If you need to use nslookup on a Debian server and you're getting an "nslookup: command not found" then use apt-get to install dnsutils.
You'll know if dnsutils & nslookup are not installed if you try a dns lookup and get this error:
$ nslookup www.example.com -bash: nslookup: command not found
Install the dnsutils package to resolve the error and lookup the domain:
$ apt-get install dnsutils
And now it works
$ nslookup www.example.com Server: 184.108.40.206 Address: 220.127.116.11#53 Non-authoritative answer: Name: www.example.com Address: 18.104.22.168