Explanation Needed for connection tracking with NAT One-Way

[Tish Best <tish.best@wichita.edu>]

I am trying to test some research ideas and am not able to perform NAT 
translation on incoming packets only. I am trying to explain why, and am 
looking for a confirmation (or correction) of what I think is happening. I 
have two Routers (B and C) in my network. I am trying to send packets directly 
from A to D through B, but route the replies through C and B using NAT 
translation.

                   D
                  / \
                 /   \
         A --- B ---- C

The gateway route of Host D is to Router C. Router C has the following 
iptables entries in the NAT PREROUTING and NAT POSTROUTING table.

iptables -t nat -A PREROUTING -s D -d A -j DNAT --to B
iptables -t nat -A POSTROUTING -s D -d B -j SNAT --to C

Router B has the following entries in its iptables NAT PREROUTING and 
POSTROUTING tables.

iptables -t nat -A PREROUTING -s C -d B -j LOG
iptables -t nat -A PREROUTING -s C -d B -j DNAT --to A
iptables -t nat -A POSTROUTING -s C -d A -j LOG
iptables -t nat -A POSTROUTING -s C -d A -j SNAT --to D

I am able to see the first TCP SYN packet travel from A through B to D. At 
this point Router B has an ip_conntrack entry from A to D. I then see the 
reply travel from D to C. C successfully performs NAT translation, and the 
packet is sent to Router B with a source of C and a destination of B. I added 
logging to the iptables entries in Router B, and I see the packet get 
translated in both the PREROUTING and the POSTROUTING tables, but the packet 
is never sent. I never see a new conntrack entry for this packet.

I think that Router B is dropping the packet through connection tracking since 
it cannot add a new conntrack entry for this packet because it already has an 
entry for packets from A to D that do not require NAT translation. If it added 
a new entry, then it would not know whether to translate the next packet from 
A to D or send it directly to D.

Is this analysis correct? Is there anyway to send packets directly to D and 
router them back to A through C and B using NAT translation the way I have 
described using iptables or otherwise? Any explanation on what criteria 
connection tracking uses to drop packets in this case would be a great help.

Thanks,
Tish Best