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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.
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 (>).
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.
In some FreeBSD, Linux distributions, and other Unix-like operating systems, the resolvconf program maintains the system information about the currently available name servers and manages the contents of the configuration file resolv.conf, which determines Domain Name System (DNS) resolver parameters.
The GNU C Library provides various operating system facilities that shell commands and other applications can call to resolve such names to the corresponding addresses or IDs, and vice versa. Some Linux distributions use an nsswitch.conf file to specify the order in which multiple resolution services are used to effect such lookups.
The Name Service Switch (NSS) is a feature found in the standard C library of various Unix-like operating systems that connects a computer with a variety of sources of common configuration databases and name resolution mechanisms. [1]
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: