On 21-04 13:20, Richard wrote: Yes, it will detect clients behind NAT if there is no SIP-ALG (something that rewrites SIP messages). If the NAT rewrites SIP messages as well then they will look like ordinary message coming from clients in the public internet and they will not be treated like NATed -- in that case the NAT will take care of the clients. If implemented properly then it will work. If a phones discovers that it can traverse the NAT using STUN then it will do so and such a client will look like a client in the public internet to ser -- neither special treatment nor rtp proxy is necessary. Clients supporting STUN will detect that they are behind symmetric NAT and should put private IPs in the messages (because symmetric NATs are not traversable by STUN and therefore STUN should not be used). In this case SER will take care of traversing the NAT (nathelper/rtp relay). The rule of thumb for user agents is: Rewrite the contents of SIP message only if you are absolutely sure that you can traverse the NAT without additional help from the server. If a SIP user agent is able to detect the type of the NAT and traverse it by any means (STUN, UPNP, whatever) then it should do it. In this case ser won't do anything special. If the user agent cannot traverse the NAT or if it is not sure that it can traverse it properly then it _should not do anything_, it should just send SIP messages containing private IPs to the server and hope that the server will be able to traverse it. In this case ser needs to get as much original information as possible -- mainly private IP addresses. Jan.