经常用tcpdump抓包来倒腾和学习，但怎么触发tcp包呢？写一个客户端程序，然后朝server上发包，肯定可以，但还有更简单的方法：
exec 8<>/dev/tcp/10.100.70.139/19006 echo -e "stats" >&8
用tcpdump抓包的结果为：
xxxxxx$ sudo tcpdump -iany port 19006 -Xnlps0
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes
23:56:54.177781 IP 10.100.70.139.34209 > 10.100.70.139.19006: Flags [S], seq 2048518213, win 43690, options [mss 65495,sackOK,TS val 1173337789 ecr 0,nop,wscale 8], length 0
0x0000: 4500 003c 5ef8 4000 4006 39e6 0a64 468b E..<^.@.@.9..dF.
0x0010: 0a64 468b 85a1 4a3e 7a19 e845 0000 0000 .dF...J>z..E....
0x0020: a002 aaaa a20c 0000 0204 ffd7 0402 080a ................
0x0030: 45ef b6bd 0000 0000 0103 0308 0000 0000 E...............
0x0040: 0000 0000 0000 0000 0000 0000 ............
23:56:54.177794 IP 10.100.70.139.19006 > 10.100.70.139.34209: Flags [S.], seq 480321333, ack 2048518214, win 43690, options [mss 65495,sackOK,TS val 1173337789 ecr 1173337789,nop,wscale 8], length 0
0x0000: 4500 003c 0000 4000 4006 98de 0a64 468b E..<..@.@....dF.
0x0010: 0a64 468b 4a3e 85a1 1ca1 1f35 7a19 e846 .dF.J>.....5z..F
0x0020: a012 aaaa a20c 0000 0204 ffd7 0402 080a ................
0x0030: 45ef b6bd 45ef b6bd 0103 0308 0000 0000 E...E...........
0x0040: 0000 0000 0000 0000 0000 0000 ............
23:56:54.177802 IP 10.100.70.139.34209 > 10.100.70.139.19006: Flags [.], ack 1, win 171, options [nop,nop,TS val 1173337789 ecr 1173337789], length 0
0x0000: 4500 0034 5ef9 4000 4006 39ed 0a64 468b E..4^.@.@.9..dF.
0x0010: 0a64 468b 85a1 4a3e 7a19 e846 1ca1 1f36 .dF...J>z..F...6
0x0020: 8010 00ab a204 0000 0101 080a 45ef b6bd ............E...
0x0030: 45ef b6bd 0000 0000 0000 0000 0000 0000 E...............
0x0040: 0000 0000 ....
23:57:05.744655 IP 10.100.70.139.34209 > 10.100.70.139.19006: Flags [P.], seq 1:7, ack 1, win 171, options [nop,nop,TS val 1173340681 ecr 1173337789], length 6
0x0000: 4500 003a 5efa 4000 4006 39e6 0a64 468b E..:^.@.@.9..dF.
0x0010: 0a64 468b 85a1 4a3e 7a19 e846 1ca1 1f36 .dF...J>z..F...6
0x0020: 8018 00ab a20a 0000 0101 080a 45ef c209 ............E...
0x0030: 45ef b6bd 6865 6c6c 6f0a 0000 0000 0000 E...hello.......
0x0040: 0000 0000 0000 0000 0000 ..........
23:57:05.744679 IP 10.100.70.139.19006 > 10.100.70.139.34209: Flags [.], ack 7, win 171, options [nop,nop,TS val 1173340681 ecr 1173340681], length 0
0x0000: 4500 0034 a98b 4000 4006 ef5a 0a64 468b E..4..@.@..Z.dF.
0x0010: 0a64 468b 4a3e 85a1 1ca1 1f36 7a19 e84c .dF.J>.....6z..L
0x0020: 8010 00ab a204 0000 0101 080a 45ef c209 ............E...
0x0030: 45ef c209 0000 0000 0000 0000 0000 0000 E...............
0x0040: 0000 0000 ....
23:57:05.744830 IP 10.100.70.139.19006 > 10.100.70.139.34209: Flags [F.], seq 1, ack 7, win 171, options [nop,nop,TS val 1173340681 ecr 1173340681], length 0
0x0000: 4500 0034 a98c 4000 4006 ef59 0a64 468b E..4..@.@..Y.dF.
0x0010: 0a64 468b 4a3e 85a1 1ca1 1f36 7a19 e84c .dF.J>.....6z..L
0x0020: 8011 00ab a204 0000 0101 080a 45ef c209 ............E...
0x0030: 45ef c209 0000 0000 0000 0000 0000 0000 E...............
0x0040: 0000 0000 ....
23:57:05.748555 IP 10.100.70.139.34209 > 10.100.70.139.19006: Flags [.], ack 2, win 171, options [nop,nop,TS val 1173340682 ecr 1173340681], length 0
0x0000: 4500 0034 5efb 4000 4006 39eb 0a64 468b E..4^.@.@.9..dF.
0x0010: 0a64 468b 85a1 4a3e 7a19 e84c 1ca1 1f37 .dF...J>z..L...7
0x0020: 8010 00ab a204 0000 0101 080a 45ef c20a ............E...
0x0030: 45ef c209 0000 0000 0000 0000 0000 0000 E...............
0x0040: 0000 0000 ....
真的是爽歪歪。 能用shell搞定的事， 绝不用C/C++.
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