We Keep Coding

python socket send data after receiving fin (stuck in LAST_ACK)

I have a python program that is supposed to receive some arbitrary bytes and send them back after receiving a fin. I already was able to implement this as you can see below.
The problem I am having is that the connection is never properly closed. Using ss -tan I can see that the connection keeps being stuck in LAST_ACK state. This is although the connection seems to be closed correctly looking at the Wireshark packet trace. I have attached an Image of the Wireshark packet trace that is the result of first sending "AAAAA" then an out of order "BBB" and then filling the hole with "CCC".
Looking at the Wireshark packets I think that all packets should be correctly acknowledged and the connection shoud terminate normally without being stuck in LAST_ACK state and without doing the retransmissions at the bottom. But I Still would guess that there is a problem with the connection closing packets.
# Echo server program
import socket
HOST = '' # Symbolic name meaning all available interfaces
PORT = 6000 # Arbitrary non-privileged port
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen(1)
conn, addr = s.accept()
print ('Connected by', addr)
data_acc = b''
while 1:
data = conn.recv(1024)
if not data: break
data_acc += data
print(data)
print("send data back")
conn.sendall(data_acc)
conn.close()

I found the answer to the problem. The sequence number of ACK on line 56 in the screenshot needs to be increased by one. By doing so Wireshark also stops interpreting it as a keep alive packet.

A request to send or receive data was disallowed because the socket is not connected in Python

I'm trying to make a console chat app in python using socket library.
Whenever I send a message to the server, the server code crashes with the following message:
OSError: [WinError 10057] A request to send or receive data was disallowed because the socket is not connected and (when sending on a datagram socket using a sendto call) no address was supplied
Server code
import socket
HOST = socket.gethostbyname(socket.gethostname()) # get the ip address of PC
PORT = 5050
ADDRESS = (HOST, PORT)
socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
socket.bind(ADDRESS)
while True:
socket.listen()
conn, addr = socket.accept()
print(f"Connected by {addr}")
while True:
data = conn.recv(64)
print(data.decode('utf-8'))
socket.send(data)
Client code
import socket
HOST = socket.gethostbyname(socket.gethostname()) # get the ip address of PC
PORT = 5050
ADDRESS = (HOST, PORT)
socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
socket.connect(ADDRESS)
while True:
msg = input("Enter your message")
socket.send(msg.encode('utf-8'))
data = socket.recv(64)
print(data.decode('utf-8'))
What I am trying to achieve is whenever I send a message to the server, the client script should print the sent message. How can I fix that?

You're attempting to send data to your own server socket. You instead want to send to the client that you accepted.
socket.send(data)
Should be:
conn.send(data)
If you think about it, if you had multiple clients, how would you send data to a specific client? By using the socket that accept gave you.
As a side note, you probably don't want to import the module as socket, and also call your variable socket. It's fine here, but if you were to make a more complicated project, you may accidentally refer to the object when you meant to refer to the module. I'd rename the socket object to sock or server_socket to avoid shadowing.

How to send socket message when Server has a preset initial response?

I'm making python binds for Blackmagic's Ethernet Control Protocol ( as documented in https://documents.blackmagicdesign.com/UserManuals/HyperDeckManual.pdf?_v=1528269592000 , page 60). Simple socket connection seems to fail however, because every commands gets rejected with the server's greeting.
This protocol documents how software can communicate with certain blackmagic devices, in this case, Blackmagic's hyperdeck, the device runs a TCP server constantly listening on port 9993, on cmd I can simply telnet to it and issue commands, you'd it expect it to be as straightforward in python, however every command gets ignored for the server's greeting message, the device's information. I have been doing socket's for at least 3 months now and i've tried several methods of code, and all seem to fail.
For the most trivial test i've used:
import socket
HOST = "device's ip"
PORT = 9993
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
s.sendall(b'play')
data = s.recv(1024)
print(data)
and a modified version to try to repeat the command:
import socket
import time
HOST = "device's ip"
PORT = 9993
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
s.sendall(b'play')
data = s.recv(1024)
time.sleep(2)
s.sendall(b'play')
It should start video playback, as documented, and as occurs when I issue the command thru telnet, however the command is completely ignored and data always equals to: b'500 connection info:\r\nprotocol version: 1.9\r\nmodel: HyperDeck Studio Mini\r\n\r\n' , the server's greeting message in byte form, it should instead be 200 ok or some sort of error / acknowledged message, as documented.
This is incredibly annoying and i've thought of using subprocess and issuing commands thru cmd as an alternative, but something tells me there's an easier workaround.

Communicating with multiple clients using one TCP socket python

I am using TCP sockets to communicate between my server and clients. The server code and socket code are as below:
server:
from socket import *
HOST = 'xx.xx.xx.xx'
PORT = 1999
serversocket = socket(AF_INET,SOCK_STREAM)
serversocket.bind((HOST,PORT))
print 'bind success'
serversocket.listen(5)
print 'listening'
while True:
(clientsocket, address) = serversocket.accept()
print ("Got client request from",address)
#clientsocket.send('True')
data = clientsocket.recv(1024)
print data
clientsocket.send('True')
clientsocket.close()
client:
import socket
import sys
# Create a TCP/IP socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# Connect the socket to the port on the server given by the caller
server_address = ('xx.xx.xx.xx', 1999)
print >>sys.stderr, 'connecting to %s port %s' % server_address
sock.connect(server_address)
try:
message = 'This is the message. It will be repeated.'
print >>sys.stderr, 'sending'
for x in range (0,1):
name=raw_input ('what is ur name')
print type(name)
sock.send(name)
print sock.recv(1024)
finally:
sock.close()
I am able to communicate with the server from client and able to send and receive data. But the problem I am facing is that I am not able to send and receive data continuously from the server. I have to restart my client code on my laptop to send and receive data again from the server. The way the above client code is working is that when I give a keyboard input, then the socket sends data to server and server responds back. But in the client code, in the for loop if I do two iterations, for the second iteration the data I enter from keyboard is not reaching server. I need to restart my client code to send data again. How do I fix this ?
Also, when once client is connected to the server, the other cannot connect to the server. Any ideas on how to do this ?

You need to design and implement a protocol that specifies what each side is supposed to do and then implement that protocol. You're expecting it to work by magic.
For example:
data = clientsocket.recv(1024)
I suspect you are expecting this to receive a "message". But TCP has no notion of messages. If you need to send and receive messages, you need to define precisely what a "message" is for your protocol and write code to send and receive them.
It may be helpful to look at the specifications for other protocols that use TCP such as HTTP, FTP, or IRC. It really is worth the time to write out a specification of your protocol before you write any code. It will save a lot of pain.

Cannot run the socket programs correctly in Centos using Python

I'm currently working on with Sockets using Python.
As a starter, I tried copying first the examples given in this (17.2.2. Example) tutorial
I put the client and the server scripts in two different machines (of course)
Now, I want to try if it works, but I'm kind of lost.
I'm thinking of running the server program continuously so that it will keep on receiving the data sent by the client program. However, when I tried to run the Server program, it is giving me this error
socket.error: (99, 'Cannot assign requested address')
and When I tried running the client program, it doesnt give me errors, however, it is printing random data, which is different from what I'm expecting because I sent the String "Hello World", So im expecting that it will receive and print "Hello World"
Shown below is the server program
# Echo server program
import socket
HOST = '192.168.104.112' # Symbolic name meaning all available interfaces
PORT = 50007 # Arbitrary non-privileged port
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen(1)
conn, addr = s.accept()
print 'Connected by', addr
while 1:
data = conn.recv(1024)
if not data: break
conn.sendall(data)
conn.close()
and the one below is the client program
# Echo client program
import socket
HOST = '192.168.104.111' # The remote host
PORT = 50007 # The same port as used by the server
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
s.sendall('Hello, world')
data = s.recv(1024)
s.close()
print 'Received', repr(data)
Assuming that the IP of the machine that runs the server program is : 192.168.104.111
while the Client program is : 192.168.104.112
Im not really sure where to get the port number so I just used the port showed in the rpyc in the terminal. how do I get the correct port number anyway?
I know I made a lot of mistakes here. I just don't which part. Could you point me the mistakes that i've done and how to correct them? and how do I run these programs?
BTW, i'm using Centos.

On the server, HOST should be either 0.0.0.0 or the server's own IP address. The server needs to bind its listening port to its own interface(s). The client connects to the server.
Your client program doesn't check for errors. So if it can't connect to the server, things go awry.