[Serusers] UA behind symmetric NAT advertising Public address

[Jiri Kuthan]

At 05:17 AM 3/18/2004, zeusng wrote:
>Hi,
>
>Is that a way to check the Top most Via rport with the sender's port?
>
>I have a UA capable of detecting its symmetric NAT's public address and
>advertise it in the SIP message. Below is an example of the message. The
>device has private address 192.168.x.x and is set to listen SIP message at
>port 5070, the NAT's public address is y.y.y.y, the register is listening at
>public address z.z.z.z port 5060.
>
>
>UA1 192.168.5.5:5070 --> NAT (y.y.y.y:60500) --> Proxy z.z.z.z:5060
>
>
>SEND >> z.z.z.z:5060
>REGISTER sip:xyz.org SIP/2.0
>Via: SIP/2.0/UDP
>y.y.y.y:5070;rport;branch=z9hG4bKA38B122163B34FA9B039C915AA2ACB43
>From: UA1 <sip:ua1 at xyz.org>
>To: UA1 <sip:ua1 at xyz.org>
>Contact: "UA1" <sip:ua1 at y.y.y.y:5070>
>Call-ID: F75DAF8081B44B62905C14B20CC7421C at xyz.org
>CSeq: 7900 REGISTER
>Expires: 1800
>Max-Forwards: 70
>User-Agent: Some SIP User Agent
>Content-Length: 0
>
>RECEIVE << z.z.z.z:5060
>SIP/2.0 200 OK
>Via: SIP/2.0/UDP
>y.y.y.y:5070;rport=60500;branch=z9hG4bK381E2433E71D4FE995B208FFD8AF8F15
>From: UA1 <sip:ua1 at xyz.org>
>To: UA1 <sip:ua1 at xyz.org>;tag=794fe65c16edfdf45da4fc39a5d2867c.8eac
>Contact: "UA1" <sip:ua1 at y.y.y.y:5070>
>Call-ID: F75DAF8081B44B62905C14B20CC7421C at xyz.org
>CSeq: 7900 REGISTER
>Contact: <sip:ua1 at y.y.y.y:5070>;q=0.00;expires=1800
>Server: Sip EXpress router (0.8.12-tcp_nonb (i386/linux))
>Content-Length: 0
>Warning: 392 z.z.z.z:5060 "Noisy feedback tells:  pid=18470
>req_src_ip=y.y.y.y req_src_port=60500 in_uri=sip:xyz.org out_uri=sip:xyz.org
>via_cnt==1"
>
>Here, the client 192.168.5.5 (not show in the message) send a message with
>source address 192.168.5.5 source port 5070 to proxy z.z.z.z port 5060.
>Because of some intellegence in the UA (STUN in this case), it determines
>that the public address of the NAT is y.y.y.y.
>
>When contructing the Register request, it use the public IP y.y.y.y in the
>Via and Contact header. The message, upon reaching the NAT, be translated to
>have source address y.y.y.y and port 60500. None of the nat_uac_test()
>function can detect that the UA is behide NAT and thus save the contact as
>sip:y.y.y.y:5070. The reply of the register is fine as it follow the rport.
>
>If however, someone invite this client, SER will return sip:y.y.y.y:5070.
>Due to the nature of symmetric NAT, port 5070 is block and cannot reach UA1.

Then "Some SIP User Agent" is broken. STUN can't be used with symmetric NATs.
y.y.y.y is of zero value.

-jiri

>I know I can setup PAT to have all incoming traffic with port 5070 to
>forward to 192.168.5.5 and make is looks like restricted cone NAT but the
>NAT device is own by the customer and I have no control.
>
>Is there a way in SER that I can check the rport with the source port, if
>different, rewrite the contact as sip:[source address]:[rport]?
>
>Regards,
>
>
>
>Zeus Ng, CISSP, CCSA
>Principal Consultant
>iSquare Technology
>Tel: +61 2 9419 3887
>Fax: +61 2 9410 2629
>Mob: 0416 135 794
>Email: zeus.ng at isquare.com.au
>
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--
Jiri Kuthan            http://iptel.org/~jiri/