[SR-Users] redirect on register

Hi all! I need to implement a kind of Location Server. The idea behind is to process the REGISTER messages and analyze the source IP address of the packet and reply with the "302 Moved Temporarily" message. What I want to achieve - provide the SIP client with the "nearest" point to use for registration and further communications. The possible approach is: 1. make some list of available registrars/SBC's mapped to appropriate IP (sub)networks. 2. configure SER to process REGISTER messages and reply with "302 Moved Temporarily" having the IP or name of "nearest" point in Contact field. Does anybody have any ideas on possible implementations (with SER or perhaps with some other tools)? Thanks a lot! PS. At this moment I can certainly send "302 Moved Temporarily" on REGISTER with SER, but I have no idea on how to insert correct Contact header, so my SIP client just stops registering.