問題現像：
[root@itzsmfgx]# tcpdump -i eth0 -s 0 port 9999 -w firstTime0911.pcap
-bash: tcpdump: command not found
解決辦法：
[root@itzsmfgx]# yum install -y tcpdump
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: mirrors.cn99.com
* extras: mirrors.163.com
* updates: mirrors.cn99.com
base | 3.6 kB 00:00:00
docker-ce-stable | 3.5 kB 00:00:00
extras | 2.9 kB 00:00:00
mysql-connectors-community | 2.5 kB 00:00:00
mysql-tools-community | 2.5 kB 00:00:00
mysql57-community | 2.5 kB 00:00:00
updates | 2.9 kB 00:00:00
Resolving Dependencies
--> Running transaction check
---> Package tcpdump.x86_64 14:4.9.2-4.el7_7.1 will be installed
--> Processing Dependency: libpcap >= 14:1.5.3-10 for package: 14:tcpdump-4.9.2-4.el7_7.1.x86_64
--> Processing Dependency: libpcap.so.1()(64bit) for package: 14:tcpdump-4.9.2-4.el7_7.1.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:
tcpdump x86_64 14:4.9.2-4.el7_7.1 base 422 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: 560 k
Installed size: 1.3 M
Downloading packages:
(1/2): libpcap-1.5.3-12.el7.x86_64.rpm | 139 kB 00:00:01
(2/2): tcpdump-4.9.2-4.el7_7.1.x86_64.rpm | 422 kB 00:00:01
---
Total 319 kB/s | 560 kB 00:00:01
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
Installing : 14:libpcap-1.5.3-12.el7.x86_64 1/2
Installing : 14:tcpdump-4.9.2-4.el7_7.1.x86_64 2/2
Verifying : 14:libpcap-1.5.3-12.el7.x86_64 1/2
Verifying : 14:tcpdump-4.9.2-4.el7_7.1.x86_64 2/2
Installed:
tcpdump.x86_64 14:4.9.2-4.el7_7.1
Dependency Installed:
libpcap.x86_64 14:1.5.3-12.el7
Complete!