It is in the nathelper module README (sip_router/modules/nathelper/README): 1.4.7. nat_uac_test(mode) Tries to guess if client's request originated behind a nat. The parameter determines what heuristics is used. If flag 1 is set, the "received" test is used -- address in Via is compared against source IP address of signaling. If flag 2 is set, Contact header field is searched for occurrence of RFC1918 addresses. If flag 4 is set the address in Via is compared with "private" RFC1918 addresses. If flag 8 is set the ip in the SDP message body is compared with RFC1918 addresses. If flag 16 is set the rport test is used: the source port of the message is compared with the source port advertised in Via (good to catch broken STUN implementations, but might cause problems for UAs with asymmetric signaling). All the flags can be bitwise combined, the test returns true if any of the tests identified a NAT (e.g. nat_uac_test(19) will return true if any of the received test, contact RFC1918 test or. rport test are true). Jan. On 07-11-2005 13:08, Maria Yndefors wrote: