在Linux命令行中使用tcpdump抓包

当然，可以使用多条件组合来筛选数据包，使用 and 以及 or 逻辑操作符来创建过滤规则。例如，筛选来自源 IP 地址 192.168.122.98 的 HTTP 数据包：

$ sudo tcpdump -i any -c5 -nn src 192.168.122.98 and port 80
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
10:08:00.472696 IP 192.168.122.98.39342 > 54.204.39.132.80: Flags [S], seq 2712685325, win 29200, options [mss 1460,sackOK,TS val 123170822 ecr 0,nop,wscale 7], length 0
10:08:00.516118 IP 192.168.122.98.39342 > 54.204.39.132.80: Flags [.], ack 268723504, win 229, options [nop,nop,TS val 123170865 ecr 522918648], length 0
10:08:00.516583 IP 192.168.122.98.39342 > 54.204.39.132.80: Flags [P.], seq 0:112, ack 1, win 229, options [nop,nop,TS val 123170866 ecr 522918648], length 112: HTTP: GET / HTTP/1.1
10:08:00.567044 IP 192.168.122.98.39342 > 54.204.39.132.80: Flags [.], ack 643, win 239, options [nop,nop,TS val 123170916 ecr 522918661], length 0
10:08:00.788153 IP 192.168.122.98.39342 > 54.204.39.132.80: Flags [F.], seq 112, ack 643, win 239, options [nop,nop,TS val 123171137 ecr 522918661], length 0
5 packets captured
5 packets received by filter
0 packets dropped by kernel

你也可以使用括号来创建更为复杂的过滤规则，但在 shell 中请用引号包含你的过滤规则以防止被识别为 shell 表达式：

$ sudo tcpdump -i any -c5 -nn "port 80 and (src 192.168.122.98 or src 54.204.39.132)"
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
10:10:37.602214 IP 192.168.122.98.39346 > 54.204.39.132.80: Flags [S], seq 871108679, win 29200, options [mss 1460,sackOK,TS val 123327951 ecr 0,nop,wscale 7], length 0
10:10:37.650651 IP 54.204.39.132.80 > 192.168.122.98.39346: Flags [S.], seq 854753193, ack 871108680, win 28960, options [mss 1460,sackOK,TS val 522957932 ecr 123327951,nop,wscale 9], length 0
10:10:37.650708 IP 192.168.122.98.39346 > 54.204.39.132.80: Flags [.], ack 1, win 229, options [nop,nop,TS val 123328000 ecr 522957932], length 0
10:10:37.651097 IP 192.168.122.98.39346 > 54.204.39.132.80: Flags [P.], seq 1:113, ack 1, win 229, options [nop,nop,TS val 123328000 ecr 522957932], length 112: HTTP: GET / HTTP/1.1
10:10:37.692900 IP 54.204.39.132.80 > 192.168.122.98.39346: Flags [.], ack 113, win 57, options [nop,nop,TS val 522957942 ecr 123328000], length 0
5 packets captured
5 packets received by filter
0 packets dropped by kernel

该例子中我们只抓取了来自源 IP 为 192.168.122.98 或者 54.204.39.132 的 HTTP （端口号80）的数据包。使用该方法就很容易抓取到数据流中交互双方的数据包了。

5、检查数据包内容

在以上的示例中，我们只按数据包头部的信息来建立规则筛选数据包，例如源地址、目的地址、端口号等等。有时我们需要分析网络连接问题，可能需要分析数据包中的内容来判断什么内容需要被发送、什么内容需要被接收等。tcpdump 提供了两个选项可以查看数据包内容，-X 以十六进制打印出数据报文内容，-A 打印数据报文的 ASCII 值。

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