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 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.