[Users] Hiding Contact or Via fields

[Marco Meinardi]

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 at reteitaly.com>

ReteItaly S.r.l.
www.reteitaly.com
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C.so Svizzera, 185 - 10149 Torino - Italy
Tel. +39 011 7767694 - Mobile +39 335 7878604
Fax +39 011 746179
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