nslookup

Definition: nslookup is a network utility program used to obtain information about Internet servers. As its name suggests, the utility finds name server information for domains by querying DNS.

Using nslookup

Most computer operating systems include a built-in command line program called nslookup.exe. Some network providers also host their own Web-based services of this same utility. These programs are all designed to perform name server look-ups against specified Internet domains.

To use the Windows version, open a command prompt and launch the tool as follows:

C:\> nslookup
Default Server: resolver1.opendns.com
Address: 208.67.222.222

\>

This first step identifies which DNS server configured on the PC is in use for look-ups.

IP Address Lookup

By default, nslookup responds with the primary IP address associated with a domain specified. To query the primary address of about.com, for example:

\> about.com
Address: 67.215.65.132

Mail Server Lookup

To search for mail server information on a particular domain, nslookup utilizes the so-called "MX record" feature of DNS. Some sites, like about.com, support both primary and backup servers while other sites only one. Mail server queries for about.com work as follows:

\> set type=mx
\> about.com
about.com MX preference = 100, mail exchanger = about.com.mail9.psmtp.com
about.com MX preference = 200, mail exchanger = about.com.mail10.psmtp.com
about.com MX preference = 300, mail exchanger = about.com.mail11.psmtp.com
about.com MX preference = 400, mail exchanger = about.com.mail12.psmtp.com

Other nslookup Queries

nslookup supports querying against other less commonly used DNS records including CNAME, PTR and SOA. Typing a question mark (?) at the prompt prints the program's help instructions. Web-based variations of the utility may offer a few additional features beyond the standard parameters.