經常用tcpdump抓包來倒騰和學習，但怎么觸發tcp包呢？寫一個客戶端程序，然后朝server上發包，肯定可以，但還有更簡單的方法：

exec 8<>/dev/tcp/10.100.70.139/19006echo -e "stats" >&8

用tcpdump抓包的結果為：

xxxxxx$ sudo tcpdump -iany port 19006 -Xnlps0tcpdump: verbose output suppressed, use -v or -vv for full protocol decodelistening on any, link-type LINUX_SLL (Linux cooked), capture size 65535 bytes23:56:54.177781 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                ....