Re: IPv6 default routes / NM vs. kernel autoconfig

[Pavel Simerda <psimerda redhat com>]

> On Tue, 2012-07-24 at 10:21 -0400, Pavel Simerda wrote:
> > > From: "Ross Vandegrift" <ross kallisti us>
> > > 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.
> 
> It isn't the kernel's preference, but the *network's*.  The kernel is
> just reporting the costs of the various routes it has heard about.
> 
> If these costs aren't useful enough, then the network is
> misconfigured,
> and the user should expect degraded performance until it is fixed.
> This is a good use case for a static workaround.
> 
> If the VPN isn't tied to at tunnel interface, then a static default
> makes sense.  But this is a case where the user has explicitly
> configured static behavior, and expects traffic to be statically
> routed
> according to that configuration.

Please feel free to file a bug report with an explanation of the benefits
of treating IPv6 different than IPv4. I guess it will be a longer discussion
with both ways having their pros and cons.

If there is some consensus reached, I'll be happy to do the patching.

> > > *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.
> 
> Do you mean NM here, or the software announcing routes?

NM, in this case. More specifically this bug is about *not implementing*
part of the IPv6 standard because of lack of time.

> In the first
> case, fair enough.  But if you mean the second, then you're saying
> that
> NM is breaking IPv6 autoconf for all users just because some other
> software might have bugs.

NM is breaking IPv6 for some users with weird routers. Patches welcome. Or
wait until someone finds time to do it for you.
 
> > > 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.
> 
> But this wasn't a simple case - there were multiple gateways
> injecting
> defaults.  NM didn't handle it because it broke the routing protocol,
> which is designed to handle more complex cases.

Nope. NM didn't handle it because it doesn't care about the default gateway
after it is once recieved. Changing it later is simply *not implemented*.

> Now, a user should surely be able to statically override a dynamic
> route.  But NM shouldn't break things for everyone

I think you will recieve better responses by not confusing the words 'me' and
'everyone'.

> just because some
> network might have a misbehaving router announcing defaults it can't
> service.

Please file a bug report with decent description to reproduce the bug and,
if you can, attach a patch. If you cannot, hopefully someone will find time
to fix it.

> > > > 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.
> 
> Fair enough, but this doesn't answer my question - what situation is
> NM
> trying to handle by overriding working routing protocol decisions
> with
> automatic static configuration that breaks things?  What about such a
> situation justifies breaking the protocol for everyone?

I'm giving no more replies to questions that misuse the word 'everyone'
just to get more attention.

Pavel