Re: IPv6 default routes / NM vs. kernel autoconfig

[Pavel Simerda <psimerda redhat com>]

----- Original Message -----
> From: "Ross Vandegrift" <ross kallisti us>
> To: "Pavel Simerda" <psimerda redhat com>
> Cc: "Phil Mayers" <p mayers imperial ac uk>, networkmanager-list gnome org
> Sent: Tuesday, July 24, 2012 3:38:04 PM
> Subject: Re: IPv6 default routes / NM vs. kernel autoconfig
> 
> On Mon, 2012-07-23 at 20:12 -0400, Pavel Simerda wrote:
> > > ::/0              fe80::21b:2bff:fec1:dcc1   UG   1   0     0 em1
> > > ::/0              fe80::217:e0ff:fe43:5941   UGDAe 1024 0     0
> > > em1
> > > ::/0              fe80::21b:2bff:fec1:dcc2   UGDAe 1024 0     0
> > > em1
> > > 
> > > Note that the lower-metric route is for the IPv6 host ending in
> > > dcc1,
> > > but the two higher-metric routes, which I assume come from the
> > > kernel
> > > autoconf, are different hosts - dcc1 and 5941.
> > 
> > The lower-metric route should be from NetworkManager and it should
> > reflect
> > the default route for the device that is used for connectivity. In
> > your
> > case it seems to be nonsense.
> 
> Why does NM do this?  Iv6 autoconf is a dynamic routing protocol that
> chooses the best available default on the network.

I guess it's because NetworkManager devels (and users) generally don't agree
that kernel's dynamic preference is useful enough. I don't think that
VPNs and such stuff could work leaving several default routes with the same
metric.

> *Any* static
> default breaks this mechanism, and should only be used to workaround
> a
> broken network configuration.
> 
> Though a router did a somewhat surprising thing (coming up with a
> different link-local address), the network here is working perfectly
> well.
> 

Software has bugs.

> > > We took a look at the router, and found that, before reboot, it
> > > had
> > > been
> > > using blahblah:dcc1 as the link-local, but after reboot it
> > > started
> > > using
> > > blahblah:dcc2.
> > 
> > Currently, NetworkManager doesn't cope well with changes like this
> > and
> > unfortunately this applies to the Git version of NM as well. I hope
> > to improve this at some point of time but even the kernel is not
> > very
> > helpful here.
> 
> Note that IPv6 autoconf would've handled this absolutely gracefully,
> if
> NM had not interfered - the box would've expired the default, and the
> other RA would become active. The right thing to do here is to let
> the
> protocol work as designed.

Unfortunately NetworkManager is there to handle many more cases than
only the simple ones.
 
> > > My main question: What should happen in this situation? Is this a
> > > bug
> > > in NM?
> > 
> > NM is working around lack of information from kernel and it works
> > only in
> > basic situations. There are solutions to this.
> 
> NM only lacks information if you require that NM track the state of
> the
> network stack.  This might be required for DHCP interfaces, since NM
> needs to manage the DHCP agent.  But what is the use-case for
> autoconf?

You can't use DHCP without router advertisements, se below.

> If someone desires different behavior, then the right thing to do is
> disable autoconf and use a different interface configuration method
> (DHCP, static config, etc).

DHCPv6 doesn't handle default routes at all. You always use router advertisement
for that in IPv6. It is a misconception to view RA and DHCP as two disjoint
configuration methods.

Cheers,

Pavel