How do we determine IP Address of interface the packet arrived on

[Valdis.Kletnieks at vt.edu]

On Wed, 24 Sep 2014 10:08:29 -0400, John de la Garza said:

> why not look at the destination ip of the ip header?

It's possible to receive a packet on the "wrong" interface.

For instance, we have a software package installed here that
insists on talking on one IP address, so we end up with this
setup:

world - 198.82.X.Y- |BOX| - 172.28.40.z - private 10G net - clientbox

and the client box does a 'route add host 198.82.x.y gw 172.28.40.z', so
when it sends packets to 198.82.x.y, it shows up on the box's 10G interface.
Said packet is then cheerfully accepted, because the destination address
matches *one* of the boxes's addresses (just on a different interface).

Another similar situation is for HA, where you'll have a pair (or more) of
boxes that each have their own IP address on a subnet, and a floating IP that's
used by client machines to actually access the service, and which moves back
and forth if a box fails or is down for maintenance. So the active one of the
HA pair has 2 addresses on the interface.

And that's not even starting on the case of "Linux-based router", where
*most* packets you handle don't have your IP address in the destination
field, because you are going to forward it.
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