netdev - Re: iproute2: adding /128 address to sit interface borks

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Message-ID: <20100227082356.67b7c0a3@nehalam>
Date:	Sat, 27 Feb 2010 08:23:56 -0800
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	Stephen Hemminger <stephen.hemminger@...tta.com>,
	netdev@...r.kernel.org
Subject: Re: iproute2: adding /128 address to sit interface borks

On Sat, 27 Feb 2010 16:03:36 +0100 (CET)
Jan Engelhardt <jengelh@...ozas.de> wrote:

> 
> On Friday 2009-07-10 17:27, Stephen Hemminger wrote:
> >On Fri, 10 Jul 2009 14:05:01 +0200 (CEST)
> >Jan Engelhardt <jengelh@...ozas.de> wrote:
> >
> >> it's impossible to do this, and the error message does not say why 
> >> that is so. No word in dmesg either.
> >> 
> >> # ip a a fc00::49/128 peer fc00::7 dev sit7
> >> RTNETLINK answers: Invalid argument
> >
> >You will need to walk through the path in the kernel.
> >In worst case, it means adding kernel printk's.
> >
> >Then if you want to be nice, make a patch to ipaddr.c in iproute
> >utilities so command line can find the error before passing to
> >kernel and give a nice error :-)
> 
> The issue here was that the peer is encoded using the broadcast
> field, and since IPv6 has no broadcast, it was rejected by
> the kernel. The error messages really need to be improved.
> 
> This year also came an article about that,
> http://lwn.net/Articles/374794/

The article overlooks the fact that netlink is a message based interface.
There is no reason that the response message can't be extended to add
additional sections to give more detailed answer. The issues are:
  * no infrastructure for this (message format, helpers, etc)
  * tedious effort to annotate all the existing error returns
  * update to command and library

The extended error reports could even be a kernel config option so the
embedded and those worried about space would not have to have it.

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