[Serusers] rtpproxy and fax clients under linux nat

[Antanas Masevicius]

Hello, 

latetly i got a bit confused while playing with rtpproxy. It is said that it supports nat clients with symmetric nat. As i know, linux is also symmetric by default. Now i get into situation where my natted clients work for voice, but does not for fax. I traced that my nat does not forward incomming T38 stream saying "destination unreachable (port unreachable)".  After looking at ip_conntrack during audio call i see: 

udp      17 180 src=192.168.0.46 dst=my.rtpproxy.ip sport=4004 dport=19006 src=my.rtpproxy.ip dst=my.external.ip sport=19006 dport=1024 [ASSURED] use=1
udp      17 29 src=192.168.0.46 dst=my.rtpproxy.ip sport=4005 dport=19007 [UNREPLIED] src=my.rtpproxy.ip dst=my.external.ip sport=19007 dport=4005 use=1
udp      17 171 src=192.168.0.46 dst=my.rtpproxy.ip sport=5069 dport=5060 src=my.rtpproxy.ip dst=my.external.ip sport=5060 dport=5069 [ASSURED] use=1

but looking at first line it doesn't seem like symmetric nat, it is clear that packets out of my.external.ip goes with low (1024) sports. Should it be like so or my linux masquarading is misconfigured? This audio session work ok for some reasons unknown to me. Can anyone explain the behaviour? 

>From second line we can see that packect can't reach 4005 port. Similar problem prevents me from receiving t38 stream i guess. How do you configure your linux natting for set up with rtpproxy?

Antanas
NTT 
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