large organization nameservers sending icmp packets to dns servers.

Valdis.Kletnieks at Valdis.Kletnieks at
Mon Aug 6 17:34:52 UTC 2007

On Mon, 06 Aug 2007 17:21:49 -0000, John Levine said:
> >> 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.

If you have sites in San Fran, London, and Tokyo, and you launch a ping from
all 3 and see which one gets there first, you'll *know* the RTT from each site.

If you just send DNS replies from all 3, you don't have a good way of telling
which one got to the destination first.

Your method works if *I* want to know which one of the 3 sites is closest
(assuming I can identify an DNS server at the 3 sites).  The problem of the
owner of the 3 sites trying to identify which one I'm closest to isn't
symmetric to it.
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