large organization nameservers sending icmp packets to dns servers.

Mon Aug 6 17:51:09 UTC 2007

On Aug 6, 2007, at 10:21 AM, John Levine wrote:

>
>>> Sounds like one of the global-scale load balancers - when you do a
>>> (presumably) recursive DNS lookup of one of their hosts, they'll  
>>> ping
>>> the nameserver from several locations and see which one gets an
>>> answer the fastest.
>
> Why would they ping rather than just sending the query to all of the
> NS and see which one answers first?  It's an IP round trip either way.
>
> I agree that pinging is harmless, but for this application it seems
> pointless, too.

Well... we're talking about recursive resolvers. There's not
really a simple way for a third party to measure the round trip time to
the recursive resolver at the dns level.

It may not respond to external queries at all, and even if it does,
what query would you send that would cause an immediate reply
without any additional processing or network latency at the resolver?

There's lots of tricks you can play to do this, but most of them are
no better than a simple ICMP ping.

Cheers,
   Steve