two interfaces one subnet

Brielle Bruns bruns at 2mbit.com
Mon May 11 16:52:19 CDT 2009


On 5/11/09 3:23 PM, Chris Meidinger wrote:
> On 11.05.2009, at 23:19, Alex H. Ryu wrote:
>
>> Unless you configure Layer 2 for two interfaces, it's not going to work.
>> It is invalid from networking principle.
>> If you have to send the traffic for host in same subnet you configured,
>> which interface it should send out ?
>> Basically it may create broadcast storm loop by putting two ip addresses
>> in same subnet in different interface.
>> It may be allowed from host-level, but from router equipment, I don't
>> think it was allowed at all.
>
> Alex, I _personally_ know that it's a problem. I was hoping for an
> RFC-reference, or similar standards document, to show to customers to
> convince them to stop trying to hack things to make it work.
>
> Chris
>

In Linux, I ran into the exact situation talked about in the link:
http://lwn.net/Articles/45373/


Basically, recent versions of Linux will respond to arp requests for IPs 
on another interface on the receiving interface.  Basically, you end up 
with traffic going in/out of unexpected interfaces. I discovered my 
iptables rules weren't quite working right and I couldn't get into one 
of my boxen because the allow was set to eth0, and the packets were 
coming in/out of eth1 even though the IP was bound to eth0.

One of the more interesting gotchas that had me stumped for hours before 
I found out what was really going on.
-- 
Brielle Bruns
The Summit Open Source Development Group
http://www.sosdg.org    /     http://www.ahbl.org




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