netdev - Re: Interface without IP address can route??

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Date:	Wed, 24 Aug 2011 18:15:57 +0200
From:	David Lamparter <equinox@...c24.net>
To:	Ben Greear <greearb@...delatech.com>
Cc:	jhs@...atatu.com, jamal <hadi@...erus.ca>,
	netdev <netdev@...r.kernel.org>
Subject: Re: Interface without IP address can route??

On Wed, Aug 24, 2011 at 06:24:54AM -0700, Ben Greear wrote:
> On 08/24/2011 06:01 AM, jamal wrote:
> > It makes sense to behave this way.
> > IPv4 addresses are owned by the system not interfaces.
> > If you want to control the forwarding behavior, control ARP so it doesnt
> > respond on the interfaces with no IP.

I agree.

> I understand your argument about IPs being owned by system instead of
> interface, but I think it's the wrong behaviour in this case.  Can
> you think of any case where this behaviour actually helps?

It's used for oddball /32 setups at server hosting farms that look like:
      /--- eth0, no ip ---- server 0.1.4.5/32, default via 0.1.2.3
router --- eth1, no ip ---- server 0.1.6.7/32, default via 0.1.2.3
      \--- eth2, no ip ---- server 0.1.8.9/32, default via 0.1.2.3
   \- eth3: 0.1.2.3/28 - to rest of internet

The general idea is to a) conserve IPs and b) not renumber servers even
when they move, so you end up with random scattered /32s on the servers
and the router has no sensible IP.

> Either way, it appears I can work around this by explicitly disabling
> forwarding for this particular interface.

I was about to suggest exactly this :)


David

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