Python原始套接字不打印接收到的数据（但正在发送）

Question

I am sending two requests to a server, code is below:

import sys
import socket

s = socket.socket ( socket.AF_INET, socket.SOCK_STREAM )
s.connect((sys.argv[1], 3232))
s.send('HELP\n')
print("1st\n" + s.recv(4096))
s.send('HELP\n')
print("2nd\n" + s.recv(4096))
s.close()

HELP should always return something; for example if using netcat:

# nc 10.x.x.x 3232

HELP
* LIST OF VALID COMMANDS:
...

The code I posted above will not always output HELP response. For example; it will only show the recv input rarely; sometimes never (below, the 1st one isnt even shown):

# python example.py 10.x.x.x
1st
Welcome to...

2nd
* LIST OF VALID COMMANDS:
...

I thought that HELP wasn't be sent, so I opened wireshark and seen the following; so it appears to be sending properly:

HELP
* LIST OF VALID COMMANDS:
...

HELP
* LIST OF VALID COMMANDS:
...

Any ideas? I'm not sure what I could be doing wrong.

Answer 1

After some experimenting I managed to fix the code. The server, upon connection sends back data; so, on initial connection you must s.recv() ; I did this and the application behaves as I would expect.

Modified code:

import sys
import socket

s = socket.socket ( socket.AF_INET, socket.SOCK_STREAM )
s.connect((sys.argv[1], 3232))
print("0th\n" + s.recv(4096))
s.sendall('HELP\n')
print("1st\n" + s.recv(4096))
s.sendall('HELP\n')
print("2nd\n" + s.recv(4096))
s.close()

This returns the following always:

# python /tmp/test.py 10.x.x.x
0th
Welcome to...


1st
* LIST OF VALID COMMANDS:
...


2nd
* LIST OF VALID COMMANDS:
...