[SR-Users] NAT problem with recvonly calls

[David Cunningham]

Hello,

We have a problem with a SIP doorbell device which sends media one way
only, and NAT at the receiving device.

When the doorbell button is pressed it makes a call to a configured
destination. Since the doorbell only sends and doesn't receive it sends the
INVITE with sendonly in the SDP, and the destination then replies with a
200 OK with recvonly in the SDP. The problem is that the destination is
behind NAT, and its reply contains a private network IP in the SDP.

Normally Asterisk when nat=yes works around that by adjusting the
destination for RTP to be the address it actually receives audio from,
however because this device is recvonly Asterisk never receives audio from
it. This means Asterisk keeps trying to send the doorbell's RTP to the
private network IP which of course fails, and the destination never gets
the RTP from the doorbell.

We haven't found a solution in Asterisk to this, so are now looking to
Kamailio which acts as a load-balancing proxy in front of Asterisk for one.
For example, maybe we could use fix_nated_sdp, but only on 200 OK's with
recvonly.

Has anyone else encountered this, and are there any recommended solutions?

Thank you in advance!

-- 
David Cunningham, Voisonics Limited
http://voisonics.com/
USA: +1 213 221 1092
New Zealand: +64 (0)28 2558 3782
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