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11-2 User Guide for the Avaya P580 and P882 Multiservice Switches, v6.0
Chapter 11
this model, the client is referred to 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 only supports a
Recursive lookup not Iterative.
There are four most common answers 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 resource 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 Authoritative 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 recursion is
enabled, that another server responded back to the preferred server
with the authority bit set.
In the Iterative approach, the client sends a query to the server. If recursion
is disabled, and the server cannot answer the query, the server will responds
with a Referral answer. The client will then use that information to query
another DNS server. This process will continue until a server responds with
an Authoritative response.
Sample
walkthrough of a
Recursive DNS
query
Typically, a network will have a local DNS server which provides
translations for devices in the local network. That server will also “know”
the IP address of the Root servers. When a client attempts to communicate
with a device by its name, the IP portion of the protocol stack will recognize
that a name (not an address) has been specified. It will then contact the local
DNS server and request the name be translated into an IP address. For
example: if a user types
ping www.avaya.com
,
that name will be sent
to the local DNS server. If the DNS server has that information, it will
respond with the appropriate IP address.
If the DNS server does not have that address translation, it will contact the
root server using the IP address for that root server. In this example, it
would use the IP address for the .COM root server.
Root servers do not have specific translations for names, but they do have
the database of master name servers. The root server would the avaya.com
master name server using the avaya.com IP address.