[SR-Users] why use received as RURI for nathelper OPTIONS ping?

[Andrew Pogrebennyk]

Some Fritzbox devices don't accept the OPTIONS ping generated by
kamailio with RURI: sip:11.22.3.4:5060. In the location table we have:
received: sip:11.22.3.4:5060
contact: sip:user at 11.22.3.4:5060;uniq=6633BC1386F4D4CC4EBD64DC7E967
path: <sip:lb at 127.0.0.1;lr;received='sip:11.22.3.4:5060'>

Kamailio version is 3.3.2 and the nathelper config is nothing fancy:
modparam("nathelper", "natping_interval", 15)
modparam("nathelper", "ping_nated_only", 1)
modparam("nathelper", "sipping_bflag", FLB_NATSIPPING)
modparam("nathelper", "sipping_from", "sip:pinger at sipwise.local")
modparam("nathelper", "received_avp", "$avp(s:received)")

What I don't understand is why kamailio sets RURI of the OPTIONS message
to value of "received" instead of the contact. I suspect a bug in the
parser somewhere along these lines:

>                 rval = ul.get_all_ucontacts(buf,cblen,(ping_nated_only?ul.nat_flag:0),
>                    ((unsigned int)(unsigned long)timer_idx)*natping_interval+iteration,
>                    natping_processes*natping_interval);
>                 if (rval != 0) {
>                         pkg_free(buf);
>                         goto done;
>                 }
>         }
> 
>         if (buf == NULL)
>                 goto done;
> 
>         cp = buf;
>         while (1) {
>                 memcpy(&(c.len), cp, sizeof(c.len));
>                 if (c.len == 0)
>                         break;
>                 c.s = (char*)cp + sizeof(c.len);
>                 cp =  (char*)cp + sizeof(c.len) + c.len;
>                 memcpy( &send_sock, cp, sizeof(send_sock));
>                 cp = (char*)cp + sizeof(send_sock);
>                 memcpy( &flags, cp, sizeof(flags));
>                 cp = (char*)cp + sizeof(flags);
>                 memcpy( &(path.len), cp, sizeof(path.len));
>                 path.s = path.len ? ((char*)cp + sizeof(path.len)) : NULL ;
>                 cp =  (char*)cp + sizeof(path.len) + path.len;
> 
>                 /* determin the destination */
>                 if ( path.len && (flags&sipping_flag)!=0 ) {
>                         /* send to first URI in path */
>                         if (get_path_dst_uri( &path, &opt) < 0) {
>                                 LM_ERR("failed to get dst_uri for Path\n");
>                                 continue;
>                         }
>                         /* send to the contact/received */
>                         if (parse_uri(opt.s, opt.len, &curi) < 0) {
>                                 LM_ERR("can't parse contact dst_uri\n");
>                                 continue;
>                         }
>                 } else {