Request for Comments: 2182
Category: Best Current Practice
Selection and Operation of Secondary DNS Servers
Abstract
The Domain Name System requires that multiple servers exist for every
delegated domain (zone). This document discusses the selection of
secondary servers for DNS zones. Both the physical and topological
location of each server are material considerations when selecting
secondary servers. The number of servers appropriate for a zone is
also discussed, and some general secondary server maintenance issues
considered.
[...]
With multiple servers, usually one server will be the primary server,
and others will be secondary servers. Note that while some unusual
configurations use multiple primary servers, that can result in data
inconsistencies, and is not advisable.
The distinction between primary and secondary servers is relevant
only to the servers for the zone concerned, to the rest of the DNS
there are simply multiple servers. All are treated equally at first
instance, even by the parent server that delegates the zone.
Resolvers often measure the performance of the various servers,
choose the "best", for some definition of best, and prefer that one
for most queries. That is automatic, and not considered here.
Earlier there was some dispute about what the RFCs say about multiple nameservers.
I found the following RFC which does have something to say about these issues.
http:// www.zoneedit. com/doc/ rfc/rfc2182. txt
Here are a couple of passages...
Request for Comments: 2182
Category: Best Current Practice
Selection and Operation of Secondary DNS Servers
Abstract
The Domain Name System requires that multiple servers exist for every
delegated domain (zone). This document discusses the selection of
secondary servers for DNS zones. Both the physical and topological
location of each server are material considerations when selecting
secondary servers. The number of servers appropriate for a zone is
also discussed, and some general secondary server maintenance issues
considered.
[...]
With multiple servers, usually one server will be the primary server,
and others will be secondary servers. Note that while some unusual
configurations use multiple primary servers, that can result in data
inconsistencies, and is not advisable.
The distinction between primary and secondary servers is relevant
only to the servers for the zone concerned, to the rest of the DNS
there are simply multiple servers. All are treated equally at first
instance, even by the parent server that delegates the zone.
Resolvers often measure the performance of the various servers,
choose the "best", for some definition of best, and prefer that one
for most queries. That is automatic, and not considered here.
[...]