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NSLOOKUP NSLOOKUP - DNS lookup utility ( man page ). Nslookup is a program to query Internet domain name servers. AUTHOR: Andrew Cherenson. Berkeley Distribution. Name server are defined in RFCs 1034 and 1035. The Domain Name System ( DNS ) is a critically important part of TCP/IP internetworks, especially the modern Internet, because it allows hosts to be accessed using easily-remembered names rather than confusing numerical addresses. Two different primary types of devices are involved in the operation of DNS: DNS name servers that store information about domains, and DNS resolvers that query DNS servers to transform names into addresses. Each time a user types a DNS name into a program such as a World Wide Web browser or FTP client —or even uses it in one of the other utilities, such as ping or traceroute—the resolver automatically performs the name resolution without the user having to ask. For this reason, there is no need for users to manually resolve DNS names into addresses. However, administrators often do need to perform a DNS resolution manually. One of the most common DNS diagnostic utilities is nslookup ( “name server lookup” ), which has been around for many years. The details of how the program is implemented of course depend on the operating system, though most of them are quite similar in operation and settings. The exact command set available in an nslookup implementation can usually be determined by issuing the command “help” or “?” at the nslookup prompt. To better understand how nslookup works, it is important to know the RFC 1034. The domain system assumes that all data originates in master files scattered through the hosts that use the domain system. These master files are updated by local system administrators. Master files are text files that are read by a local name server, and hence become available through the name servers to users of the domain system. The user programs access name servers through standard programs called resolvers. The name server uses the master files or copies to load its zones. For resolvers, the configuration data identifies the name servers which should be the primary sources of information. The domain name system ( DNS ) defines procedures for accessing the data and for referrals to other name servers. The domain system also defines procedures for caching retrieved data and for periodic refreshing of data defined by the system administrator. The system administrators provide: the definition of zone boundaries, Master files of data, updates to master files, statements of the refresh policies desired.
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