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命令简介
tcpdump 命令是一款类 Unix/Linux 环境下的抓包工具。
tcpdump 采纳命令行形式对接口的数据包进行筛选抓取，如果不带任何选项的 tcpdump，默认会抓取第一个网络接口，且只有将 tcpdump 过程终止才会进行抓包。
[root@CentOS7-1 ~]# tcpdump
-bash: tcpdump: command not found
[root@CentOS7-1 ~]# yum install tcpdump -y
语法格局
[root@CentOS7-1 ~]# tcpdump --help
tcpdump version 4.9.2
libpcap version 1.5.3
OpenSSL 1.0.2k-fips 26 Jan 2017
Usage: tcpdump [-aAbdDefhHIJKlLnNOpqStuUvxX#] [-B size] [-c count]
[-C file_size] [-E algo:secret] [-F file] [-G seconds]
[-i interface] [-j tstamptype] [-M secret] [--number]
[-Q|-P in|out|inout]
[-r file] [-s snaplen] [--time-stamp-precision precision]
[--immediate-mode] [-T type] [--version] [-V file]
[-w file] [-W filecount] [-y datalinktype] [-z postrotate-command]
[-Z user] [expression]
选项阐明
抓包选项
-c #指定要抓取的包数量
-i interface #指定要监听的接口
-n #对地址以数字形式显式
-n #端口显示为数值
-N #不打印出 host 的域名局部
-P #指定要抓取的包是流入还是流出的包
-s len #设置数据包抓取长度为 len，默认将会是 65535 字节
输入选项
-e #输入的每行中都将包含数据链路层头部信息
-q #疾速打印输出
-X #输入包的头部数据
-XX #输入包的头部数据
-v #具体的输入
-vv #比 - v 更具体的输入
-vvv #比 -vv 更具体的输入
其余选项
-D #查问能够抓包的接口
-F #从文件中读取抓包的表达式
-w #将抓包的数据输入到文件
-r #从指定的数据包文件中读取数据
举荐给你：值得珍藏！Linux 零碎常用命令速查手册
所以罕用的选项也就这几个：
tcpdump -D
tcpdump -c num -i int -nn -XX -vvv
利用举例
实例
[root@CentOS7-1 ~]# tcpdump -i ens33
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes
08:43:32.649405 IP CentOS7-1.ssh > 192.168.1.93.62148: Flags [P.], seq 1603116601:1603116813, ack 87129926, win 273, length 212
08:43:32.650284 IP CentOS7-1.54879 > public1.alidns.com.domain: 63951+ PTR? 93.1.168.192.in-addr.arpa. (43)
08:43:32.679205 IP public1.alidns.com.domain > CentOS7-1.54879: 63951 NXDomain 0/1/0 (120)
08:43:32.680996 IP CentOS7-1.50467 > public1.alidns.com.domain: 7677+ PTR? 100.1.168.192.in-addr.arpa. (44)
08:43:32.693832 IP 192.168.1.93.62148 > CentOS7-1.ssh: Flags [.], ack 212, win 4101, length 0
08:43:32.708977 IP public1.alidns.com.domain > CentOS7-1.50467: 7677 NXDomain 0/1/0 (121)
08:43:32.709897 IP CentOS7-1.54341 > public1.alidns.com.domain: 22823+ PTR? 5.5.5.223.in-addr.arpa. (40)
08:43:32.710391 IP CentOS7-1.ssh > 192.168.1.93.62148: Flags [P.], seq 212:392, ack 1, win 273, length 180
08:43:32.731500 IP public1.alidns.com.domain > CentOS7-1.54341: 22823 1/0/0 PTR public1.alidns.com. (72)
08:43:32.733069 IP CentOS7-1.ssh > 192.168.1.93.62148: Flags [P.], seq 392:1260, ack 1, win 273, length 868
08:43:32.733632 IP 192.168.1.93.62148 > CentOS7-1.ssh: Flags [.], ack 1260, win 4106, length 0
08:43:32.733936 IP CentOS7-1.ssh > 192.168.1.93.62148: Flags [P.], seq 1260:1520, ack 1, win 273, length 260
08:43:32.734278 IP CentOS7-1.ssh > 192.168.1.93.62148: Flags [P.], seq 1520:1684, ack 1, win 273, length 164
08:43:32.734619 IP 192.168.1.93.62148 > CentOS7-1.ssh: Flags [.], ack 1684, win 4104, length 0
抓取 Ping 数据包
[root@CentOS7-1 ~]# tcpdump -c 5 -nn -i ens33 icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes
08:53:52.652906 IP 192.168.1.199 > 192.168.1.100: ICMP echo request, id 1368, seq 1, length 64
08:53:52.654987 IP 192.168.1.100 > 192.168.1.199: ICMP echo reply, id 1368, seq 1, length 64
08:53:53.659034 IP 192.168.1.199 > 192.168.1.100: ICMP echo request, id 1368, seq 2, length 64
08:53:53.659095 IP 192.168.1.100 > 192.168.1.199: ICMP echo reply, id 1368, seq 2, length 64
08:53:54.667539 IP 192.168.1.199 > 192.168.1.100: ICMP echo request, id 1368, seq 3, length 64
5 packets captured
7 packets received by filter
0 packets dropped by kernel
剖析数据包
[root@CentOS7-1 ~]# tcpdump -c 2 -q -XX -vvv -nn -i ens33 tcp dst port 22
tcpdump: listening on ens33, link-type EN10MB (Ethernet), capture size 262144 bytes
08:55:43.606727 IP (tos 0x0, ttl 64, id 27235, offset 0, flags [DF], proto TCP (6), length 40)
192.168.1.93.62148 > 192.168.1.100.22: tcp 0
0x0000: 000c 2925 626f f875 a47d f4ec 0800 4500 ..)%bo.u.}....E.
0x0010: 0028 6a63 4000 4006 4c5b c0a8 015d c0a8 .(jc@.@.L[...]..
0x0020: 0164 f2c4 0016 0531 cb7e 5f9f 9479 5010 .d.....1.~_..yP.
0x0030: 1005 641a 0000 0000 0000 0000 ..d.........
08:55:43.651422 IP (tos 0x0, ttl 64, id 27236, offset 0, flags [DF], proto TCP (6), length 40)
192.168.1.93.62148 > 192.168.1.100.22: tcp 0
0x0000: 000c 2925 626f f875 a47d f4ec 0800 4500 ..)%bo.u.}....E.
0x0010: 0028 6a64 4000 4006 4c5a c0a8 015d c0a8 .(jd@.@.LZ...]..
0x0020: 0164 f2c4 0016 0531 cb7e 5f9f 964d 5010 .d.....1.~_..MP.
0x0030: 100a 6241 0000 0000 0000 0000 ..bA........
2 packets captured
3 packets received by filter
0 packets dropped by kernel
每天学一个 Linux 命令（70）：dig
每天学一个 Linux 命令（71）：traceroute
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