How to tell if something is anycasted?

Tue May 16 22:45:19 UTC 2006

On Tue, 16 May 2006, David Hubbard wrote:

> So I'm looking at a company who offers anycasted DNS;
> how do I tell if it's really anycasted?  Just hop on
> different route servers to see if I can find different
> AS paths and then do traceroutes to see if they suggest
> the packets are not ending in the same location?
>> From my routers' perspective I don't see a difference,
> but then I don't think I should, correct?

I was looking at this a lot for the DNS infrastructure distribution 
research I presented at the last NANOG.

Seeing that something is anycasted at all is sometimes pretty easy.  If 
you do traceroutes from geographically distant sources, you should 
hopefully end up in different places.  However, that doesn't always work, 
particularly if most of the anycast nodes are "local" nodes, which serve 
only small areas that you don't have access to.

Finding all the anycast locations, or at least being sure you've found all 
the locations, is much harder.  No matter how much testing you do, you'll 
never be sure you haven't missed some sites.

The approach I settled on was to ask lots of questions, and then do some 
traceroutes to verify once I knew where to look.  If I knew there was 
supposed to be a server in location x, a looking glass near location x 
would probably find it for me.

If you drop me a note off-list saying which anycast network you're looking 
at, what you're looking for may be information I have.

-Steve