netdev - Re: [PATCH net-next RFC] ipv4 neigh: match add/del dev lookup behaviour to SIOCSARP/SIOCDARP

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Date:	Tue, 24 Feb 2015 16:03:23 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	roopa@...ulusnetworks.com
Cc:	hadi@...atatu.com, netdev@...r.kernel.org,
	hannes@...essinduktion.org
Subject: Re: [PATCH net-next RFC] ipv4 neigh: match add/del dev lookup
 behaviour to SIOCSARP/SIOCDARP

From: roopa@...ulusnetworks.com
Date: Mon, 23 Feb 2015 21:42:52 -0800

> From: Roopa Prabhu <roopa@...ulusnetworks.com>
> 
> Today neigh adds and dels using the netlink RTM_NEWNEIGH/RTM_DELNEIGH
> messages require dst device to be specified. The equivalent ipv4 neigh
> entry add/del using SIOCSARP/SIOCDARP does not require a device. It finds
> the dst dev by doing a route lookup (ip_route_output).
> 
> This patch tries to match netlink ipv4 neigh add/del behaviour with
> SIOCSARP/SIOCDARP for the case where user does not specify a device.
> If user has not specified a dst device, it looks for the device via a
> route lookup. If the route lookup fails, the behaviour is unchanged,
> the user will get an -EINVAL as before
> (This can be changed to return -ENODEV if needed. This patch leaves it at
> -EINVAL to match current behaviour).
> 
> Testing this change requires an iproute2 patch to make dev optional
> during ipv4 neigh adds/dels.
> 
> Reported-by: Jamal Hadi Salim <hadi@...atatu.com>
> Suggested-by: Jamal Hadi Salim <hadi@...atatu.com>
> Signed-off-by: Roopa Prabhu <roopa@...ulusnetworks.com>

I think the horse has already exited the barn on this one.

If people start depending upon this new behavior, they will write apps
that do not work on %99.9999999 of the kernels out there.

We are therefore stuck with the current semantics, for good or for
bad.
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