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resolv.conf is a computer file used in various operating systems to configure the system's Domain Name System (DNS) resolver.The file is a plain-text file usually created by the network administrator or by applications that manage the configuration tasks of the system.
nslookup operates in interactive or non-interactive mode. When used interactively by invoking it without arguments or when the first argument is - (minus sign) and the second argument is a hostname or Internet address of a name server, the user issues parameter configurations or requests when presented with the nslookup prompt (>).
It can operate based on command line option and flag arguments, or in batch mode by reading requests from an operating system file. When a specific name server is not specified in the command invocation, it uses the operating system's default resolver, usually configured in the file resolv.conf. Without any arguments it queries the DNS root zone.
The resolver, or another DNS server acting recursively on behalf of the resolver, negotiates use of recursive service using bits in the query headers. DNS servers are not required to support recursive queries. The iterative query procedure is a process in which a DNS resolver queries a chain of one or more DNS servers. Each server refers the ...
For example, to do a reverse lookup of the IP address 8.8.4.4 the PTR record for the domain name 4.4.8.8.in-addr.arpa would be looked up, and found to point to dns.google. If the A record for dns.google in turn pointed back to 8.8.4.4 then it would be said to be forward-confirmed.
However, if the resolver is specifically told to look for CNAME records, the canonical name (right-hand side) is returned, rather than restarting the query. The canonical name that a CNAME record points to can be anywhere in the DNS, whether local or on a remote server in a different DNS zone. For example, consider a DNS zone as follows:
GNU adns is a C library that provides easy-to-use DNS resolution functionality. The library is asynchronous, allowing several concurrent calls. The package also includes several command-line utilities for use in scripts.
A public recursive name server (also called public DNS resolver) is a name server service that networked computers may use to query the Domain Name System (DNS), the decentralized Internet naming system, in place of (or in addition to) name servers operated by the local Internet service provider (ISP) to which the devices are connected.