[SR-Users] Problem on multiple interfaces and conversion between UDP and TCP

[Klaus Darilion]

Am 16.03.2011 15:06, schrieb David:
> Hello,
> 
> I have a Kamailio 3.0 server with two interfaces. 192.168.x.x and a
> public interface.
> 
> I send PUBLISH out on the private interface to my presence server.
> 
> Everything else happens on the public address.
> 
> I did not use the "listen=" option in the config, so Kamailio was
> automatically detecting my interfaces to bind to.

I always specify explicitly the sockets, thus I do not know hwo the
"auto" feature works.

> Last night I had a provider sending packets in on TCP and Kamailio was
> sending out on UDP. Unfortunately, it kept using the private interface
> despite that the IP was unreachable on this interface.
> 
> It looks like Kamailio uses the first interface it finds when converting
> between TCP and UDP, and I believe that on the server, the private
> interface was being listed first.
> 
> I finally added listen= in kamailio with all 4 interfaces ( tcp/udp
> public and tcp/udp private ). I listed the public UDP interface first,
> restarted and the packets started going out on the public interface.
> 
> Is this a feature or a bug ?

Probably it is just by design. If you do not tell Kamailio which
interface to use (force_send_socket) it somehow has to guess. If usually
tries to use the same socket on which the message was received. Probably
in case of protocol change that does not work.
> 
> How does Kamailio decide which interface to use ? What if I sent a
> packet to an IP that is only reachable on one interface?

Either force the interface, or use mhomed=true (multihomed).

Actually, I always thought that the interface on which the request will
be sent is chosen by the OS (based on routing table) - Kamailio just
decides which src-IP address to use in the SIP message. If you set
mhomed=yes, then it opens a new socket to find out which IP will be
choosen be the OS, close the socket, use the IP in the message header,
and send it on the respective socket.

> At the very least, I think that a note about this difficulty should be
> included in the default config so that future users don't have the same
> issue I did.

Patches are welcome.

regards
Klaus