The nf-nat-sip module interferes with VoIP calls

I have been using ekiga occasionally to call my parents back home. The default VoIP provider in ekiga did not provide a very good service initially but have improved lately. A call to a landline in Bulgaria is less than $0.04/min and the quality is acceptable. Still, the service is not as good as I wish it to be.

I decided to give Gizmo a chance. I do have a Gizmo account, charged with a $0.25 sign-up bonus. I thus decided to make a call home and even though I could hear my wife very clearly (much better than with Diamondcard) she could not hear me. After some bugging with Gizmo's echo test I found the culprit.

When I send a request for the user "echo" to the proxy server it sends a reply with the address of the echo test -- a different IP, same port (5060). I really have no idea what happens next, but my outgoing voice packets are obviously not sent to the echo address.

The following is my speculation. Traffic going to/from port 5060 is being handled by the nf-nat-sip kernel module which has already seen that the current session is established between my internal machine and the proxy. Being very helpful, the module does its magic on the packets carrying my voice and (probably) redirects them from the echo address to the proxy address.

Anyway, unloading the offending modules cleanly solved my problem.

And just so I don't forget -- a list with some useful Gizmo numbers.


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