新装的系统是默认没有安装这些命令的，但是实际开发过程中肯定要使用到这些命令，我们可以通过yum简单的命令来安装：
yum install bind-utils
使用host：
host命令使用说明：
Usage: host [-aCdlriTwv] [-c class] [-N ndots] [-t type] [-W time]
[-R number] [-m flag] hostname [server]
-a is equivalent to -v -t ANY
-c specifies query class for non-IN data
-C compares SOA records on authoritative nameservers
-d is equivalent to -v
-l lists all hosts in a domain, using AXFR
-i IP6.INT reverse lookups
-N changes the number of dots allowed before root lookup is done
-r disables recursive processing
-R specifies number of retries for UDP packets
-s a SERVFAIL response should stop query
-t specifies the query type
-T enables TCP/IP mode
-v enables verbose output
-w specifies to wait forever for a reply
-W specifies how long to wait for a reply
-4 use IPv4 query transport only
-6 use IPv6 query transport only
-m set memory debugging flag (trace|record|usage)
