[SR-Users] about kamailio.cfg NAT test

[Ben Kaufman]

Shouldn't *TCP* use an ephemeral source port, and expect replies to be to the source port? 

Ben Kaufman


-----Original Message-----
From: sr-users <sr-users-bounces at lists.kamailio.org> On Behalf Of Daniel-Constantin Mierla
Sent: Tuesday, March 24, 2020 9:01 AM
To: Kamailio (SER) - Users Mailing List <sr-users at lists.kamailio.org>; Juha Heinanen <jh at tutpro.com>
Subject: Re: [SR-Users] about kamailio.cfg NAT test

Hello,

this is the test to detect devices behind NAT that use STUN, so they discover properly the public IP of the NAT router, but the port allocation is different for STUN and SIP traffic.

If you have an asymmetric signalling client, then this test is not useful -- actually nat traversal cannot be done for asymmetric signalling (when client sends from one address (ip+port) and expects traffic back to another address). You haven't included the contact address, but anyhow probably the reply is not expected on port 44717 as it is indicated in Via. The phone expects some natted environment, as it requested "rport" handling (send back the reply to the port from where the request was received). If it wanted asymmetric signalling, then rport is not needed. If it knew it sends and receives to the same port, also rport would not be needed.

Maybe this is just baresip approach not to detect local port of the tcp connection, putting a random value in Via and setting rport parameter.

You can tune the test for your specific environment/client apps, but for the vast majority of the cases I encountered over the years, this test is the best one to detect clients behind nat that are using stun.

Cheers,
Daniel

On 24.03.20 13:01, Juha Heinanen wrote:
> In kamailio/etc/kamailio.cfg NAT test is based on nat_uac_test("19").
> 19 includes test 16:
>
>   16 - Test if the source port is different from the port in the “Via”
>   header. If the “Via” header contains no port, it uses the default SIP
>   port 5060
>
> Based on a couple of tests using baresip, looks like that results in 
> false positive when UA connects to SIP proxy via TCP.
>
> An example:
>
> T 2020/03/24 13:57:12.685441 192.26.134.10:38940 -> 192.26.134.1:5060 
> [AP] #127 REGISTER sip:test.tutpro.com SIP/2.0.
> Via: SIP/2.0/TCP 192.26.134.10:44717;branch=z9hG4bK5b6aa423104ef942;rport.
>
> Does someone know why test 16 is included?
>
> -- Juha
>
> _______________________________________________
> Kamailio (SER) - Users Mailing List
> sr-users at lists.kamailio.org
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--
Daniel-Constantin Mierla -- www.asipto.com www.twitter.com/miconda -- www.linkedin.com/in/miconda


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