New subject: Best approach for TCP/TLS connection re-use for nated Contacts

9 Mar 2012

Hi all,
I have a scenario where it's not clear to me what the best approach should be
(Kamailio 3.2.0 on Squeeze, although I think it's a matter of routing logic only).

I have a client behind NAT that sends an initial REGISTER request with a Contact
containing private ip:port (e.g.
Contact: "GV" <sip:12345@192.168.1.2:54321;transport=tcp>
)
However, after it's challenged for authentication with a 401, it sends the following
REGISTER request with Authorization header and with a Contact containing ip:port equal to
the 'received' SIP URI (with a combination of 'rport' and
'received' parameters (e.g.
Contact: "GV" <sip:12345@[receivedIP]:[rport];transport=tcp>
)
In this case kamailio's routing logic is not marking the REGISTER as natted (no
Received field in location and no NAT flags set).

You can imagine I have a fairly standard NAT handling:

route[NATDETECT] {
        force_rport();
        if (nat_uac_test("19")) {
                if (is_method("REGISTER")) {
                        fix_nated_register();
                } else {
                        fix_nated_contact();
                }
                setflag(FLT_NATS);
        }
        return;
}

FLT_NATS won't be set, and so it won't FLB_NATB in the following processing.

This client receives a call: Kamailio relays the INVITE to it using the stored Contact.
The INVITE is correctly received by the client, because the stored Contact refers to the
connected socket ([received]:rport).
The trouble starts when then the client replies with a 200 OK containing the private
ip:port in the Contact header field (again 192.168.1.2:54321).
The ACK will then have that private ip:port in the R-URI, and its relaying will fail.

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.

I know that without traces and the full .cfg this is quite a broad question, but I wonder
if you have already had to deal with this kind of client behaviour and can provide advice
on the best practices.
This is in particular useful for the re-use of TCP-based connections.

Thanks in advance,
Giacomo

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