11-2 User Guide for the Avaya P580 and P882 Multiservice Switches, v6.1
Chapter 11
Recursive vs.
Iterative The goal of DNS is to resolve a fully qualified domain name (FQDN) to an
IP address. This work can either be done by the DNS server or the DNS
client. These approaches are referred to as Recursive or Iterative,
respectively.
In the Recursive approach, a client sends a query to the server. Assuming
recursion is enabled, the server then looks for resolution first locally in its
own database, then in its local cache, finally by going through the DNS tree
until it finds a server that can give an authoritative answer to the query. In
this model, the client is referred t o as a Stub Resolver. Typically, Stub
Resolvers are implemented on devices with limited resources such as
embedded systems or Personal Computers.
The Avaya Multiservice switch is a Stub Resolver, i.e. it on ly supports a
Recursive lookup not Iterative.
There are four most common answ ers a DNS server can provide are:
■Authoritative - a positive answer returned to the client with the
Authoritative Answer (AA) bit set in the response.
■Positive - an answer that contains the reso urce record (RR) or list of
RRs that match the query.
■Referral - an answer that contains a list of alternate servers the
client can use to resolve the name. This type of answer is given if
Recursion is not supported.
■Negative - this answer indicates that an Authorit ative server
reported that the name (or record type) does not exist in the DNS
name space. The server can be the preferred server or, if recur sion is
enabled, that another server responded back to the preferred server
with the authority bit set.
In the Iterative approach, the clien t sends a query to the server. If recursion
is disabled, and the server cannot answer the query , the server will r esponds
with a Referral answer. The client will then us e that information to query
another DNS server. This process will conti nue until a server responds with
an Authoritative response.