[Xorp-users] Unreachable default route.

Pavlin Radoslavov pavlin at icir.org
Thu Sep 13 16:11:57 PDT 2007


Ben Greear <greearb at candelatech.com> wrote:

> Pavlin Radoslavov wrote:
> 
> > You could use the following configuration on Linux to configure a
> > discard interface and a static route that is blackhole:
> > 
> > interfaces {
> >     interface my_discard {
> >         discard: true
> >         vif my_discard {
> >         }
> >     }
> > }
> 
> [snip]
> 
> Ok, this does indeed create a blackhole route.  But, it seems this will just
> silently eat packets.  What I really want is unreachable, which will return
  ~~~~~~~~~~~~~~~~~~~~

Correct. This is the definition of "blackhole" :)

> the proper ICMP packet saying the destination is unreachable.
> 
> Any idea how hard it would be to add this functionality?

You need to install a different type of route in the kernel which I
believe in Linux is RTN_UNREACHABLE instead of RTN_BLACKHOLE.
However, XORP doesn't support such routes.
You could try experimenting with such routes by replacing all
references (I counted two references) of RTN_BLACKHOLE with
RTN_UNREACHABLE inside file
fea/data_plane/fibconfig/fibconfig_entry_set_netlink_socket.cc

This is not the right solution, but allows you to play with such
routes.

Just curious, could you describe your particular scenario you have
that requires installing RTN_UNREACHABLE routes.

> In the meantime, I'll work on a patch that makes the 'static' priority
> configurable with an environment variable.

I should tell you upfront that configurable admin distances in RIB
has been on our TODO list for quite some time. However it is not
trivial if we want to do it properly by taking into account various
considerations.
E.g., one of the goals is to be able to configure the priorities (on
the fly) inside the XORP config file.

Hence, most likely we won't use a solution that is based on
setting an environmental variable (or something like this).
In other words, don't be offended if your patch is not applied to
the CVS.
Though, if I were in your position I would use such shortcut in my
local XORP copy.

Regards,
Pavlin

> Thanks,
> Ben
> 
> -- 
> Ben Greear <greearb at candelatech.com>
> Candela Technologies Inc  http://www.candelatech.com
> 
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