[SR-Users] sip response not handled by kamailio

Hello, We are trying to use Kamailio 4.2 with RTPEngine 3.3 behind NAT. Somehow SIP responses (200 OK) are not handled by Kamailio. The following INVITE is send to the receiving end (WebRTC Client): INVITE sip:evQzk4l62MdF3G7U6eudj@whtest3.24dial.com SIP/2.0 Record-Route: <sip:104.155.11.255:5060;nat=yes;lr=on> Via: SIP/2.0/UDP 104.155.11.255:5060 ;branch=z9hG4bK865.9db0c023bec2f314e89ae0f18b78d2f0.0 Via: SIP/2.0/UDP 10.240.215.73:5060 ;rport=5060;received=146.148.113.245;branch=z9hG4bK644600e2 Max-Forwards: 69 From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=as68f0a26a To: <sip:evQzk4l62MdF3G7U6eudj@whtest3.24dial.com> Contact: <sip:anonymous@10.240.215.73:5060;alias=146.148.113.245~5060~1> Call-ID: 368f6ec901a3552e3b37a70907634f4f@10.240.215.73:5060 CSeq: 102 INVITE User-Agent: Asterisk PBX 12.5.0 Date: Fri, 24 Oct 2014 08:38:09 GMT Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY, INFO, PUBLISH, MESSAGE Supported: replaces, timer Content-Type: application/sdp Content-Length: 738 P-hint: outbound ....... The client response looks like this: SIP/2.0 200 OK Via: SIP/2.0/UDP 104.155.11.255:5060 ;branch=z9hG4bK865.9db0c023bec2f314e89ae0f18b78d2f0.0;rport=5060;received=104.155.11.255 , SIP/2.0/UDP 10.240.215.73:5060 ;rport=5060;received=146.148.113.245;branch=z9hG4bK644600e2 From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=as68f0a26a To: <sip:evQzk4l62MdF3G7U6eudj@whtest3.24dial.com Contact: <sip:evQzk4l62MdF3G7U6eudj@whtest3.24dial.com Call-ID: 368f6ec901a3552e3b37a70907634f4f@10.240.215.73:5060 CSeq: 102 INVITE Record-Route: <sip:104.155.11.255:5060;nat=yes;lr=on> Content-Type: application/sdp Content-Length: 938 ....... I have a xlog call as the first statement in kamailio routing script. I can see the request, which get's handled correctly but the response does not appear in kamailio log, though it arrives at the machine. I am totally helpless, what might be wrong. If we use kamailio on a public accessable machine everything works as expected. Maybe anyone might have had similiar problems using kamailio behind NAT. Kind regards, Marko