[asterisk-users] Asterisk behind NAT and Kamailio --> Internal IP in SDP and not "externip"

[Daniel-Constantin Mierla]

Hello,

On 6/28/13 4:29 PM, Johan Wilfer wrote:
> Hi,
>
> We have some Asterisk servers that we are moving behind a NAT to 
> preserve public addresses and make room for growth. This is Asterisk 1.4
>
> NAT works very good with the externip/localnet-setting when we are 
> connected directly to our teleco. But when I try to use NAT and put 
> them behind our Kamailio something interesting happens: The 
> media-address in the SDP is the internal ip and not the external.
>
>
> This is the setup:
>
> Teleco <-> Kamailio <-> Asterisk
>   SIP -->  1.2.3.4
>            10.0.0.1 --> 10.0.0.2
>
>                         externip=1.2.3.5
>                         localnet=10.0.0.0/255.255.255.0
>
>
>   RTP <----------------> 1.2.3.5 (NAT:ed to 10.0.0.2)
>
>
> On an incomming call from the teleco -> to kamailio (public addr) -> 
> to asterisk in the private net. Asterisk responds with the following SDP:
>
> v=0
> o=root 1889 1889 IN IP4 10.0.0.2
> s=session
> c=IN IP4 10.0.0.2
> t=0 0
> m=audio 23344 RTP/AVP 8 0 101
> a=rtpmap:8 PCMA/8000
> a=rtpmap:0 PCMU/8000
> a=rtpmap:101 telephone-event/8000
> a=fmtp:101 0-16
> a=ptime:20
> a=sendrecv
>
> Asterisk seems to think that because the proxy is on the localnet, the 
> media is too, so it doesn't use the externip as the RTP-ip.
>
> This is a incomming call and the RTP ip of the other leg is another 
> public address. So the RTP-ip should the public address (externip).
>
> If I connect to the teleco directly from the pbx (bypassing kamailio) 
> Asterisk correctly uses the externip as the rtp-ip in the SDP.
>
>
> I know this is an old and unsupported version of Asterisk, but any 
> input on the topic is welcome. If this is supported in later versions 
> we can maybe work around until we migrate later.
what I did when I had similar scenario was to let asterisk completely 
behind NAT, using only the local IP. I used rtpproxy running on the same 
host as kamailio to bridge the rtp between external and internal networks.

Cheers,
Daniel

-- 
Daniel-Constantin Mierla - http://www.asipto.com
http://twitter.com/#!/miconda - http://www.linkedin.com/in/miconda