewall but I can't do that until i 
can get this bit working.

Thanks in advance
Martin