Aliases and Cannonical Names
A number of servers typically have many names for convenience. For case yamuna.iitk.ernet.in recognize the same server. In addition numerous mailboxes might be provided by some organizations. Most of these machines have a notion that one of the equivalent set of names is the canonical or key name and all others are aliases.
When a name server fails to find a desired RR in the resource set linked with the domain name, it checks to observe if the resource set consists of a CNAME record with a matching class. If so, then the name server includes the CNAME record in the reply and restarts the query at the domain name particular in the data field of the CNAME record.
Name Servers
Name servers are the repositories of information that build up the domain database. The database is splitted up into sections called zones, which are dispersed among the name servers. Name servers can answer queries in a easy manner; the response could always be generated using only local data, and either contains the answer to the query or a referral to other name servers "closer" to the preferred information. The way that the name server answers the query depends upon whether it is operating in recursive mode or iterative mode:
- The simplest form for the server is non-recursive, because it can answer queries using only local information: the reply contains an error, the answer, or a recommendation to some other server "closer" to the answer. All name servers have to implement non- recursive queries.
- The simplest mode for the client is recursive, as in this mode the name server acts
in the role of a resolver and returns either an error or the answer, however never referrals. This service is optional in a name server, and the name server might also choose to restrict the clients which can use recursive mode.
Recursive Query vs Iterative Query
If the server is supposed to answer a recursive quesry then the response is either the reource record data or a error code. A server which is operating in this mode will never return the name of any forwarding name server but will contact the appropiate name server itself and try to get the information.
In iterative mode, alternatively, if the server does not have the information requested locally then it return the address of some name server who could have the information about the query. It is then the accountability of the contacting application to contact the subsequently name server to resolve its query and do this iteratively until gets an answer or error.
Relative Names
In place of giving complete DNS names such as cu2.cse.iitk.ac.in or bhaskar.cc.iitk.ac.in one could give just cu2 or bhaskar. This could be used by the server side as well as the client side. However for this one has to manually identify these extensions in the database of the servers holding the resource records.
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