[Users] Hiding Contact or Via fields

Dear Sirs, being new to SIP and OpenSer, I have read the SIP introduction and Admin's Guide, but there still is an issue that I could not find information about. I would like to set up a SIP infrastructure made of many UAs and one SIP proxy (OpenSer). I want to bypass any NAT configuration so that I will use UAs with STUN support and mediaproxy module on OpenSer; I will perform billing (accounting) on the SIP proxy, so I will use record route on OpenSer. I want to make sure that my accounting will *never* be bypassed. This seems quite difficult because the fields 'Contact' and 'Via' contain the real address of the UA (or the translated one in case of NAT configuration) and the SIP header will be fowarded to the other UA. Sniffing the network for SIP packets, any user will be able to know the real address of the UA and make direct call bypassing the SIP proxy. How could I secure this situation, avoiding to forward Contact and Via fields containing the real UA address? I was thinking if it is possible to replace UA address with SIP proxy address, or would I break any protocol rule? Thank you in advance!! Best regards, -- Marco Meinardi <m.meinardi(a)reteitaly.com> ReteItaly S.r.l. www.reteitaly.com _____________________________________________ C.so Svizzera, 185 - 10149 Torino - Italy Tel. +39 011 7767694 - Mobile +39 335 7878604 Fax +39 011 746179 _____________________________________________