________________________________
From: Zabaluev Mikhail (Nokia-D/Helsinki)
Sent: Monday, February 09, 2009 5:14 PM
To: drostow...@airbiquity.com; sofia-sip-devel@lists.sourceforge.net
Subject: Re: [Sofia-sip-devel] Multiple ethernet interface support?
How does Sofia SIP handle multiple Ethernet interface support? I’m having a
problem where my linux box has eth0=172.16.1.1, and eth1=192.168.15.115. When
Sofia sends an INVITE out to the destination 192.168.15.13, my expectation was
that the source IP address should be 192.168.15.115. However, Ethereal captures
show the source IP address is actually 172.16.1.1. The INVITE is actually being
sent out eth1 as I have physically unplugged 172.16.1.1. How does Sofia’s SIP
socket determine to use eth0’s source IP address? It should be using eth1 as
the source IP address.
The stack has no clue by default. It binds to INADDR_ANY and uses the first
interface entry for the source address. There is no standardized way to ask the
system which route will be taken to reach a destination address (not to mention
this would be inherently racy). We tried out some Linux-specific magic in our
team, but this is utterly non-portable and may even break between kernel
releases. We're not at the stage where it works in a Sofia-SIP branch.
To think of it, a simple heuristic would be to look at the routing table and
see which interface has the default route. This should work in most cases, e.g.
VPN. Pekka, what do you think?
Best regards,
Mikhail
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