Strange RTCP issue - sr-users

27 Apr 2023

Hello,

We have a transparent proxy in front of Asterisk boxes. The media is handled by rtpengine,
and the "manage" work is done by Kamailio in the route[NATMANAGE] section like
this:

        if(nat_uac_test("8")) {

                # Packet from backend server
                if(ds_is_from_list()){
                    if (rtpengine_manage("replace-origin replace-session-connection
direction=internal direction=pub")) {
                        xdbg("NATMANAGE --- Success rtpengine_manage() RTP from
INTERNAL to EXTERNAL $ru");
                    }
                }
                else
                {
                    if (rtpengine_manage("replace-origin replace-session-connection
direction=pub direction=internal")) {
                        xdbg("NATMANAGE --- Success rtpengine_manage() RTP from
EXTERNAL to INTERNAL $ru");
                    }
                }

        } else {

                # Packet from backend server
                if(ds_is_from_list()){
                    if (rtpengine_manage("replace-origin replace-session-connection
trust-address direction=internal direction=pub")) {
                        xdbg("NATMANAGE --- Success rtpengine_manage() RTP from
INTERNAL to EXTERNAL $ru");
                    }
                }
                else
                {
                    if (rtpengine_manage("replace-origin replace-session-connection
trust-address direction=pub direction=internal")) {
                        xdbg("NATMANAGE --- Success rtpengine_manage() RTP from
EXTERNAL to INTERNAL $ru");
                    }
                }

        }

Everything is working fine, but with some clients (like Grandstream phone) the RTCP
session wants to go tot he private address of the phone. Here is the log of one of these
strange calls:

Apr 27 16:54:38 rtp1 rtpengine[2273]: INFO:
[312ed76c31f21b71452e91e5184ad25b@172.16.2.210:5060]: [core] --------- Port 
178.238.213.14:11088 <>    81.183.216.3:5068 , SSRC 77c19488, 625 p, 107500 b, 0 e,
29 ts
Apr 27 16:54:38 rtp1 rtpengine[2273]: INFO:
[312ed76c31f21b71452e91e5184ad25b@172.16.2.210:5060]: [core] --------- Port 
178.238.213.14:11089 <>      10.0.5.192:5069  (RTCP), SSRC 0, 0 p, 0 b, 0 e, 43 ts

As you can see the RTP itself is okay, but the RTCP will somehow go to the private
address. If we tracing the sdp messages in sngrep, then we can see that Kamailio
transforming IP addresses for the backend servers to the public (and the private address
of the rtpengine) addresses.

We are missing something?

Thank you!
With kind regards,
Zoltan