|
North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical RE: large organization nameservers sending icmp packets to dns servers.
Hi Guys, All things being equal (which they're usually not) you could use the ACK response time of the TCP handshake if they've got TCP DNS resolution available. Though again most don't for security reasons... -J -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Monday, August 06, 2007 11:35 AM To: John Levine Cc: [email protected] Subject: Re: large organization nameservers sending icmp packets to dns servers. 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.
|