static default route not always pushed to kernel (2.0.4)

[Kenth Eriksson]

On Mon, 2019-04-29 at 14:46 +0200, Ondrej Zajicek wrote:
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> On Mon, Apr 29, 2019 at 12:14:56PM +0000, Kenth Eriksson wrote:
> > Hi!
> > 
> > There are cases where bird does not push a static default route to
> > the
> > kernel. One way to see this is as follows;
> > 
> > Next remove both static routes, configure, add one of them back
> > again
> > and both are shown.
> > 
> > bird> show route for 0.0.0.0
> > Table master4:
> > 0.0.0.0/0            unicast [static1 14:13:16.762] * (255)
> >       via 10.210.137.1 on eth1
> >                      unicast [kernel1 13:46:07.958] (215)
> >       via 10.210.137.1 on eth1
> 
> Hi
> 
> Yes, BIRD on Linux intentionally avoids replacing any existing non-
> BIRD
> (alien) routes in kernel tables.
> 
> The reason is: Consider route A from BIRD and alien route B, both for
> the
> same network. A is preferred in BIRD and therefore pushed to kernel,
> where it replaces route B. Therefore, route B is overwritten and
> disappears. Then route A is for some reason removed, but route B no
> longer exists, so it cannot be restored and we end with no route.
> 
> There are two ways how to fix that: one way is that kernel would
> remember
> all alien routes it learned, even when they were replaced by BIRD
> route.
> This would solve the problem above, but has other issues (e.g. the
> original source of alien route may want update or remove it, but it
> is
> no longer in kernel table).
> 
> The approach we use is that we expect to have dedicated kernel metric
> value (by default 32) that is not used by alien routes (as kernel
> table
> keeps multiple routes with different kernel metric). This works well
> with IPv6, where default kernel metric used by alien routes is higher
> (256 or 1024) but in IPv4 the default kernel metric is 0 (most
> preferred),
> so we cannot override such routes without replacing them. The answer
> is
> to create alien routes with higher kernel metric (easy when created
> with
> 'ip' tool, perhaps harder in other cases). It would be great if there
> existed sysctl option for default IPv4 route metric.
> 

There is no overwrite involed here. The default route in the kernel
here has metric 100. As you said, bird pushes with metric 32 so it
should push again. 

kenth ~ # ip route show
default via 10.210.137.1 dev eth1  proto bird  metric 32 
default via 10.210.137.1 dev eth1  metric 100 

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> Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
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> "To err is human -- to blame it on a computer is even more so."