DNS Protocol Overview:

DNS is the abbreviation of The Domain Name Service (or System or Server) Protocol. DNS is documented in related RFCs. DNS protocol is an internet service that translates domain names into IP addresses. DNS operates over TCP/UDP connections, usually to port 53. DNS protocol usually has two operations: lookup and zone transfer.

DNS Lookups

Normal Resource Records (RR) lookups are done with UDP. The protocol itself is stateless; all the information needed is contained in a single message, fully documented in RFC 1035 �4.1, and having the following format:

    Header 
    Question     the question for the name server
    Answer       RRs answering the question
    Authority     RRs pointing toward an authority
    Additional   RRs holding additional information

Questions are always Name, Type, Class tuples. For Internet applications, the Class is IN, the Type is a valid RR type, and the Name is a fully-qualified domain name, stored in a standard format. Names can't be wildcarded, but Types and Classes can be. In addition, special Types exist to wildcard mail records and to trigger zone transfers. The question is the only section included in a query message; the remaining sections being used for replies.

Answers are RRs that match the Name, Type, Class tuple. If any of the matching records are CNAME pointers leading to other records, the target records should also be included in the answer. There may be multiple answers, since there may be multiple RRs with the same labels.

Authority RRs are type NS records pointing to name servers closer to the target name in the naming hierarchy. This field is completely optional, but clients are encouraged to cache this information if further requests may be made in the same name hierarchy.

Additional RRs are records that the name server believes may be useful to the client. The most common use for this field is to supply A (address) records for the name servers listed in the Authority section.

Zone Transfers

Sometimes, it is necessary to efficiently transfer the resource records of an entire DNS zone. This is most commonly done by a secondary name server having determined the need to update its database.

The operation of a zone transfer is almost identical to a normal DNS query, except that TCP is used (due to large quantity of reply records) and a special Class exists to trigger a zone transfer. A DNS query with Name=FreeSoft.org, Class=IN, Type=AXFR will trigger a zone transfer for FreeSoft.org. The end of a zone transfer is marked by duplicating the SOA RR that started the zone.

Zone transfers are discussed in more detail in RFC 1034 �4.3.5.

Lower-Level Transport

Either TCP or UDP can be used to transport DNS protocol messages, connecting to server port 53 for either. Ordinary DNS requests can be made with TCP, though convention dictates the use of UDP for normal operation. TCP must be used for zone transfers, however, because of the danger of dropping records with an unreliable delivery protocol such as UDP.

References:

Domain Name System (DNS) Overview

Various DNS Tools like: tracert, ping, whois, etc.

DNS How-To

DNS Troubshooting In Configuration and NSLOOKUP

How To Setup Linux DNS service