读计算机应该就同说过TCP/IP三次握手，但是都没有去验证过，今天心血来潮，去验证了一下，于是乎写下了这篇博客，可能写的可能有问题，还请多多指教
包括我学习，还有从很多资料来看资料，第三次握手，应该会返回ack(上一个seq+1)，但是我从抓包，并没有发现，第三次只会返回一个ack，希望有人来解答一下这个问题
抓包如下(用黄色的勾画起来的):
:: > localhost.localdomain.http: Flags [S], , win , options [mss ,nop,wscale ,nop,nop,sackOK], length E..4x.@.@.@z...n...m...P..._........r............... ::: Flags [S.], , ack , win , options [mss ,nop,nop,sackOK,nop,wscale ], length E....@.@......m...n.P...@>G...`..r..R.............. :: > localhost.localdomain.http: Flags [.], ack , win , length E..(x.@.@.@....n...m...P...`.@>HP............. :: > localhost.localdomain.http: Flags [P.], :, ack , win , length : HTTP: GET /favicon.ico HTTP/1.1 E...x @.@.?....n...m...P...`.@>HP.......GET /favicon.ico HTTP/1.1
这次我们就利用tcpdump来验证一下TCP/IP三次握手
要使用tcpdump抓包，首先要清楚工具输出的信息，默认tcpdump输出格式为
src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len
src > dst 目标源IP 端口 和 目标IP 端口
tcpflags
Tcpflags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or '.' (ACK), or 'none' if no flags are set.
S: SYN 同步 SYN = 1 表示这是一个连接请求或连接接受报文
F: FIN 用来释放一个连接.FIN=1 表明此报文段的发送端的数据已发送完毕,并要求释放运输连接
P: PUSH 接收 TCP 收到 PSH = 1 的报文段,就尽快地交付接收应用进程,而不再等到整个缓存都填满了后再向上交付
R: RST 当 RST=1 时,表明 TCP 连接中出现严重差错（如由于主机崩溃或其他原因）,必须释放连接,然后再重新建立运输连接
.: 为ACK,表示响应
使用tcpdump抓取本机80端口的包
[root@localhost ~]# tcpdump -i enp0s8 port 80 -A tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on enp0s8, link-type EN10MB (Ethernet), capture size 262144 bytes 09:52:52.462194 IP 192.168.0.110.54420 > localhost.localdomain.http: Flags [S], seq 3925850975, win 65535, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0 E..4x.@.@.@z...n...m...P..._........r............... 09:52:52.462222 IP localhost.localdomain.http > 192.168.0.110.54420: Flags [S.], seq 2302688839, ack 3925850976, win 29200, options [mss 1460,nop,nop,sackOK,nop,wscale 7], length 0 E..4..@.@......m...n.P...@>G...`..r..R.............. 09:52:52.462491 IP 192.168.0.110.54420 > localhost.localdomain.http: Flags [.], ack 1, win 1024, length 0 E..(x.@.@.@....n...m...P...`.@>HP............. 09:52:52.462628 IP 192.168.0.110.54420 > localhost.localdomain.http: Flags [P.], seq 1:223, ack 1, win 1024, length 222: HTTP: GET /favicon.ico HTTP/1.1 E...x @.@.?....n...m...P...`.@>HP.......GET /favicon.ico HTTP/1.1 Accept: */* UA-CPU: AMD64 Accept-Encoding: gzip, deflate User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; Trident/7.0; rv:11.0) like Gecko Host: 192.168.0.109 Connection: Keep-Alive 09:52:52.462638 IP localhost.localdomain.http > 192.168.0.110.54420: Flags [.], ack 223, win 237, length 0 E..(.&@.@..}...m...n.P...@>H...>P....F.. 09:52:52.463084 IP localhost.localdomain.http > 192.168.0.110.54420: Flags [P.], seq 1:325, ack 223, win 237, length 324: HTTP: HTTP/1.1 404 Not Found E..l.'@.@..8...m...n.P...@>H...>P.......HTTP/1.1 404 Not Found Server: nginx/1.14.2 Date: Wed, 17 Apr 2019 13:52:52 GMT Content-Type: text/html Content-Length: 169 Connection: keep-alive <html> <head><title>404 Not Found</title></head> <body bgcolor="white"> <center><h1>404 Not Found</h1></center> <hr><center>nginx/1.14.2</center> </body> </html> 09:52:52.463219 IP 192.168.0.110.54420 > localhost.localdomain.http: Flags [.], ack 325, win 1022, length 0 E..(x!@.@.@....n...m...P...>.@?.P............. 09:52:54.652738 IP 192.168.0.110.54420 > localhost.localdomain.http: Flags [R.], seq 223, ack 325, win 0, length 0 E..(x#@.@.@....n...m...P...>.@?.P............. ^C 8 packets captured 8 packets received by filter 0 packets dropped by kernel [root@localhost ~]#
分析: :: > localhost.localdomain.http: Flags [S], , win , options [mss ,nop,wscale ,nop,nop,sackOK], length 客户端向发送SYN请求，seq为2538461989 ::: Flags [S.], , ack , win , options [mss ,nop,nop,sackOK,nop,wscale ], length 服务器接受来自客户端的请求ack为2538461990(上次请求的seq + ) ， 并且发送 seq 为643447264 :: > localhost.localdomain.http: Flags [.], ack , win , length 客户端向服务器发送一个为ack
使用Wireshark分析
# tcpdump -i enp0s8 -w dump.pcap
可以清晰的看到TCP/IP的三次交互