Hello, there is not a central switch to configure TCP transport etc.. But check your configuration, I guess that it uses t_relay() for forwarding. You can specify the wanted protocol in the t_relay call, there are special function (t_relay_to_tcp() etc..) for this. http://kamailio.org/docs/modules/stable/modules/tm.html#tm.f.t_relay About the SRV query - check it e.g. with tcpdump if Kamailio does the SRV query. Is there is a port specified in the forwarding target, it will not do it. Cheers, Henning -- Henning Westerholt – https://skalatan.de/blog/ Kamailio services – https://gilawa.com -----Original Message----- From: sr-users &lt;sr-users-bounces(a)lists.kamailio.org&gt; On Behalf Of BALL SUN Sent: Friday, June 5, 2020 10:36 AM To: sr-users(a)lists.kamailio.org Subject: [SR-Users] Kamailio IMS using returned SRV records for transport slelection Hi All I am new to Kamailio IMS, and I just setup a testing environment where having P-CSCF, I-CSCF and S-CSCF. I would like to use make below works 1. SIP client <-> P-CSCF (either UDP or TCP, based on the transport tag) 2. P-CSCF <-> I-CSCF (TCP by default) 3. I-CSCF <-> S-CSCF (TCP by default) For point 2, I observed that P-CSCF actually will send DNS SRV query to DNS server for resolving _sip._udp. and _sip._tcp. I believed we can just disable the "_sip._udp." so that P-CSCF will always use TCP to communication wih I-CSCF. However, for point 3, I didnt notice I-CSCF send DNS SRV query, My question is how we can force I-CSCF to do that or is there any configuration that we should enable such that I-CSCF always use TCP to S-CSCF? - RBK _______________________________________________ Kamailio (SER) - Users Mailing List sr-users(a)lists.kamailio.org https://lists.kamailio.org/cgi-bin/mailman/listinfo/sr-users