Thank you for the reply Karsten! I'm afraid the options_reply() function won't work for me. The reason is that it seems to only handle OPTIONS where the RURI username is not set, and in my case I need to be able to handle these cases. Moreover, it is not evident from the siputils module documentation if it will perform NAT traversal on the side of kamailio, which is my original problem. I'm beginning to think I can't serve NATted endpoints' requests statelessly and that I'd have to handle the reply statefully, am I right in thinking that? BR, George On Fri, 5 Jul 2019 at 09:37, Karsten Horsmann &lt;khorsmann(a)gmail.com <mailto:khorsmann@gmail.com>> wrote: Hi George, i use something like that (shameless stolen from other configuration files within request-route). Thats for answering options to my internal kamailio. But Alex Balashov wrote an nice blog poste about NAT/OPTION Pings  with kamailio: http://www.evaristesys.com/blog/server-side-nat-traversal-with-kamailio-the…     ### only initial requests (no To tag)     if (is_method("OPTIONS"))     {         if (uri=~"sip:.*[@]+.*")  {             sl_send_reply("501", "Not Implemented");         }         else {             options_reply();             exit;         }     } Cheers Karsten Am Do., 4. Juli 2019 um 10:29 Uhr schrieb George Diamantopoulos &lt;georgediam(a)gmail.com <mailto:georgediam@gmail.com>>: Hello all, I've been trying to handle OPTIONS statelessly, but proper handling fails for UACs behind NAT without ALG. I thought I'd overcome this by setting the $du pseudovariable, but apparently I'm missing something. The following does not work:         if ($Rp == "6050") {             force_send_socket(udp:10.10.10.10:6050 <http://10.10.10.10:6050>);         }         $du = "sip:" + $si + ":" + $sp;         sl_send_reply("200","OK");         exit; I'm now realising that I put this in request_route() so force_send_socket() and $du have no use whatsoever, since the request is not forwarded anywhere. Is this the reason? So the question is, how can I do the following things for a stateless reply: a) choose a sending socket for the reply and b) change the destination for the reply to something other than the URI in the first Via header. Thanks! BR, George _______________________________________________ Kamailio (SER) - Users Mailing List sr-users(a)lists.kamailio.org <mailto:sr-users@lists.kamailio.org> https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users -- Mit freundlichen Grüßen *Karsten Horsmann* _______________________________________________ Kamailio (SER) - Users Mailing List sr-users(a)lists.kamailio.org <mailto:sr-users@lists.kamailio.org> https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users _______________________________________________ Kamailio (SER) - Users Mailing List sr-users(a)lists.kamailio.org https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users