socket.error: [Errno 111] when trying to connect to a socket

Question

I was trying to write a code where a client connects to server on a default port number, the server then sends another port number to the client. The client now connects to the new port number.

Client:

import socket
import sys
import os
import signal
import time
s = socket.socket()
s.connect(("127.0.0.1", 6667))
line = s.recv(1024)
if line.strip():
    port = int(line)
    s.close()
    soc = socket.socket()
    soc.connect(("127.0.0.1", port))
    print soc.recv(1024)
    soc.close()
else:
    s.close()

Server:

import socket
import sys
import os
import signal
import time
port = 7777
s = socket.socket()
s.bind(("127.0.0.1", 6667))
s.listen(0)
sc, address = s.accept()
print address
sc.send(str(port))
sc.close()
s.close()
sock = socket.socket()
sock.bind(("127.0.0.1", port))
soc, addr = sock.accept()
print addr
soc.send("Success")
soc.close()
sock.close()

When I execute this code, I am getting following errors on client and server sides.

Server:

('127.0.0.1', 36282)
Traceback (most recent call last):
  File "server.py", line 17, in <module>
    soc, addr = sock.accept()
  File "/usr/lib/python2.7/socket.py", line 202, in accept
    sock, addr = self._sock.accept()
socket.error: [Errno 22] Invalid argument

Client:

Traceback (most recent call last):
  File "client.py", line 13, in <module>
    soc.connect(("127.0.0.1", port))
  File "/usr/lib/python2.7/socket.py", line 224, in meth
    return getattr(self._sock,name)(*args)
socket.error: [Errno 111] Connection refused

Can someone explain me the reason for these errors and provide a solution for these errors.

Answer 1

Before you can listen to a TCP/IP socket (a connection based streaming socket) you need to use bind to assign a socket (created with socket.socket()) . Then you need to do listen to prepare it for incoming connections and then finally you do accept on the prepared socket.

You appear to be missing sock.listen(0) after your call to sock.bind(("127.0.0.1", port)). The Python documentation is short on details but it does say this about TCP/IP:

Note that a server must perform the sequence socket(), bind(), listen(), accept() (possibly repeating the accept() to service more than one client), while a client only needs the sequence socket(), connect(). Also note that the server does not sendall()/recv() on the socket it is listening on but on the new socket returned by accept().

Python bases its socket module on a Berkeley Socket model. You can find some more detailed information on Berkeley Sockets at this link . In particular it says this about bind:

bind() assigns a socket to an address. When a socket is created using socket(), it is only given a protocol family, but not assigned an address. This association with an address must be performed with the bind() system call before the socket can accept connections to other hosts.

Also consider what would happen if your client gets sent a port number (and tries to connect) before the server starts listening for connections (on port 7777 in this case). Although not the cause of your problems, I wanted to point out the scenario for completeness. Something you may consider is not closing the port 6667 socket until after you have called listen on the port 7777 socket. After calling listen you can then close down the first socket. On the client after reading the port you can wait until the first connection (port 6667) is closed down by the server and then connect to port 7777.