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

[Johan Wilfer]

2013-07-01 15:04, Daniel-Constantin Mierla skrev:
> 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
>

I think that you are right that this should be done with Kamailio.
Maybe the nathelper-module in Kamilio would do the trick in modifying 
the SDP/Contact to the NAT:ed address instead of using rtpproxy.

Thanks for the feedback!

/Johan