res_pjsip_nat: Don't rewrite Contact on REGISTER responses.

When sending a SIP response to an incoming REGISTER request
we don't want to change the Contact header as it will
contain the Contacts registered to the AOR and not our own
Contact URI.

ASTERISK-29235

Change-Id: I35a0723545281dd01fcd5cae497baab58720478c
(cherry picked from commit d65041ebea)

res/res_pjsip_nat.c

@@ -406,12 +406,24 @@ static pj_status_t process_nat(pjsip_tx_data *tdata)
 	}
 
 	if (!ast_sockaddr_isnull(&transport_state->external_signaling_address)) {
-		/* Update the contact header with the external address */
-		if (uri || (uri = nat_get_contact_sip_uri(tdata))) {
-			pj_strdup2(tdata->pool, &uri->host, ast_sockaddr_stringify_host(&transport_state->external_signaling_address));
-			if (transport->external_signaling_port) {
-				uri->port = transport->external_signaling_port;
-				ast_debug(4, "Re-wrote Contact URI port to %d\n", uri->port);
+		pjsip_cseq_hdr *cseq = PJSIP_MSG_CSEQ_HDR(tdata->msg);
+
+		/* Update the Contact header with the external address. We only do this if
+		 * a CSeq is not present (which should not happen - but we are extra safe),
+		 * if a request is being sent, or if a response is sent that is not a response
+		 * to a REGISTER. We specifically don't do this for a response to a REGISTER
+		 * as the Contact headers would contain the registered Contacts, and not our
+		 * own Contact.
+		 */
+		if (!cseq || tdata->msg->type == PJSIP_REQUEST_MSG ||
+			pjsip_method_cmp(&cseq->method, &pjsip_register_method)) {
+			/* We can only rewrite the URI when one is present */
+			if (uri || (uri = nat_get_contact_sip_uri(tdata))) {
+				pj_strdup2(tdata->pool, &uri->host, ast_sockaddr_stringify_host(&transport_state->external_signaling_address));
+				if (transport->external_signaling_port) {
+					uri->port = transport->external_signaling_port;
+					ast_debug(4, "Re-wrote Contact URI port to %d\n", uri->port);
+				}
 			}
 		}