UDP communication using aliased IP address

[Ashwani Wason <ashwas@eternal-systems.com>]

I have this UDP client-server system in which the server has bind'ed to 
an aliased IP address. The client is sending requests and I see (using 
tcpdump) that the replies have the source IP as the aliased IP address. 
The client does a recvfrom and validates that the source address is the 
address of the server. I move the server to another machine, for which I 
delete the alias on the first one. I see that client receives a reply 
from a server that was sent by the server application before the move 
and that carries the source IP address as the machine's actual address 
and not the aliased address! The client rejects this packet (and I know 
it is fine because this is UDP and all..)

But I was wondering why was the source address not the aliased address?
Buffering seems to be the reason to me. I always thought, on the sender 
side the UDP packets are not buffered except by the I/F driver and when 
the IP datagram is there it should not be changed..

This is reproducible every time using my relatively complex 
applications. But I dont know how to reproduce this "buffering" (if any) 
with a simple UDP client-server. I have tested and confirmed that if the 
server sends a message on the socket after the alias has been deleted it 
gets error 22: invalid argument, but what about the packets that are 
already in the kernel (if they are)? what happens to those? does message 
size have something to do with it?

Thank you.

--
Ashwani Wason
ashwas@eternal-systems.com
Tel: +1 (805) 696 9051 x244; Fax: +1 (805) 696 9083
Eternal Systems, Inc.
5290 Overpass Road, Building D, Santa Barbara, CA 93111
www.eternal-systems.com

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