RE: Bug with ARP - request source address on wrong subnet

From: Richard Underwood (richard@aspectgroup.co.uk)
Date: Fri Aug 15 2003 - 08:02:08 EST


David S. Miller wrote:
>
> > I have a problem with ARP on Linux 2.4.20 (RedHat
> 2.4.20-18.8 if it
> > matters) which I believe to be a bug. While I'm willing to
> upgrade the
> > kernel, it appears to be a generic problem.
>
> No standard says we cannot behave the way we do.
>
No standard, perhaps - but how about common sense?

Unless you're suggesting that Linux has been been designed to work
best with networks where you run all your subnets over the same bit of
copper, I don't understand where you're coming from.

ARP is local to the ethernet broadcast domain and (hopefully)
therefore to a single subnet. What possible meaning could there be to an ARP
request coming from an IP address on another subnet? Or are you suggesting
that ARP responses should be routed???

No, it's a meaningless packet, and quite rightly dropped by the
router.

> You can control the behavior by setting the preferred source
> on your routes using the 'ip' command and then setting the
> /proc/sys/net/ipv4/conf/*/arp_filter sysctl values to '1'
> for the interfaces where you'd like the preferred source to
> determine what arp responses will occur.
>
As it happens, arp_filter IS set to 1 for eth0. Since we're talking
about incoming connections I don't see the relevance of using the 'ip'
command.

Also note that the documentation for arp_filter concerns ARP
*replies* not ARP requests. It is the request that is in error.

Thanks for trying to help, though.

Richard
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