On Mon, 28 Sep 2009, Jason Frisvold wrote:

On Sep 28, 2009, at 4:55 PM, Nathan Stratton Treadway wrote:

Well, as I understand it, the kernel uses the source IP that matches the
outgoing route entry that it has selected for sending the packet in
question.  When the routes are choosing between interfaces on separate
subnets, this generally does exactly what you want, since it means the
source IP of the outgoing packet is the one assigned to the interface
actually used to send the packet, and a return packet would thus come
back in via that same interface.

I haven't investigated this as closely in the context of two IP numbers
assigned to the same physical interface, but by analogy it would seem
that if the kernel it choosing the other IP, it's now choosing a
different route entry for sending the packet.  Since both entries point
to the same subnet, it might be making the selection using some very
minor criteria.  (For example, if both routes have the same netmask, it
might be as simple as which route appears first in the routing table.)

Well, it used to just use the primary IP, the one assigned to eth0. I'm not sure if the newer kernel I'm running is causing this odd behavior, or if something else is affecting it. I've been running this kernel for a while, but I've only rebooted it once, so maybe the latest reboot cause it to switch for some odd reason. I'll dig more into it with a non-production machine and see what I can figure out.

(When you say "this solves my problem", what exactly did you do to get
things working?)

I deleted the existing default route using ip route and added a new one, to the same place, but putting in the proper src address.

                                                Nathan

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Jason 'XenoPhage' Frisvold
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friz AT godshell DOT com
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