TCP/IP Tutorial and Technical Overview - IBM Redbooks

authority, it returns all of the requested information in a response with the
authoritative answer flag set. When it receives a query for a domain over which it
does not have authority, its actions depend on the setting of the recursion
desired flag in the query:
► If the recursion desired flag is set and the server supports recursive queries, it
will direct its query to another name server. This will either be a name server
with authority for the domain given in the query, or it will be one of the root
name servers. If the second server does not return an authoritative answer
(for example, if it has delegated authority to another server), the process is
repeated.
► When a server (or a full resolver program) receives a response, it will cache it
to improve the performance of repeat queries. The cache entry is stored for a
maximum length of time specified by the originator in a 32-bit time-to-live
(TTL) field contained in the response. A typical TTL value is 86,400 seconds
(one day).
► If the recursion desired flag is not set or the server does not support recursive
queries, it will return whatever information it has in its cache and also a list of
additional name servers to be contacted for authoritative information.
Domain name server operation
Each name server has authority for zero or more zones. There are three types of
name servers:
Primary A primary name server loads a zone's information from
disk and has authority over the zone.
Secondary A secondary name server has authority for a zone, but
obtains its zone information from a primary server using a
process called zone transfer. To remain synchronized, the
secondary name servers query the primary on a regular
basis (typically three hours) and re-execute the zone
transfer if the primary has been updated. A name server
can operate as a primary or a secondary name server for
multiple domains, or a primary for some domains and as a
secondary for others. A primary or secondary name
server performs all of the functions of a caching-only
name server.
Caching-only A name server that does not have authority for any zone
is called a caching-only name server. A caching-only
name server obtains all of its data from primary or
secondary name servers as required. It requires at least
one NS record to point to a name server from which it can
initially obtain information.
Chapter 12. Directory and naming protocols 435