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Date:	Tue, 12 Jun 2012 07:14:44 -0400
From:	Neil Horman <>
To:	David Miller <>,
Subject: Re: [PATCHv2 net-next] ipv4: Add interface option to enable routing

On Tue, Jun 12, 2012 at 06:44:01AM -0400, Thomas Graf wrote:
> Routing of 127/8 is tradtionally forbidden, we consider
> packets from that address block martian when routing and do
> not process corresponding ARP requests.
> This is a sane default but renders a huge address space
> practically unuseable.
> The RFC states that no address within the 127/8 block should
> ever appear on any network anywhere but it does not forbid
> the use of such addresses outside of the loopback device in
> particular. For example to address a pool of virtual guests
> behind a load balancer.
> This patch adds a new interface option 'route_localnet'
> enabling routing of the 127/8 address block and processing
> of ARP requests on a specific interface.
> Note that for the feature to work, the default local route
> covering 127/8 dev lo needs to be removed.
> Example:
>   $ sysctl -w net.ipv4.conf.eth0.route_localnet=1
>   $ ip route del dev lo table local
>   $ ip addr add dev eth0
>   $ ip route flush cache
> V2: Fix invalid check to auto flush cache (thanks davem)
> Signed-off-by: Thomas Graf <>
Just out of curiosity, would it be more efficient to implement this by
optionally adding a prohibit route to the local table for to every
interface that was brought up, based on weather or not that interfaces
route_localnet bool was true or not?  It would save the additional checks in the
routing path I think.  Not sure how much a savings that is, but I thought I
would ask.


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