如果你导出了一个 .pcap 文件，你就会知道不能使用文本编辑器来读取文件内容。因此，你应该使用-r file.pcap选项。它读取现有捕获的文件并将它们显示出来。

# 导出.pcap文件
[root@localhost ~]# tcpdump -c 4 -i any port 53 -nn -w dns.pcap -v
dropped privs to tcpdump
tcpdump: listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
4 packets captured
8 packets received by filter
0 packets dropped by kernel

# 使用-r选项读取.pcap文件
[root@localhost ~]# tcpdump -r dns.pcap 
reading from file dns.pcap, link-type LINUX_SLL (Linux cooked)
dropped privs to tcpdump
19:33:54.533792 IP localhost.localdomain.48048 > _gateway.domain: 30912+ A? www.bai. (25)
19:33:54.533835 IP localhost.localdomain.48048 > _gateway.domain: 51681+ AAAA? www.bai. (25)
19:33:54.537733 IP _gateway.domain > localhost.localdomain.48048: 51681 NXDomain 0/1/0 (100)
19:33:54.539312 IP _gateway.domain > localhost.localdomain.48048: 30912 NXDomain 0/1/0 (100)

如果要过滤特定主机的流量，可以使用host选项后面添加ip 或者主机名来捕获特定主机的数据包。

[root@localhost ~]# tcpdump host redhat.com -i any -c5
dropped privs to tcpdump
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
20:27:19.762717 IP localhost.localdomain.59096 > redirect.redhat.com.https: Flags [S], seq 2565597156, win 29200, options [mss 1460,sackOK,TS val 178658343 ecr 0,nop,wscale 7], length 0
20:27:19.977742 IP redirect.redhat.com.https > localhost.localdomain.59096: Flags [S.], seq 1933719472, ack 2565597157, win 64240, options [mss 1460], length 0
20:27:19.977773 IP localhost.localdomain.59096 > redirect.redhat.com.https: Flags [.], ack 1, win 29200, length 0
20:27:19.983584 IP localhost.localdomain.59096 > redirect.redhat.com.https: Flags [P.], seq 1:518, ack 1, win 29200, length 517
20:27:19.983781 IP redirect.redhat.com.https > localhost.localdomain.59096: Flags [.], ack 518, win 64240, length 0
5 packets captured
9 packets received by filter
0 packets dropped by kernel

可以使用 src 或 dst 关键字告诉 tcpdump 捕获的数据包是否应包含源地址或目标地址中的主机。列入下面获取源主机地址为redhat.com的数据包，和获取目的主机地址为redhat.com的数据包：

[root@localhost ~]# tcpdump src host redhat.com -i any -c5
dropped privs to tcpdump
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
20:29:23.090360 IP redirect.redhat.com.https > localhost.localdomain.59098: Flags [S.], seq 1420240111, ack 1139421965, win 64240, options [mss 1460], length 0
20:29:23.096748 IP redirect.redhat.com.https > localhost.localdomain.59098: Flags [.], ack 518, win 64240, length 0
20:29:23.353159 IP redirect.redhat.com.https > localhost.localdomain.59098: Flags [P.], seq 1:1381, ack 518, win 64240, length 1380
20:29:23.353434 IP redirect.redhat.com.https > localhost.localdomain.59098: Flags [.], seq 1381:2841, ack 518, win 64240, length 1460
20:29:23.353461 IP redirect.redhat.com.https > localhost.localdomain.59098: Flags [P.], seq 2841:3407, ack 518, win 64240, length 566
5 packets captured
9 packets received by filter
0 packets dropped by kernel

[root@localhost ~]# tcpdump dst host redhat.com -i any -c5
dropped privs to tcpdump
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
20:29:35.661917 IP localhost.localdomain.59100 > redirect.redhat.com.https: Flags [S], seq 4207740141, win 29200, options [mss 1460,sackOK,TS val 178783790 ecr 0,nop,wscale 7], length 0
20:29:35.886845 IP localhost.localdomain.59100 > redirect.redhat.com.https: Flags [.], ack 1287594187, win 29200, length 0
20:29:35.892463 IP localhost.localdomain.59100 > redirect.redhat.com.https: Flags [P.], seq 0:517, ack 1, win 29200, length 517
20:29:36.121990 IP localhost.localdomain.59100 > redirect.redhat.com.https: Flags [.], ack 1461, win 32120, length 0
20:29:36.122028 IP localhost.localdomain.59100 > redirect.redhat.com.https: Flags [.], ack 2921, win 35040, length 0
5 packets captured
10 packets received by filter
0 packets dropped by kernel

tcpdump 支持and/or/not运算符作为关键字，例如，tcpdump -i ens33 “host lrxjmw.cn and (port 80 or port 443)”。在复合表达式周围使用引号是很有必要的，这样 bash 就不会试图解释括号。

[root@localhost ~]# tcpdump -i any "host lrxjmw.cn and (port 80 or port 443)" -s64 -c5
dropped privs to tcpdump
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 64 bytes
20:37:12.398299 IP localhost.localdomain.52754 > 140.249.61.18.https: Flags [S], seq 3304304157, win 29200, options [mss 1460,sackOK,TS[|tcp]>
20:37:12.408805 IP 140.249.61.18.https > localhost.localdomain.52754: Flags [S.], seq 2112965730, ack 3304304158, win 64240, options [mss 1460], length 0
20:37:12.408842 IP localhost.localdomain.52754 > 140.249.61.18.https: Flags [.], ack 1, win 29200, length 0
20:37:12.414672 IP localhost.localdomain.52754 > 140.249.61.18.https: Flags [P.], seq 1:518, ack 1, win 29200, length 517
20:37:12.414948 IP 140.249.61.18.https > localhost.localdomain.52754: Flags [.], ack 518, win 64240, length 0
5 packets captured
9 packets received by filter
0 packets dropped by kernel

net 关键字可用于捕获一个网段的数据包。net关键字还可以将 src 和 ds与逻辑运算符一起使用，并更精确地过滤包。下面实例是获取源net为192.168.0.0/16网段，并且目标net不包含192.168.0.0/16网段的数据包。

[root@localhost ~]# tcpdump -i any -n "src net 192.168.0.0/16 and not dst net 192.168.0.0/16" -c4
dropped privs to tcpdump
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
20:43:25.558537 IP 192.168.43.131.34562 > 140.249.61.18.http: Flags [S], seq 3720011773, win 29200, options [mss 1460,sackOK,TS val 4199917698 ecr 0,nop,wscale 7], length 0
20:43:25.571477 IP 192.168.43.131.34562 > 140.249.61.18.http: Flags [.], ack 1199844, win 29200, length 0
20:43:25.571693 IP 192.168.43.131.34562 > 140.249.61.18.http: Flags [P.], seq 0:82, ack 1, win 29200, length 82: HTTP: GET / HTTP/1.1
20:43:25.587514 IP 192.168.43.131.34562 > 140.249.61.18.http: Flags [.], ack 546, win 29975, length 0
4 packets captured
4 packets received by filter
0 packets dropped by kernel

可以使用 ip6 关键字捕获 IPv6 流量。下面是一个例子：

[root@localhost ~]# tcpdump -i any ip6 host  fe80::9520:7b41:7099:c6f7 -c4
dropped privs to tcpdump
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
22:17:08.182720 IP6 localhost.localdomain > fe80::9520:7b41:7099:c6f7: ICMP6, echo request, seq 12, length 64
22:17:08.183329 IP6 fe80::9520:7b41:7099:c6f7 > localhost.localdomain: ICMP6, echo reply, seq 12, length 64
22:17:09.207102 IP6 localhost.localdomain > fe80::9520:7b41:7099:c6f7: ICMP6, echo request, seq 13, length 64
22:17:09.207555 IP6 fe80::9520:7b41:7099:c6f7 > localhost.localdomain: ICMP6, echo reply, seq 13, length 64
4 packets captured
4 packets received by filter
0 packets dropped by kernel

tcpdump 用于收集有关网络流量数据的出色工具。数据包捕获为故障排除和安全分析提供了有用的信息。

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