So you've just run a barebones installation of CentOS 6, and you run a host command to check DNS resolution and you get the following: whois: command not found By default the barebones CentOS installation lacks even the most basic network diagnostic tools. Use yum to install the following packages to get a few basic tools back on your server: yum install bind-utils (installs nslookup and host) yum install jwhois (installs whois)