[SR-Users] Fwd: INVITE routing issue when using Private IP (RFC1918) network

Hello, I have a Kamailio IMS instance setup, using different hosts for each IMS function (P-CSCF,S-CSCF,I-CSCF etc) and am able to register clients successfully. However, when Client-A tries to call Client-B (both registered to the same S-CSCF), the call is failing with the "403 Forbidden - You must register first with a S-CSCF" error. . All hosts & clients are running on the same subnet using RFC1918 IP addresses, there is no NAT involved. I've seen the same issue on both 4.2 and Master branches. I have the nathelper 'nat_uac_test' parameter set to 18. Looking at the debug prints and SIP traces, I can see that the 403 Error is being generated by the P-CSCF when it receives the INVITE from the S-CSCF (for forwarding to Client-B). Digging deeper into the debug logs, it looks like this is a result of a failed contact match - I've experimented with different values for "hashing_type", which do change the routing logic but still I see the 403 Error. Here's 2 excerpts from the debug prints for different values of hashing_type : *modparam("ims_usrloc_pcscf", ""hashing_type",hashing_type", 0) :* 2(19636) DEBUG: ims_registrar_pcscf [service_routes.c:183]: getContactP(): 2(19636) DEBUG: ims_usrloc_pcscf [udomain.c:618]: get_pcontact_by_src(): 2(19636) DEBUG: ims_usrloc_pcscf [udomain.c:465]: get_pcontact(): 2(19636) DEBUG: ims_usrloc_pcscf [usrloc.c:187]: get_aor_hash(): Returning 2(19636) DEBUG: ims_registrar_pcscf [service_routes.c:186]: getContactP(): 2(19636) DEBUG: ims_usrloc_pcscf [udomain.c:465]: get_pcontact(): 2(19636) DEBUG: ims_usrloc_pcscf [usrloc.c:187]: get_aor_hash(): Returning 2(19636) DEBUG: ims_registrar_pcscf [service_routes.c:119]: 2(19636) ERROR: *** cfgtrace:request_route=[Orig_Initial] *modparam("ims_usrloc_pcscf", ""hashing_type",hashing_type", 2) :* 1(19401) DEBUG: ims_registrar_pcscf [service_routes.c:183]: getContactP(): >> > 1(19401) DEBUG: ims_usrloc_pcscf [udomain.c:618]: get_pcontact_by_src(): >> > 1(19401) DEBUG: ims_usrloc_pcscf [udomain.c:465]: get_pcontact(): >> > 1(19401) DEBUG: ims_usrloc_pcscf [usrloc.c:97]: >> get_alias_host_from_contact(): no params > > 1(19401) DEBUG: ims_usrloc_pcscf [usrloc.c:183]: get_aor_hash(): using >> host for hash [10.133.202.17] > > 1(19401) DEBUG: ims_usrloc_pcscf [usrloc.c:187]: get_aor_hash(): >> Returning hash slot: [-1564914093] > > 1(19401) DEBUG: ims_registrar_pcscf [service_routes.c:186]: >> getContactP(): No entry in usrloc for 10.133.202.17:5060 (Proto 1) found! > > 1(19401) DEBUG: ims_usrloc_pcscf [udomain.c:465]: get_pcontact(): >> Searching for contact in P-CSCF usrloc [sip:john@10.133.202.61:64013 >> ;transport=udp] > > 1(19401) DEBUG: ims_usrloc_pcscf [usrloc.c:176]: get_aor_hash(): Looks >> like this contact is natted - contact URI: [10.133.202.61] but came from >> received_host: [10.133.202.17] so will use received_host for hash > > 1(19401) DEBUG: ims_usrloc_pcscf [usrloc.c:187]: get_aor_hash(): >> Returning hash slot: [-1564914093] > > 1(19401) ERROR: *** cfgtrace:request_route=[Orig_Initial] >> > Can anyone offer any hints as to how to resolve this? I am attaching a SIP call flow - I can supply a complete Kamailio debug trace if needed. Thanks Phill