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 From:  "Chris Buechler" <cbuechler at gmail dot com>
 Cc:  m0n0wall at lists dot m0n0 dot ch
 Subject:  Re: [m0n0wall] Fritzbox Fon WLAN with SIP Proxy
 Date:  Wed, 9 Jan 2008 17:25:59 -0500
On Jan 9, 2008 11:27 AM, Henning Andreseck <AndreseckH at gymszbad dot de> wrote:
>  Hi,
>  i'm new to this sip-proxy and thought maybe i can use it with the fritzbox
> i have here. now i use the nat to connect the fritzbox. this works, but not
> really good. as example: it's not possible to have 2 calls at the same time.
>  so is it possbile to use the sip-proxy with the fritz.box?

If it uses SIP, I would assume so. I'm not familiar with fritz box.

> and, the
> question's a bit stupid, but what actualley does the sip-proxy?

SIP traffic uses source and destination port 5060, and rewriting the
source port generally breaks SIP. Because m0n0wall's underlying NAT
only works at layers 3 and 4, it can't NAT from multiple internal
sources to the same external destination using the same source port,
only the first connection works. The SIP proxy intercepts the SIP
traffic and tracks it in a fashion that works around this limitation.