命令是：yum install -y nc

[root@master2 opt]# yum install -y nc
Loaded plugins: fastestmirror, product-id, search-disabled-repos, subscription-manager

This system is not registered with an entitlement server. You can use subscription-manager to register.

Loading mirror speeds from cached hostfile
 * base: mirrors.nju.edu.cn
 * extras: mirror.bit.edu.cn
 * updates: mirrors.nju.edu.cn
Resolving Dependencies
--> Running transaction check
---> Package nmap-ncat.x86_64 2:6.40-19.el7 will be installed
--> Processing Dependency: libpcap.so.1()(64bit) for package: 2:nmap-ncat-6.40-19.el7.x86_64
--> Running transaction check
---> Package libpcap.x86_64 14:1.5.3-12.el7 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

=================================================================================================================================================================================================================
 Package                                            Arch                                            Version                                                  Repository                                     Size
=================================================================================================================================================================================================================
Installing:
 nmap-ncat                                          x86_64                                          2:6.40-19.el7                                            base                                          206 k
Installing for dependencies:
 libpcap                                            x86_64                                          14:1.5.3-12.el7                                          base                                          139 k

Transaction Summary
=================================================================================================================================================================================================================
Install  1 Package (+1 Dependent package)

Total download size: 345 k
Installed size: 740 k
Downloading packages:
(1/2): libpcap-1.5.3-12.el7.x86_64.rpm                                                                                                                                                    | 139 kB  00:00:00     
(2/2): nmap-ncat-6.40-19.el7.x86_64.rpm                                                                                                                                                   | 206 kB  00:00:00     
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Total                                                                                                                                                                            925 kB/s | 345 kB  00:00:00     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
  Installing : 14:libpcap-1.5.3-12.el7.x86_64                                                                                                                                                                1/2 
  Installing : 2:nmap-ncat-6.40-19.el7.x86_64                                                                                                                                                                2/2 
  Verifying  : 2:nmap-ncat-6.40-19.el7.x86_64                                                                                                                                                                1/2 
  Verifying  : 14:libpcap-1.5.3-12.el7.x86_64                                                                                                                                                                2/2 

Installed:
  nmap-ncat.x86_64 2:6.40-19.el7                                                                                                                                                                                 

Dependency Installed:
  libpcap.x86_64 14:1.5.3-12.el7                                                                                                                                                                                 

Complete!