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

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  ..+...."+<^/n...
0x0020:  5018 0732 d8e3 0000 0000 00d0 d1ce 67d9  P..2..........g.
0x0030:  b8e9 5171 dd56 bfbb 2d3e 7ce7 9a9b 60a5  ..Qq.V..->|...`.
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...+<_#
0x0020:  5010 1007 5f2c 0000 0000 0000 0000       P..._,........
3 packets captured
9 packets received by filter
0 packets dropped by kernel

使用 -A 选项，将显示 ASCII字符。

[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

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

下面是使用-v选项：

[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

下面是使用-vv选项：

[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

下面是使用-vvv选项：

[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.^..
0x0080:  12b5 e838 1c81 9e4d c070 a523 1274 4b02  ...8...M.p.#.tK.
0x0090:  4cf0 e7e1 5c70 1be1 8170 0ef0 8026 903e  L...\p...p...&.>
0x00a0:  4920 abbf fcc0 57a5 92b0 ed6f fd68 ed96  I.....W....o.h..
0x00b0:  53a1 3c7e 96bd 9f9d b95a 8dad 998b db5f  S.<~.....Z....._
0x00c0:  9ae9 ea52 37bd 4eaa effd 3aa7 7db8 2b3b  ...R7.N...:.}.+;
0x00d0:  d601 8d7a 84a4 bfd5 8e3e be22            ...z.....>."
1 packet captured
7 packets received by filter
0 packets dropped by kernel

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

下面是过滤出TCP协议的端口为443的数据包：

[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

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

下面是打印时间戳的一些常见选项。

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

可以看到每行数据最前面不显示时间戳了。

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

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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