[tcp] 使用socket模拟分析tcp协议

写两段简单的 python 代码，然后来抓包分析tcp协议

服务端IP:172.16.196.145

客户端IP:172.16.196.142

TCP三次握手、四次挥手

server端代码

import socket

s = socket.socket()
s.bind(('172.16.196.145',60000))
s.listen(5)

while 1:
    conn, addr = s.accept()
    date = conn.recv(1024)
    if date == 'get':
        conn.send('200 ok')
    conn.close()
    print 'Connected by', addr, 'now closed'

client端代码

import socket
s = socket.socket()
s.connect(('172.16.196.145',60000))
s.send('get')
print s.recv(1024)
s.close()

tcpdump抓包

1 12:43:46.905723 IP 172.16.196.142.41334 > 172.16.196.145.60000: Flags [S], seq 3255498564, win 14600, options [mss 1460,sackOK,TS val 1412272238 ecr 0,nop,wscale 7], length 0
2 12:43:46.905751 IP 172.16.196.145.60000 > 172.16.196.142.41334: Flags [S.], seq 4195434198, ack 3255498565, win 14480, options [mss 1460,sackOK,TS val 1425611003 ecr 1412272238,nop,wscale 7], length 0
3 12:43:46.905987 IP 172.16.196.142.41334 > 172.16.196.145.60000: Flags [.], ack 4195434199, win 115, options [nop,nop,TS val 1412272238 ecr 1425611003], length 0
4 12:43:46.906031 IP 172.16.196.142.41334 > 172.16.196.145.60000: Flags [P.], seq 3255498565:3255498568, ack 4195434199, win 115, options [nop,nop,TS val 1412272238 ecr 1425611003], length 3
5 12:43:46.906041 IP 172.16.196.145.60000 > 172.16.196.142.41334: Flags [.], ack 3255498568, win 114, options [nop,nop,TS val 1425611003 ecr 1412272238], length 0
6 12:43:46.906265 IP 172.16.196.145.60000 > 172.16.196.142.41334: Flags [P.], seq 4195434199:4195434205, ack 3255498568, win 114, options [nop,nop,TS val 1425611003 ecr 1412272238], length 6
7 12:43:46.906305 IP 172.16.196.145.60000 > 172.16.196.142.41334: Flags [F.], seq 4195434205, ack 3255498568, win 114, options [nop,nop,TS val 1425611003 ecr 1412272238], length 0
8 12:43:46.906406 IP 172.16.196.142.41334 > 172.16.196.145.60000: Flags [.], ack 4195434205, win 115, options [nop,nop,TS val 1412272239 ecr 1425611003], length 0
9 12:43:46.906487 IP 172.16.196.142.41334 > 172.16.196.145.60000: Flags [F.], seq 3255498568, ack 4195434206, win 115, options [nop,nop,TS val 1412272239 ecr 1425611003], length 0
10 12:43:46.906500 IP 172.16.196.145.60000 > 172.16.196.142.41334: Flags [.], ack 3255498569, win 114, options [nop,nop,TS val 1425611004 ecr 1412272239], length 0

逐行分析

第一行: 客户端172.16.196.142，端口(后面用点表示)41336向服务端172.16.196.145.60000发起SYN主动请求，seq为3255498564

第二行: 服务端172.16.196.145.60000给客户端172.16.196.142.41336确认ACK ack为3255498564+1=3255498565，并同时也发起SYN同步

第三行: 客户端回复服务端的SYN确认

第四行: 客户端调用s.send('get')，发送数据，因此为P标记(PST)

第五行: 服务端回复客户端ACK确认标记

第六行: 服务端调用conn.send('200 ok')给客户端回复数据，为P标记(PST)

第七行: 服务端发送回复数据后，关闭连接，发起FIN主动关闭

第八行: 客户端回复服务端数据的确认ACK(ack=4195434205)

第九行: 客户端发送FIN关闭连接

第十行: 服务端发送客户端FIN的确认，由此，整个连接关闭

从分析来看，这是一个正常的三次握手建立连接之后传输数据，但是说好的四次挥手呢？

第七行、第九行、第十行，其实只有三次挥手

为什么是三次挥手

把上面最后断开连接的包放下来看

6 12:43:46.906265 IP 172.16.196.145.60000 > 172.16.196.142.41334: Flags [P.], seq 4195434199:4195434205, ack 3255498568, win 114, options [nop,nop,TS val 1425611003 ecr 1412272238], length 6
7 12:43:46.906305 IP 172.16.196.145.60000 > 172.16.196.142.41334: Flags [F.], seq 4195434205, ack 3255498568, win 114, options [nop,nop,TS val 1425611003 ecr 1412272238], length 0
8 12:43:46.906406 IP 172.16.196.142.41334 > 172.16.196.145.60000: Flags [.], ack 4195434205, win 115, options [nop,nop,TS val 1412272239 ecr 1425611003], length 0
9 12:43:46.906487 IP 172.16.196.142.41334 > 172.16.196.145.60000: Flags [F.], seq 3255498568, ack 4195434206, win 115, options [nop,nop,TS val 1412272239 ecr 1425611003], length 0
10 12:43:46.906500 IP 172.16.196.145.60000 > 172.16.196.142.41334: Flags [.], ack 3255498569, win 114, options [nop,nop,TS val 1425611004 ecr 1412272239], length 0

看看上面最后关闭tcp连接抓到的封包

为什么上面抓包只有三次挥手呢，因为服务端在conn.send('200 ok')后马上就调用cnn.close()发起关闭连接了，而客户端收到后还得调用s.recv(1024)，还没有发收到数据包的确认就马上又收到了服务端的FIN，于是客户端先是回复了服务端[P.]的ACK确认，然后此时客户端已经知道了服务端要关闭连接了，所以干脆合并FIN和ACK回复一个包，省掉一步，减少通信

抓到正确的四次挥手

既然知道是因为服务端太快的关闭了连接，因此修改服务端代码

import socket
import time

s = socket.socket()
s.bind(('172.16.196.145',60000))
s.listen(5)

while 1:
    conn, addr = s.accept()
    date = conn.recv(1024)
    if date == 'get':
        conn.send('200 ok')
        time.sleep(0.1) #暂停0.1秒后关闭连接
    conn.close()
    print 'Connected by',addr,'now closed'

我们暂停1秒后关闭连接，再抓包

1 20:01:49.251026 IP 172.16.196.142.41424 > 172.16.196.145.60000: Flags [S], seq 3935290883, win 14600, options [mss 1460,sackOK,TS val 1524954581 ecr 0,nop,wscale 7], length 0
2 20:01:49.251069 IP 172.16.196.145.60000 > 172.16.196.142.41424: Flags [S.], seq 3178165011, ack 3935290884, win 14480, options [mss 1460,sackOK,TS val 1538293348 ecr 1524954581,nop,wscale 7], length 0
3 20:01:49.251291 IP 172.16.196.142.41424 > 172.16.196.145.60000: Flags [.], ack 3178165012, win 115, options [nop,nop,TS val 1524954581 ecr 1538293348], length 0
4 20:01:49.251334 IP 172.16.196.142.41424 > 172.16.196.145.60000: Flags [P.], seq 3935290884:3935290887, ack 3178165012, win 115, options [nop,nop,TS val 1524954582 ecr 1538293348], length 3
5 20:01:49.251358 IP 172.16.196.145.60000 > 172.16.196.142.41424: Flags [.], ack 3935290887, win 114, options [nop,nop,TS val 1538293349 ecr 1524954582], length 0
6 20:01:49.251544 IP 172.16.196.145.60000 > 172.16.196.142.41424: Flags [P.], seq 3178165012:3178165018, ack 3935290887, win 114, options [nop,nop,TS val 1538293349 ecr 1524954582], length 6
7 20:01:49.251663 IP 172.16.196.142.41424 > 172.16.196.145.60000: Flags [.], ack 3178165018, win 115, options [nop,nop,TS val 1524954582 ecr 1538293349], length 0
8 20:01:49.251781 IP 172.16.196.142.41424 > 172.16.196.145.60000: Flags [F.], seq 3935290887, ack 3178165018, win 115, options [nop,nop,TS val 1524954582 ecr 1538293349], length 0
9 20:01:49.291441 IP 172.16.196.145.60000 > 172.16.196.142.41424: Flags [.], ack 3935290888, win 114, options [nop,nop,TS val 1538293389 ecr 1524954582], length 0
10 20:01:49.351812 IP 172.16.196.145.60000 > 172.16.196.142.41424: Flags [F.], seq 3178165018, ack 3935290888, win 114, options [nop,nop,TS val 1538293449 ecr 1524954582], length 0
11 20:01:49.352041 IP 172.16.196.142.41424 > 172.16.196.145.60000: Flags [.], ack 3178165019, win 115, options [nop,nop,TS val 1524954682 ecr 1538293449], length 0

6 服务端调用conn.send('200 ok')，seq=3178165012:3178165018

7 客户端收到封包的确认，ack=3178165018

8 此时发现跟之前不一样了，由于服务端暂停了0.1秒，因此主动关闭发送FIN标记的变成了客户端(172.16.196.142)，seq:3935290887

9 服务端回复客户端的FIN确认ACK，ack:3935290888

10 服务端发起FIN关闭，seq:3178165018

11 行客户端回复服务端的FIN确认ACK,ack:3178165019

因此以上抓包抓到了TCP完整的四次挥手

CLOSING状态的tcpdump包

由于CLOSING状态是两边同时发FIN，而且还要在收到对方的ACK前收到FIN，因此代码很难实现，我倒是在web上面很容易就抓到了这种封包

1 19:40:34.831216 IP 10.29.64.142.443 > 10.25.137.230.46556: Flags [F.], seq 3633446994, ack 1764274713, win 131, length 0
2 19:40:34.832085 IP 10.25.137.230.46556 > 10.29.64.142.443: Flags [F.], seq 1764274713, ack 3633446994, win 480, length 0
3 19:40:34.832107 IP 10.29.64.142.443 > 10.25.137.230.46556: Flags [.], ack 1764274714, win 131, length 0
4 19:40:34.832395 IP 10.25.137.230.46556 > 10.29.64.142.443: Flags [.], ack 3633446995, win 480, length 0

1 10.29.64.142 发起了FIN，seq: 3633446994

2 10.25.137.230 发起了FIN，seq: 1764274713

3 10.29.64.142 发送确认ACK，ack: 1764274714

4 10.25.137.230 发送确认ACK，ack: 3633446995

这种封包就是同时发FIN，没有先收到ACK，而是收到对方的FIN，双方进入CLOSING状态的封包，不过随着对方很快发送ACK，因此双方进入TIME_WAIT状态