tcpdump命令可以对网络上的数据包进行截获和分析。它支持针对网络层、协议、服务器、网络或端口的过滤，并提供and、or、not等逻辑语句来帮助你去掉无用的信息。
基于 TCP 标志的过滤器
可以根据各种 tcp 标志过滤 TCP 流量。这是一个基于tcp-ack标志的过滤示例。
[root@localhost ~]# tcpdump -i any "tcp[tcpflags] & tcp-ack !=0" -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 16:25:08.738925 IP localhost.localdomain.ssh > 192.168.43.1.39970: Flags [P.], seq 725364803:725365047, ack 1854457395, win 1842, length 244 16:25:08.739562 IP 192.168.43.1.39970 > localhost.localdomain.ssh: Flags [.], ack 244, win 4106, length 0 16:25:08.742750 IP localhost.localdomain.ssh > 192.168.43.1.39970: Flags [P.], seq 244:552, ack 1, win 1842, length 308 16:25:08.742822 IP localhost.localdomain.ssh > 192.168.43.1.39970: Flags [P.], seq 552:732, ack 1, win 1842, length 180 16:25:08.742882 IP localhost.localdomain.ssh > 192.168.43.1.39970: Flags [P.], seq 732:912, ack 1, win 1842, length 180 5 packets captured 5 packets received by filter 0 packets dropped by kernel 格式化输出内容
tcpdump 还可以通过对十六进制使用 -X选项或对 ASCII 使用 -A选项来调整输出格式。
[root@localhost ~]# tcpdump -i any -c3 -X 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 16:37:30.318137 IP localhost.localdomain.ssh > 192.168.43.1.39970: Flags [P.], seq 725376559:725376803, ack 1854460843, win 1842, length 244 0x0000: 4548 011c 0faf 4000 4006 5210 c0a8 2b83 EH....@.@.R...+. 0x0010: c0a8 2b01 0016 9c22 2b3c 5e2f 6e88 d3ab ..+...."+|...`. 0x0040: 152d 4295 9f8f d6ba dec2 895e 3921 2d76 .-B........^9!-v 0x0050: c5c6 5b6b 7161 61eb 0b30 1eae b622 2f14 ..[kqaa..0..."/. 0x0060: dfe5 0afc b91a 8a16 e3f1 62ae df5a 6728 ..........b..Zg( 0x0070: 4b9f 942d b762 a178 9d5e 5f70 96c2 fbad K..-.b.x.^_p.... 0x0080: 53f3 1bc5 80da 0e14 394c e31b 6b6a 02fc S.......9L..kj.. 0x0090: 203e 9a22 75c3 02ea c8d5 a2ec 5d30 60db .>."u.......]0`. 0x00a0: 64bf 4819 f2d4 ae88 c593 3b0c 90a2 273d d.H.......;...'= 0x00b0: 8f42 bf91 27bf b324 4f5f aec6 5d57 c27f .B..'..$O_..]W.. 0x00c0: 3c72 77de 6da5 97b9 52e8 7695 a964 d2a2 b.. 16:37:30.318540 IP localhost.localdomain.50573 > _gateway.domain: 47072+ PTR? 1.43.168.192.in-addr.arpa. (43) 0x0000: 4500 0047 a7e5 4000 4011 baea c0a8 2b83 E..G..@.@.....+. 0x0010: c0a8 2b02 c58d 0035 0033 d81a b7e0 0100 ..+....5.3...... 0x0020: 0001 0000 0000 0000 0131 0234 3303 3136 .........1.43.16 0x0030: 3803 3139 3207 696e 2d61 6464 7204 6172 8.192.in-addr.ar 0x0040: 7061 0000 0c00 01 pa..... 16:37:30.318743 IP 192.168.43.1.39970 > localhost.localdomain.ssh: Flags [.], ack 244, win 4103, length 0 0x0000: 4500 0028 538d 4000 8006 cf6d c0a8 2b01 E..(S.@....m..+. 0x0010: c0a8 2b83 9c22 0016 6e88 d3ab 2b3c 5f23 ..+.."..n...+<_ class="hljs-comment" style="color: #998;font-style: italic;line-height: 26px"># 0x0020: 5010 1007 5f2c 0000 0000 0000 0000 P..._,........ 3 packets captured 9 packets received by filter 0 packets dropped by kernel [root@localhost ~]# tcpdump -i any -c4 -A 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 16:38:36.499869 IP localhost.localdomain.ssh > 192.168.43.1.39970: Flags [P.], seq 725380591:725380835, ack 1854462375, win 1842, length 244 EH....@.@.Q...+...+...."+..3}s..#...[.@....l..8..xN........P....V%.B.......O..7.`.B!...O........R....%.L.m..RMx........m..3.0.H.`).....^....r..n*+/...p'..f....s...7...l.b2..Q....i@...M.X. 16:38:36.500384 IP localhost.localdomain.57135 > _gateway.domain: 50676+ PTR? 1.43.168.192.in-addr.arpa. (43) E..Gh.@.@.....+...+../.5.3...............1.43.168.192.in-addr.arpa..... 16:38:36.500580 IP 192.168.43.1.39970 > localhost.localdomain.ssh: Flags [.], ack 244, win 4106, length 0 E..(S.@....;..+...+.."..n...+ localhost.localdomain.57135: 50676 NXDomain 0/1/0 (78) E..j..........+...+..5./.VRs.............1.43.168.192.in-addr.arpa..........................p.... . :...Q. 4 packets captured 9 packets received by filter 0 packets dropped by kernel 详细信息输出
tcpdump 使用 -v、-vv 或 -vvv 来提供不同级别的详细信息。
下面是默认输出：
[root@localhost ~]# tcpdump -i any -c1 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 19:40:24.112322 IP localhost.localdomain.ssh > 192.168.43.1.39970: Flags [P.], seq 725383083:725383327, ack 1854472047, win 1842, length 244 1 packet captured 6 packets received by filter 0 packets dropped by kernel [root@localhost ~]# tcpdump -i any -c1 -v dropped privs to tcpdump tcpdump: listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes 19:41:00.606276 IP (tos 0x48, ttl 64, id 4249, offset 0, flags [DF], proto TCP (6), length 220) localhost.localdomain.ssh > 192.168.43.1.39970: Flags [P.], cksum 0xd8a3 (incorrect -> 0x314c), seq 725383979:725384159, ack 1854472375, win 1842, length 180 1 packet captured 7 packets received by filter 0 packets dropped by kernel [root@localhost ~]# tcpdump -i any port 443 -c1 -vv dropped privs to tcpdump tcpdump: listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes 19:51:18.409014 IP (tos 0x0, ttl 64, id 14543, offset 0, flags [DF], proto TCP (6), length 60) localhost.localdomain.52470 > 180.101.49.12.https: Flags [S], cksum 0xd1cb (incorrect -> 0x3f8f), seq 895899993, win 29200, options [mss 1460,sackOK,TS val 1518996680 ecr 0,nop,wscale 7], length 0 1 packet captured 1 packet received by filter 0 packets dropped by kernel [root@localhost ~]# tcpdump -i any -c1 -X -vvv dropped privs to tcpdump tcpdump: listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes 19:51:55.583214 IP (tos 0x48, ttl 64, id 4909, offset 0, flags [DF], proto TCP (6), length 220) localhost.localdomain.ssh > 192.168.43.1.39970: Flags [P.], cksum 0xd8a3 (incorrect -> 0x59f9), seq 725558447:725558627, ack 1854498815, win 1842, length 180 0x0000: 4548 00dc 132d 4000 4006 4ed2 c0a8 2b83 EH...-@.@.N...+. 0x0010: c0a8 2b01 0016 9c22 2b3f 24af 6e89 67ff ..+...."+?$.n.g. 0x0020: 5018 0732 d8a3 0000 0000 0090 ef64 ad4b P..2.........d.K 0x0030: ae12 dc9c 5d75 4136 b631 e567 d66e 4043 ....]uA6.1.g.n@C 0x0040: 5315 e0c7 9153 dec2 b406 3fda 915a 998b S....S....?..Z.. 0x0050: e504 7172 cb26 e560 1a51 1cf7 925b 16f2 ..qr.&.`.Q...[.. 0x0060: b7d3 35f4 01f6 cbc8 456a 4b62 52e9 bbbf ..5.....EjKbR... 0x0070: 8fa5 1a5c 3c18 ea42 dc08 1e44 e85e b111 ...<..b...d.> 0x00a0: 4920 abbf fcc0 57a5 92b0 ed6f fd68 ed96 I.....W....o.h.. 0x00b0: 53a1 3c7e 96bd 9f9d b95a 8dad 998b db5f S.." 1 packet captured 7 packets received by filter 0 packets dropped by kernel 按照协议过滤
可以使用协议名称来过滤特定协议的数据包。下面是过滤出UDP协议的数据包：
[root@localhost ~]# tcpdump udp -i any -c3 -nn 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 19:57:15.123051 IP 192.168.43.131.55682 > 192.168.0.12.123: NTPv4, Client, length 48 19:57:15.124002 IP 192.168.0.12.123 > 192.168.43.131.55682: NTPv4, Server, length 48 19:57:41.494061 IP 192.168.43.131.68 > 192.168.43.254.67: BOOTP/DHCP, Request from 00:0c:29:71:df:91, length 276 3 packets captured 3 packets received by filter 0 packets dropped by kernel [root@localhost ~]# tcpdump tcp and port 443 -i any -c 2 -nn 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 14:41:53.357110 IP 192.168.43.131.56320 > 180.101.49.11.443: Flags [S], seq 1415602203, win 29200, options [mss 1460,sackOK,TS val 1913450260 ecr 0,nop,wscale 7], length 0 14:41:53.378144 IP 180.101.49.11.443 > 192.168.43.131.56320: Flags [S.], seq 1535386750, ack 1415602204, win 64240, options [mss 1460], length 0 2 packets captured 3 packets received by filter 0 packets dropped by kernel [root@localhost ~]# 下面实例是筛选出不包括icmp-echo和icmp-echoreply类型的icmp数据包：
[root@localhost ~]# tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply' -c4 dropped privs to tcpdump tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on ens160, link-type EN10MB (Ethernet), capture size 262144 bytes 14:57:47.675667 IP localhost.localdomain > 192.168.43.1: ICMP host localhost.localdomain unreachable - admin prohibited filter, length 68 14:57:48.677588 IP localhost.localdomain > 192.168.43.1: ICMP host localhost.localdomain unreachable - admin prohibited filter, length 68 14:57:49.680887 IP localhost.localdomain > 192.168.43.1: ICMP host localhost.localdomain unreachable - admin prohibited filter, length 68 14:57:50.686504 IP localhost.localdomain > 192.168.43.1: ICMP host localhost.localdomain unreachable - admin prohibited filter, length 68 4 packets captured 4 packets received by filter 0 packets dropped by kernel -q 选项简化输出信息
如果想要简化输出内容，请使用 -q选项更快速、安静的输出。
[root@localhost ~]# tcpdump -i any -c5 -q 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 15:00:26.440699 IP localhost.localdomain.ssh > 192.168.43.1.55202: tcp 244 15:00:26.441052 IP localhost.localdomain.39876 > _gateway.domain: UDP, length 43 15:00:26.441220 IP 192.168.43.1.55202 > localhost.localdomain.ssh: tcp 0 15:00:26.447406 IP _gateway.domain > localhost.localdomain.39876: UDP, length 78 15:00:26.447835 IP localhost.localdomain.41058 > _gateway.domain: UDP, length 45 5 packets captured 9 packets received by filter 0 packets dropped by kernel 时间戳选项
下面是打印时间戳的一些常见选项。
移除时间戳
使用-t选项删除时间戳：
[root@localhost ~]# tcpdump tcp -i any -c4 -t 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 IP localhost.localdomain.ssh > 192.168.43.1.55202: Flags [P.], seq 743668214:743668458, ack 3963265225, win 343, length 244 IP 192.168.43.1.55202 > localhost.localdomain.ssh: Flags [.], ack 244, win 4102, length 0 IP localhost.localdomain.ssh > 192.168.43.1.55202: Flags [P.], seq 244:520, ack 1, win 343, length 276 IP localhost.localdomain.ssh > 192.168.43.1.55202: Flags [P.], seq 520:684, ack 1, win 343, length 164 4 packets captured 4 packets received by filter 0 packets dropped by kernel 输出本行和前一行的时间差,而不是时间
下面例子使用-ttt选项，显示了6行icmp数据包，可以看到每行之间的时间差：
[root@localhost ~]# tcpdump icmp -i any -c6 -ttt -nn 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 00:00:00.000000 IP 192.168.43.131 > 172.16.1.5: ICMP echo request, id 2986, seq 1, length 64 00:00:00.251269 IP 172.16.1.5 > 192.168.43.131: ICMP echo reply, id 2986, seq 1, length 64 00:00:00.749532 IP 192.168.43.131 > 172.16.1.5: ICMP echo request, id 2986, seq 2, length 64 00:00:00.253396 IP 172.16.1.5 > 192.168.43.131: ICMP echo reply, id 2986, seq 2, length 64 00:00:00.747521 IP 192.168.43.131 > 172.16.1.5: ICMP echo request, id 2986, seq 3, length 64 00:00:01.051634 IP 192.168.43.131 > 172.16.1.5: ICMP echo request, id 2986, seq 4, length 64 6 packets captured 6 packets received by filter 0 packets dropped by kernel
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