netdev - Re: [RFC] arp announce, arp_proxy and windows ip conflict verification

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Date:	Thu, 02 Jul 2009 13:36:55 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Denys Fedoryschenko <denys@...p.net.lb>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [RFC] arp announce, arp_proxy and windows ip conflict verification

Denys Fedoryschenko <denys@...p.net.lb> writes:

> On Wednesday 01 July 2009 20:40:08 Eric W. Biederman wrote:
>> The only case where I can imagine proxying the default route would even
>> approach being correct is on a point to point link.  But that seems
>> pointless as you could simply have a default route to the other side.
>>
>> Eric
>
> It seems Linux proxy_arp behavior is also RFC 1027 non-conformant.
>
> Quote from RFC:
>
>     In 4.3BSD (and probably in other operating systems), a default route
>     is possible.  This default route specifies an address to forward a
> !   packet to when no other route is found.  The default route must not
> !   be used when checking for a route to the target host of an ARP
> !   request.  If the default route were used, the check would always
>     succeed.  But the host specified by the default route is unlikely to
>     know about subnet routing (since it is usually an Internet gateway),
>     and thus packets sent to it will probably be lost.  This special
>     case in the routing lookup method is the only implementation change
>     needed to the routing mechanism.
>
>
>
> If i have linux host with 
> eth0 10.0.0.1/24
> eth1 10.0.1.1/24
>
> sysctl -w net.ipv4.conf.eth1.proxy_arp=1
>
> from other host in same ethernet segment eth1
>
> Xpernet_137 ~ # arping -I eth1 2.4.6.8
> ARPING to 2.4.6.8 from 1.0.0.1 via eth1
> Unicast reply from 2.4.6.8 [0:5:5d:2f:9b:ba] 501.685ms
> Unicast reply from 2.4.6.8 [0:5:5d:2f:9b:ba] 0.198ms
>
> And RFC define to not do that (use default route for proxy_arp).

We agree that proxy_arp for the default route is problem behavior.

The question is can you configure proxy_arp to not do that in your
configuration?

There is the medium_id tunable and the per interface proxy_arp tunable.

Eric

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