An DNS inquiry (or: DNS Request) can be answered in three different procedures:

• autoritativ (the server gets the data from a local zone file)
• not autoritativ
  • recursively (the server gets the data of another name server)
  • iterative (the server answers with a reference to other name servers)

The administrator name servers can define whether this name server can work on inquiries recursively or not. Normally name servers do not work recursively, there some resolvers with an iterative answer anything to begin can. With servers strongly charged to capacity (e.g. DNS root servers) the recursion is however deactivated.

Whether a Request was answered autoritativ (thus from a local zone file), a flag in the DNS Headern becomes with inquiries by and defines answers, the Autoritative Response flag. The resolver can infer thus from the answer of the server, which of the three methods specified above was used.

The recursive answer

The recursion behavior is determined by further flags. A resolver sets the Recursion Desired flag - RD abbreviated - in the DNS Request header, if it wishes a recursive dissolution of its Requests. The name server sets the Recursion Available flag - RA - in its answer, if it is in principle ready for the recursion. Only if both with a Request, and with the Response these flags are set, and/or recursively one worked. Otherwise do not differentiate between themselves autoritative and recursive answer.

The iterative answer

An iterative answer contains in place of the data (e.g. IP address) or several references to other name servers. A such reference contains the name of the other server, which names of the domain and - if admits - IP address (n). Here a complex example of an iterative answered DNS inquiry with Nslookup:

       C:\ >nslookup test.example.com

       name: test.example.com Served by: - dns01.extern.com 172.27.182.11, 172.27.158.208 example.com - dns02.extern.com example.com - dns03.extern.com 172.27.157.16 example.com 

The name server communicates to the resolver in this example that it cannot dissolve the name test.example.com that it knows however three name servers, which possess information to this name. For the name server dns01.extern.com are provided two IP addresses and for dns03.extern.com one.

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