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Re: How to tell if something is anycasted?

  • From: Steve Gibbard
  • Date: Tue May 16 18:52:07 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.