[SR-Users] Best approach for TCP/TLS connection re-use for nated Contacts

[Giacomo Vacca]

Thanks for your answers.
I thought I could make an example with your suggestions, for documentation (I see others are interested at the moment).

Using add_contact_alias, when the 200 OK for an INVITE is received, e.g. from 10.0.9.49:61756:

SIP/2.0 200 OK
[...]
Contact: <sip:giacomo at 10.0.9.49:61750;transport=tcp>

add_contact_alias detects that received ip:port is different than the one in the Contact, so it appends an alias before relaying the 200 OK:

SIP/2.0 200 OK
[...]
Contact: <sip:giacomo at 10.0.9.49:61750;alias=10.0.9.49~61756~2;transport=tcp>

This will then be used when generating the ACK:

ACK sip:giacomo at 10.0.9.49:61750;alias=10.0.9.49~61756~2;transport=tcp SIP/2.0
[...]

which Kamailio will change with handle_uri_alias(), removing the alias and setting the received ip:port as destination:

ACK sip:giacomo at 10.0.9.49:61750;transport=tcp SIP/2.0

sent correctly to 10.0.9.49:61756.

With fix_nated_contact, same scenario, Kamailio would modify the Contact before relaying the 200 OK:
SIP/2.0 200 OK
[...]
Contact: <sip:giacomo at 10.0.9.49:61756;transport=tcp>

Which will then be used as R-URI for the ACK:

ACK sip:giacomo at 10.0.9.49:61756;transport=tcp SIP/2.0
[...]

And sent without modifications to 10.0.9.49:61756.

As stated in other references, the difference is that with the add_contact_alias approach the invited client sees its published Contact in the ACK's R-URI.

I hope this is useful.
Regards,
Giacomo


From: Klaus Darilion [mailto:klaus.mailinglists at pernau.at]
Sent: 11 March 2012 13:10
To: SIP Router - Kamailio (OpenSER) and SIP Express Router (SER) - Users Mailing List
Cc: Giacomo Vacca
Subject: Re: [SR-Users] Best approach for TCP/TLS connection re-use for nated Contacts


Am 09.03.2012 20:56, schrieb Giacomo Vacca:
One of the solutions I've found is using always add_contact_alias() in onreply_route when handling the 200 OK, and then use handle_ruri_alias() when defining the destination for the ACK.

Yes, that the pragmatic approach I always use. IMO it is also a bit of security. I do not like to blindly trust user provided data (e.g. contact header), thus I always use the IP:port from where the message was received.

regards
Klaus
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