Stupid question

[Roy Arends]

On Fri, 17 Aug 2001, no mail wrote:

> How does DNS decide which root server from the cache.dns file to send its
> query to?  Obviously if all requests tried to go to A.root-servers.net
> first, it would have an unfair load.
>
> How's this work?

A bind 8 caching resolver does this as follows. It keeps track of the
round trip time per nameserver. Whenever it has a choice from a list of
nameservers, it will take the fastest (shortest rtt). It will then
calculate new rtt's as followes:

   rtt(queried ns)       = ( .7 * old-rtt) + (.3 * cur-rtt)
   rtt(non-responsive ns)= (1.2 * old-rtt)
   rtt(other)            = (.98 * old-rtt)

The values can be found in src/bin/named/ns_defs.h:

#define ALPHA    0.7   /* How much to preserve of old response time */
#define BETA     1.2   /* How much to penalize response time on failure */
#define GAMMA    0.98  /* How much to decay unused response times */

These values are used in src/bin/named/ns_resp.c

DJBDNS picks a server at random, using a prng.

For other brands, I don't know, though I'm curious.

Note that for instance the Netscape applications use their own
DNS-resolver (dns-helper).

Roy Arends
Nominum
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0-14-023750-X dcrpt ths 43.0D.01 01.05.0C 84.18.03 8A.13.04 2D.0B.0A