Hi all,

I just started deploying Kamailio release 1.5.4, and I think there's an issue with how Kamailio identifies an outgoing interface when mhomed is enabled under Linux.

I use Kamailio as a call distributor/proxy between a soft-switch/SBC and a voicemail platform. It runs on a CentOS 5.3 (Linux 2.6 kernel) host with two network interfaces and is configured such that it listens on both interfaces. One interface (public interface) handles traffic with the SBC, the other (private interface) handles with the VM platform. The 'mhomed' option is enabled.

After upgrading from 1.5.3 to 1.5.4, I started noticing problems with UDP packets coming out of the public interface. After looking at some ngrep captures on that interface, I noticed that some packets had the source IP address of the private interface and also had Record-Route and Via headers for the private interface only - no headers for the public interface were there.

Usually when I see the wrong source IP in a UDP packet, it's an issue with how routes are set up on the host. However, I had our network engineer double check them, and they seem fine (no ambiguous routes). The fact that I captured these messages on the public interface also indicates to me that the kernel is routing the message correctly. The missing Record-Route and Via for the public interface, however, lead me to believe that the proxy didn't correctly identify the outgoing interface in the first place.

After looking at the ChangeLog for 1.5.4, I noticed that the some new logic was put in to improve performance when mhomed is enabled (r5971) in forward.c, and I think this is the issue.

As I understand it, prior to 1.5.4, when mhomed was enabled, Kamailio determined the outgoing interface by creating a temporary UDP socket, invoking connect() on the socket with the packet destination, then checking the source IP of the socket that the kernel assigned using getsockname(). After the source address was determined, the temp socket was closed closed. As of 1.5.4, this was modified to reuse the temporary socket and just re-invoking connect() with a new destination address.

The problem with the enhancement is that Linux (again, at least in the 2.6 kernels I'm using) doesn't seem to rebind a new source address to the socket when connect() is called more than once on a UDP socket. Instead, it keeps the original one, and thus the wrong interface is assumed.

I wrote a small program to confirm this - basically creates a UDP socket, calls connect()/getsockname() multiple times using different destination addresses. I ran it on several 2.6 kernels, including Centos4.x and Centos5. The result was always that the source address of the socket wasn't changed after the first connect(), regardless of the destination address. The only way I could get it work as required was to first do a connect() using a zero'd out AF_UNSPEC address before doing the connect() to the remote address. I also ran it on Solaris and it worked. Go figure.

I've downloaded the latest stable release (3.1.2) but I think the issue is still there, and I don't see anything in the user groups that addresses this.

Any help would be appreciated.

Thanks,

Sean