netdev - Re: [PATCH net-next] ipv6: addrconf: fix mcast route for GRE devices

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Message-ID: <20140730173544.GM801478@jupiter.n2.diac24.net>
Date:	Wed, 30 Jul 2014 19:35:44 +0200
From:	David Lamparter <equinox@...c24.net>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	David Lamparter <equinox@...c24.net>, netdev@...r.kernel.org,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH net-next] ipv6: addrconf: fix mcast route for GRE devices

On Wed, Jul 30, 2014 at 06:52:21PM +0200, Hannes Frederic Sowa wrote:
> On Mi, 2014-07-30 at 18:31 +0200, David Lamparter wrote:
> > On Wed, Jul 30, 2014 at 06:09:27PM +0200, Hannes Frederic Sowa wrote:
> > [cut]
> > > > On Wed, Jul 30, 2014 at 05:14:42PM +0200, Hannes Frederic Sowa wrote:
> > > > > On Mi, 2014-07-30 at 02:55 +0200, David Lamparter wrote:
> > > > > > GRE devices, for some reason, were coming up with an autoconfigured
> > > > > > address, but no ff00::/8 route in the local table.  This breaks any kind
> > > > > > of multicast, in particular OSPFv3, mDNS, - and ND.  In fact, IPv6 only
> > > > > > works at all because there is little need for ND on PtP devices.
> > > > > > 
> > > > > > Adding any other IPv6 address on the device would rectify this issue
> > > > > > through inet6_addr_add()/addrconf_add_dev() - and would leave the route
> > > > > > around even if the address was later removed.  (This is probably why
> > > > > > this issue was not discovered earlier.  AFAICS it has been there from
> > > > > > the beginning, e.g. aee80b5 "generate link local address for GRE
> > > > > > tunnel")
> > > > > 
> > > > > Yep, this is poor, but changing this will break user space...
> > > > 
> > > > How exactly will this break user space?
> > > 
> > > Because the multicast routes will always be restored after e.g. a route
> > > flush or manual route deletion. Scripts might depend on this.
> > 
> > Sorry, I still don't get it.  Without this patch you end up in an
> > inconsistent state, where a LL addr exists, but multicast doesn't work
> > (since ff00::/8 is missing from RT6_TABLE_LOCAL).
> 
> Sure, people can remove addresses and routes at will.

That's not the point.

user manually adds address on any if: ff00::/8 added.
kernel link-up autoconf addr on !gre: ff00::/8 added.
kernel link-up autoconf addr on gre:  no ff00::/8.

This is about automatic kernel behaviour in device up/change
notifications.  The user can always shoot himself in the foot.  This is
the kernel shooting the user in the foot in one particular and quite
specific instance and without a request to do so.

> > Userspace is not supposed to touch RT6_TABLE_LOCAL in general, and, the
> > kernel will actually refuse installing the ff00::/8 route into the local
> > table from userspace (because there will be other ff00::/8 routes from
> > other interfaces, so you get "File exists").  You can delete the route
> > (and thus break mcast), but not add it.  The only way to add it is to
> > add an address.
> 
> People really do flush the routing table.

This is RT6_TABLE_LOCAL.  Most people aren't even aware it exists.  And
even though I can't find a reference for it, my memory tells me that
"table local" is supposed to be under the kernel's authority.

> I'll have a look why the addition of the multicast route throws an
> error.

While that may be interesting, it won't fix the issue at hand.

(It's probably because the output device is not part of uniqueness
considerations for ff00::/8.  Might need to be treated as "scope link".)

> > Not changing this behaviour keeps breaking userspace;  ospf6d among
> > other things assumes an interface has working IPv6 when a link-local
> > address is present.
>
> Yeah, but in the end, people also can drop specific packets and we
> cannot do anything.

See above, this isn't the user screwing himself, this is the kernel
doing the wrong thing in one specific corner case.  (Which shouldn't
even be a corner case.)


-David
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