SNAT/Masquerade binding ports generate conflicts

[Olivier GRALL <olivier.grall@neotip.com>]

Hi,

I have a problem using iptables as a NAT box. I'm testing a p2p VoIP 
system trying to establish UDP channels using something like STUN.

                      
------------         snat                        ------------    
                      ------------
|      A        |-----| X |-----<-->------ |   Server S  
|---<----->-----|        B      |
------------(p1)         \(P1)                ------------               
/  (P2) ------------
                                   \------<----->-------<---->-------/

X is iptables with a MASQUERADE (or SNAT) rule

1- A sends a packet to S from port p1
2- X opens port P1 (should be equal to p1)
3- S answers to A with a message containing UDP port of B
4- B knows port P1 of X thanks to S too
=> Sometimes A sends the first packets of the communication to B through 
X, then B can send packets to A too. Otherwise B sends the first packets 
and X seems to drop them (we can see icmp). Then A sends UDP packets to 
B and at this time iptables opens a new port 1024 for instance different 
from P1.
      It is like the uplet 
[(src=(@A,p1),dst=(@B,P2)),(src=(@X,P1),dst=(@B,P2))] wasn't  available 
anymore and so iptables changes P1 to P1'. Doing this, only one part of 
the uplet changes. I thought that conntrack forces the fact that for 
(@A,p1),(@B,P2) there is only a unique couple on the other side (= at 
the end of iptables treatments).

I used iptables 1.2.7a for that test. Anyone can tell me if  a recent 
version can solve this problem ?

Thanks.

Olivier GRALL.