[Asterisk-Users] Asterisk, Configuration of SDP in SIP messages

[Brian Cuthie]

Alex,

The media ports are configured in rtp.conf.  Also, note that Asterisk 
sends RTP packets out the same ports it expects them to return on. This 
has the effect of creating a NAT mapping for that 5-tuple, as well as 
opening a hole in your firewall (naturally, YMMV depending on exactly 
what you're running for a firewall).

One interesting consequence of the way Asterisk works is that if you 
don't have anything behind the NAT/Firewall that's generating RTP 
packets (ie, no audio) no hole gets made and incoming packets will get 
rejected.  I recently ran into an interesting problem with two SIP 
phones trying to talk through Asterisk behind a (non-NAT) firewall. 

The problem was both phones were sending RTP to the Asterisk box but the 
firewall was blocking both RTP streams because Asterisk never sent any 
RTP out those ports. And the reason Asterisk hadn't sent RTP out those 
ports was because it was waiting for RTP from each of the two SIP 
phones. This was the classic chicken-and-egg scenario. 

I resolved it by opening up the firewall for the range of ports I had 
configured Asterisk to use for RTP.  A better solution would be fore 
Asterisk to always send a "starter" RTP packet so that it can ensure 
that the firewall opens up.

-brian


Alexander Simeonidis wrote:

> Hello everybody,
>
> I'm new to Asterisk and I'm trying to configure the SIP side.
>
> I use Asterisk under the following configuration:
>
> SIP Proxy ---- INTERNET ---- | NAT FIREWALL | ---- Asterisk ---- SIP 
> Phone A
>
> I'm trying to put a call from SIP Phone A through Asterisk to the SIP 
> Proxy. I'm able to deliver messages to SIP Proxy. However, I have 
> noticed that the port used to deliver the audio changes randomly. I 
> would like to fix that to a specific range of ports so that I can tell 
> to NAT Firewall to port forward these particalar ports to Asterisk. I 
> have searched on documentation and the only thing that I found was how 
> to change the SIP port but not the media port. Has anybody any ideas 
> on how to solve that problem or where to look for a solution?
>
> Regards,
>
> Alex.
>
>
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