netdev - Re: The mystery of optimistic ipv6 DAD handling

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Message-ID: <20111228192150.GA16499@neilslaptop.think-freely.org>
Date:	Wed, 28 Dec 2011 14:21:50 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: The mystery of optimistic ipv6 DAD handling

On Wed, Dec 28, 2011 at 01:39:33PM -0500, David Miller wrote:
> From: Neil Horman <nhorman@...driver.com>
> Date: Wed, 28 Dec 2011 10:19:28 -0500
> 
> > Yeah, If the premise that the test never triggers is true (and it seems like
> > you've confirmed that), then theres no harm in removing it.  It still
> > seems like we might still need the test somewhere (maybe on a manual route add).
> > I've family in town, so I'm not able to look at this closely today, but I'd say this
> > is at least a harmless change right now.  Lets take this change, and I'll
> > re-read the RFC and look for appropriate code paths where we may need this
> > test in a few days.  Thanks Dave!
> > 
> > Acked-By: Neil Horman <nhorman@...driver.com>
> 
> Thanks for reviewing Neil, I've applied this patch to net-next.
No problem, I got some time this afternoon, and I've reread the relevant section
of RFC 4429, and clearly I made a mistake in the above test, but I'm a little
confused over what the test should be.  The relevant language from the RFC is:

3.3 Modifications to RFC 2462 Stateless Address Autoconfiguration

   * (modifies section 5.5) A host MAY choose to configure a new address
        as an Optimistic Address.  A host that does not know the SLLAO
        of its router SHOULD NOT configure a new address as Optimistic.
        A router SHOULD NOT configure an Optimistic Address.

So my read of that is that, when we are adding a new address to the host, we
should not make the address optimistic if:

1) The interface is acting as a router - We catch this with the !forwarding
check in addrconf_prefix_rcv, although we should perhaps move that to a common
code path, as we won't catch that if we manually add an ipv6 address via
inet6_rtm_newaddr

2) If the interface has not recieved a router advertizement, and that
solicitation did not contain a source link layer option in it.

Clearly I misread the RFC the first time through and the test as it existed is
incorrect.  I'm wondering though if what we need is a way to lookup if we've
received a RA on the interface, and check to see if it has a source link layer
option attached to it.

Thoughts?
Neil

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