[libvirt] Static Route for IPv6

[Gene Czarcinski]

In the process of adding the code to support static routes for IPv4 and 
IPv6, I hit a problem when I tried to specify a static route for ::/0 
(that is "default") ... the error was "file exists." and the problem is 
that this is a feature and not a bug.  Here is the comment from the 
bugzilla report which explains things:
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yeah, this isn't a bug as far as I can see.  The magic that your looking for is
in fib6_add_rt2node().  When adding a new route, we search for duplicate
routes, and should we find one, we compare them to ensure that when the ipv6
stack does a route lookup, they have some way to select which route should be
used.  To do that they use the metric/preference number (I admit that use of
the term metric in the iproute2 tool is a bit misleading, but if you check the
ip-route(8) man page, you'll see its a synonym for preference).  At any rate,
since you already have a default gateway (which the stack sees as a duplicate
for the one your adding), and the existing route has the same metric/preference
as the one you are adding, you get the EXIST error.

Ways to work around this:

1) Use the src option in the ip route add command to differentiate and prevent
the existing default gw from being used (might be useful if you want different
virt guests to use different default gw).

2) instead of using ip route add, use ip route replace, which will overwrite
the existing default gw.

3) Use a different metric value than the existing default gw.  Both routes will
be added, but only the higest priority route will be used.
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As I understand the situation now, the error I got for ::/0 was valid 
and should not be ignored or overriden by default.  However, there is 
bound to be someone who has a valid reason to do this so there needs to 
be a way to do it.

Proposal:

Add a new attribute metric to the <route> subelement.  The default for 
metric will be metric='1' so that it will be the same as it is now.  
However, the use will be able to specify an arbitrary unsigned 32 bit 
number.

There are two other suggestions up there.  Any comments?

When I started this, my goal was simply to get some static routes for 
"regular" networks so that the virtualization host would know where to 
route packets.  I am not sure that it is a good idea that folks fiddle 
with default routes in libvirt virtual-network definitions but I am sure 
that someone, sometime will have a valid reason to do so.

BTW, just because some static route variation is not implemented as part 
of the libvirt definition does not mean that a user cannot issue the 
ip-route-add command manually.

Gene