[Users] newbie wants to forward REGISTERs through DMZ to internal SIP server

Hi All, I have an internal SIP server that handles registrations, and is used for incoming and outgoing phone calls. It does not do any authentication. I'd like to set up SER in my DMZ, and have it: 1. authenticate all requests. (this should be easy - I think I can handle it, although I would like to get digest -> RADIUS auth against an Active directory going). 2. forward REGISTER requests to the internal SIP server, both for phones on the internal network and for phones on the external network. 3. forward INVITE and ACK requests performing the correct handling of NAT, using RTP helper to forward media when needed. Right now I'd love to just get the REGISTER requests doing the right thing. Any help, or even better sample config scripts would be much appreciated. Simon