Description of problem:
I have the impression that iptables SNAT does not work correctly in 2.6.19 kernels. 

Our context is that we are running xen with the virtual domains running in a
private network (as described here:
http://lists.xensource.com/archives/html/xen-users/2006-09/msg00925.html )
The domU hosts get an internal IP address (say, 192.168.0.x), dom0 has a virtual
veth0 with IP address 192.168.0.1 acting as default gateway on the domU's. It's
the idea that ip forwording and SNAT on dom0 should give the right access to the
domU's. This approach seems to work on 2.6.18 kernels, but not on 2.6.19

Version-Release number of selected component (if applicable):
Kernels tested and ok: 
- kenel-xen-2.6.18-1.2869 (x86_64)
- kernel-xen-2.6.18-1.2849 (i386)
Kernels test and NOT ok:
- kernel-xen-2.6.19-1.2911 (x86_64)
- kernel-xen-2.6.19-1.2911.6.5 (i386)



How reproducible:
Always

Steps to Reproduce:
1. Start xen with '(network-script network-private)' in xend-config.sxp
(network-private script is coming from above link - modify the veth0 address to
10.0.0.1 in the file)
2. On dom0:
iptables -t nat -A POSTROUTING -s 192.168.0.0/24 -j SNAT --to <eth0 address of dom0>
echo "1" > /proc/sys/net/ipv4/ip_forward
3. On domU
wget http://www.google.com or whatever
  
Actual results:
Approach works on 2.6.18, no (or sometimes very slow) connectivity in domU if
dom0 runs 2.6.19. In fact, executing traceroute on domU shows nothing further
than 10.0.0.1


Expected results:
normal working wget, ftp, ssh...

Additional info:
- This might be completley independent of xen and a pure iptables problem.
However, I don't have the equipment to test this.
- Executing an ftp session on domU to an external system, shows the greeting
message, but not the login. Which makes me think that packets are going out
correctly, but not coming back, or maybe only the first package manages to get
back. It also reinforces me of my idea that this is likely not a xen issue.
- This stuff is a bit at the edge of my knowledge, so please give some
explanation if you want me to execute some tests.
- And here's a wget output:
[root@ninja ~]# wget http://193.239.211.46
--12:08:09--  http://193.239.211.46/
Connecting to 193.239.211.46:80... connected.
HTTP request sent, awaiting response... No data received.
Retrying.


Thanks for your help and kind regards,

Bruno