Next-hop check not needed?

[Vincent Bernat]

 ❦ 28 juin 2017 16:12 +0200, Vincent Bernat <bernat at luffy.cx> :

>> Note that when learning route from the kernel you could workaround it by
>> using 'onlink' route flag.
>
> Great!
>
> It seems to work:
>
> $ ip route show table public dev eth2
> 203.0.113.1 scope link metric 10
> 203.0.113.10 via 203.0.113.1 metric 10 onlink
>
> bird> show route table public
> 203.0.113.10/32    via 203.0.113.1 on eth2 [kernel_public 15:58:09] * (200)
> 203.0.113.1/32     dev eth2 [kernel_public 15:58:09] * (200)
>
> But BIRD (1.6.3) still seems to be a bit confused, since I got this
> message in a loop:
>
> 2017-06-28 16:01:20 <ERR> KRT: Received route 203.0.113.10/32 with strange next-hop 203.0.113.1
> 2017-06-28 16:01:20 <WARN> Netlink: File exists
>
> Other than that, everything works as expected. I don't know how to
> interpret the first message (is it receiving two routes?) but the second
> message seems to say that it tries to reinstall the same route it
> received.

After investigating a bit more, the problem is triggered by the fact
that BIRD doesn't install the route with the "onlink" attribute. I am
using this kind of configuration to maintain a stripped version of the
routing table for local use:

table local_out;
protocol kernel kernel_local_out {
  persist;
  import none;
  export filter {
    krt_prefsrc = loopback_private;
    accept;
  };
  scan time 10;
  kernel table 100;
  table local_out;
  device routes yes;
  merge paths yes;
}
protocol pipe private_local_out {
  table private;
  peer table local_out;
  import none;
  export all;
}
protocol pipe public_local_out {
  table public;
  peer table local_out;
  import none;
  export filter {
    if proto = "kernel_public" then accept;
    reject;
  };
}

So, when the route is copied to the "local_out" table, it is copied
without the "onlink" parameter:

$ ip route show table public dev eth2
203.0.113.1 scope link metric 10
203.0.113.10 via 203.0.113.1 metric 10 onlink
$ ip route show table local-out dev eth2
203.0.113.1 proto bird scope link src 172.22.2.1 metric 10
203.0.113.10 via 203.0.113.1 proto bird src 172.22.2.1 metric 10

On the next pass, BIRD doesn't recognize it (KRT: Received route
203.0.113.10/32 with strange next-hop 203.0.113.1) and tries to install
it again but it already exists (Netlink: File exists). I'll dig more to
find a workaround.
-- 
"Elves and Dragons!" I says to him.  "Cabbages and potatoes are better
for you and me."
		-- J. R. R. Tolkien