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Things to consider while failproofing socket communciation

I have a simple client/server socket program written in python. So far I think I have managed to deal with two types of certain socket errors.
server is not alive when the client is open (Connectionrejected)
server shuts down while the client is open (Connectionreset)
I am wondering what other errors I should be accounting for. When the client sends a message and the server is closed before the send happens, the client waits until it can reconnect and sends the message however I am not sure if that covers all the cases where the message can be lost or not transmitted completely.
server.py
import socket
import threading
HEADER = 8
PORT = 5050
SERVER = "127.0.0.1" #socket.gethostbyname(socket.gethostname())
ADDR = (SERVER, PORT)
FORMAT = 'utf-8'
DISCONNECT_MESSAGE = "exit"
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
#server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
server.bind(ADDR)
def close():
server.shutdown(socket.SHUT_RDWR)
server.close()
print ("closed")
def handle_client(conn, addr):
print(f"[NEW CONNECTION] {addr} connected.")
connected = True
while connected:
msg_length = conn.recv(HEADER).decode(FORMAT)
if msg_length:
msg_length = int(msg_length)
msg = conn.recv(msg_length).decode(FORMAT)
if msg == DISCONNECT_MESSAGE:
connected = False
print(f"[{addr}] {msg}")
conn.send("Barcode received".encode(FORMAT))
conn.shutdown(socket.SHUT_RDWR)
conn.close()
def start():
server.listen()
print(f"[LISTENING] Server is listening on {SERVER}")
while True:
conn, addr = server.accept()
thread = threading.Thread(target=handle_client, args=(conn, addr))
thread.start()
print(f"[ACTIVE CONNECTIONS] {threading.active_count() - 1}")
print("[STARTING] server is starting...")
start()
client.py
import socket
import datetime
import time
HEADER = 8
PORT = 5050
FORMAT = 'utf-8'
DISCONNECT_MESSAGE = "exit"
SERVER = "127.0.0.1"
ADDR = (SERVER, PORT)
def check_connection(ADDR):
print("Trying to connect to the server...")
connected = False
client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
while not connected:
try:
client.connect(ADDR)
except socket.error as error:
connected = False
time.sleep(0.01)
print("Can't connect.Retrying...")
#print(connected)
else:
connected = True
print("Connected to the server")
return client
def send(msg,client,ADDR):
message = msg.encode(FORMAT)
msg_length = len(message)
send_length = str(msg_length).encode(FORMAT)
send_length += b' ' * (HEADER - len(send_length))
sending = True
while sending :
try:
cl_ls[0].send(send_length)
cl_ls[0].send(message)
print(cl_ls[0].recv(2048).decode(FORMAT))
sending = False
except socket.error as error:
cl_ls[0] = check_connection(ADDR)
cl_ls[0].send(send_length)
cl_ls[0].send(message)
sending = False
print(cl_ls[0].recv(2048).decode(FORMAT))
cl_ls = ["client"]
client = check_connection(ADDR)
cl_ls[0] = client
accept_more_barcodes = True
while accept_more_barcodes:
input_value = input("Type 'exit' to close input box ")
today = datetime.datetime.now()
date_time = today.strftime("%m/%d/%Y, %H:%M:%S")
input_value += ' *** ' + date_time
if (input_value).startswith("exit"):
accept_more_barcodes = False
else:
with open("barcodes.txt", "a") as f:
f.write(input_value + "\n")
send(input_value,cl_ls[0],ADDR)

How to limit number of connections to server socket

I'm trying to have multiple clients connect to a server socket and transfer data between them, however I want to limit the number of connections that can be made to the server/ number of client processes to 3. How can I do this?
server:
import socket
from _thread import *
PORT = 5050
HOST = socket.gethostbyname(socket.gethostname())
ADDR = (HOST, PORT)
ThreadCount = 0
tcp = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
try:
tcp.bind(ADDR)
except socket.error as e:
print(str(e))
print("Awaiting connection")
tcp.listen(5)
def threaded_client(connection):
connection.send(str.encode('Welcome to sv'))
while True:
data = connection.recv(2048)
reply = 'Server returns: ' + data.decode('utf-8')
if not data:
break
connection.sendall(str.encode(reply))
connection.close()
while True:
Client, address = tcp.accept()
print('Connected to: ' + address[0] + ':' + str(address[1]))
start_new_thread(threaded_client, (Client, ))
ThreadCount += 1
print('Number of threads: ' + str(ThreadCount))
ServerSocket.close()
client:
import socket
Client = socket.socket(socket.AF_INET, socket. SOCK_STREAM)
PORT = 5050
HOST = socket.gethostbyname(socket.gethostname())
ADDR = (HOST, PORT)
print('Awaiting connection')
try:
Client.connect(ADDR)
except socket.error as e:
print(str(e))
Response = Client.recv(1024)
while True:
Input = input('Sends Info: ')
Client.send(str.encode(Input))
Response = Client.recv(1024)
print(Response.decode('utf-8'))
Cliente.close()

python socket listener not receiving data

I am programming a decentralised script to track the IPs of other computers running the script, to explore decentralisation. This script isolates the problem. The code consists of 2 scripts, one main program which sends its IP to an IP provided if one is provided, and a listener program which is run as a subscript and listens for data and pipes that data back to the main program. The main script appears to be working, the data is sent over the network, but the listener does not receive it.
This is the main script
import socket
from subprocess import Popen, PIPE
from time import sleep
def getIP():
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
try:
s.connect(('8.8.8.4', 1))
IP = s.getsockname()[0]
except Exception:
IP = '127.0.0.1'
finally:
s.close()
return IP
def sendfyi(target, ownIP):
toSend = 'fyi' + ':' + ownIP
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((target, 50000))
s.send(toSend.encode())
s.close()
print('sent fyi')
otherIPs = []
ownIP = getIP()
targetIP = input('enter ip or 0: ')
if targetIP != '0':
otherIPs.append(targetIP)
sendfyi(targetIP, ownIP)
listener = Popen(['python3', 'testlistener.py'], stdout=PIPE, stderr=PIPE)
i = 0
while i == 0:
sleep(1)
listenerPipe = listener.stdout.readline()
print(listenerPipe)
This is the sub process:
import socket
def getIP():
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
try:
# doesn't even have to be reachable
s.connect(('8.8.8.4', 1))
IP = s.getsockname()[0]
except Exception:
IP = '127.0.0.1'
finally:
s.close()
return IP
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((getIP(), 50000))
i = 1
while i == 1:
s.listen(1)
conn, addr = s.accept()
print('conected', flush=True)
data = conn.recv(1024)
print('data receved', flush=True)
out = data.decode()
print('data decoded', flush=True)
print(out, flush=True)
conn.close()

Incorect bind statement
bind(('', 50000))

Python socket client doesnt receive anything, hangs by recv.from(datasize)

I have to make a simple udp echo client and server, currently my problem is that the client hangs itself up at the code "data, address = sock.recvfrom(dataSize)". Here is my client and server code (I removed some api functions, so it aint long). I tried same ports and different ports and the same goes with the ip's but i dont get any message back. I tried the original file from a friend and his version works, I have the same port and ip and the same methods on both files, but it still keeps hanging at the echoClient method receiveMSG at the first line.
echoServerUDP.py
0<0# : ^
'''
#echo off
python "%~f0" %*
pause
exit /b 0
'''
#!/usr/bin/env python
import socket
import json
host = '0.0.0.0'
sport = 11111 # own port
dataSize = 1024
ip_adresses = {}
def echo_server():
receiveSock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
receiveSock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
receiveSock.bind((host,sport))
print("Starting up echo server on %s port %s" % (host, sport))
while True:
print("Waiting to receive message")
print("Generate a Session ...")
data, address = receiveSock.recvfrom(dataSize)
data = data.decode("utf-8")
if data:
print("receive data: %s from %s" % (data,address))
json_object = json.loads(data)
operation=json_object["operation"]
if operation == "register":
register(json_object["name"],json_object["value"],json_object["sid"])
json_message={"ergebnis":"ok"}
dump = json.dumps(json_message)
sendMSG(bytes(dump,encoding="utf-8"),address)
print("er")
if operation == "unregister":
unregister(json_object["name"],json_object["sid"])
if operation == "query":
query(json_object["sid"])
if operation == "reset":
reset(json_object["sid"])
print("sent %s bytes back to %s" % (data,address))
def sendMSG(data,address):
sendSock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sendSock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sendSock.sendto(data,address)
sendSock.close()
if __name__ == '__main__':
echo_server()
and here the echoClientUDP.py
0<0# : ^
'''
#echo off
python "%~f0" %*
pause
exit /b 0
'''
#!/usr/bin/env python
import socket
import time
import json
from random import randint
host = '127.0.0.1'
sport = 11111
dataSize = 1024
sid= randint(1,10000)
name=socket.gethostname()
own_ip = socket.gethostbyname(name)
def echo_client():
sendSock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
try:
json_message = {"operation":"register","name":name,"value":own_ip,"sid":sid}
dump = json.dumps(json_message)
print("Sending %s to %s:%s" % (dump,host,sport))
sendMSG(sendSock, dump)
data = receiveMSG(sendSock)
if data:
print("Received: %s" % data)
except Exception as err:
print("Socket error: %s" %str(err))
finally:
print("Closing connection to the server")
sendSock.close()
def sendMSG(sendSock, data):
sendSock.connect((host, sport))
sendSock.sendall(bytes(data,encoding="utf-8"))
def receiveMSG(sock):
data, address = sock.recvfrom(dataSize)
print(data)
return data
if __name__ == '__main__':
echo_client()

Please put more effort in some areas like you have not added any headers and you have gone more complicated i have also created a similar working script see and observe that script and make changes to your script
client.py :-
import socket
import sys
HEADER = 64
PORT = 65432
FORMAT = 'utf-8'
DISCONNECT_MESSAGE = "!DISCONNECT"
SERVER = socket.gethostbyname(socket.gethostname())
ADDR = (SERVER, PORT)
client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client.connect(ADDR)
def send(msg):
message = msg.encode(FORMAT)
msg_length = len(message)
send_length = str(msg_length).encode(FORMAT)
send_length += b' ' * (HEADER - len(send_length))
client.send(send_length)
client.send(message)
print(client.recv(2048).decode(FORMAT))
def chat():
while True:
try:
a = input()
if 'quit' in a:
sys.exit()
else:
send(a)
except Exception as e:
print(e)
sys.exit()
chat()
server.py :-
import socket
import threading
HEADER = 64
PORT = 65432
SERVER = socket.gethostbyname(socket.gethostname())
ADDR = (SERVER, PORT)
FORMAT = 'utf-8'
DISCONNECT_MESSAGE = "Bye"
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind(ADDR)
def handle_client(conn, addr):
print(f"[NEW CONNECTION] {addr} connected.")
connected = True
while connected:
msg_length = conn.recv(HEADER).decode(FORMAT)
if msg_length:
msg_length = int(msg_length)
msg = conn.recv(msg_length).decode(FORMAT)
if msg == DISCONNECT_MESSAGE:
connected = False
print(f"[{addr}] left")
print(f"[{addr}] {msg}")
conn.send("Msg received".encode(FORMAT))
conn.close()
def start():
server.listen()
print(f"[LISTENING] Server is listening on {SERVER}")
while True:
conn, addr = server.accept()
thread = threading.Thread(target=handle_client, args=(conn, addr))
thread.start()
print(f"[ACTIVE CONNECTIONS] {threading.activeCount() - 1}")
print("[STARTING] server is starting...")
start()
and i have used threading here to make the process more fast.
First run the server.py in background and then run client.py to connect to the server.

How to send data to client with conn.send()

Im trying to create a simple chat server. I've been able to send information to the server through the client using 'client.send()' but I cannot seem to do the same server->client
I have tried using methods such as conn.send() and conn.sendall(), but (I guess since the code is in a try) they seem to get skipped after the initial conn.send(str.encode("Connected"))
Server code
import socket
from _thread import *
import sys
server = "192.168.0.4"
port = 5555
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
try:
s.bind((server, port))
except socket.error as e:
str(e)
s.listen(2)
print("Waiting for a connection, Server Started")
def threaded_client(conn):
conn.send(str.encode("Connected"))
reply = ""
while True:
conn.send(str.encode(str(reply)))
try:
data = conn.recv(2048*1)
reply = data.decode("utf-8")
if not data:
print("Disconnected")
break
else:
print("Received: ", reply)
print("Sending : ", reply)
conn.sendall(str.encode(reply)) #Where I want to send information to the client
except:
break
print("Lost connection")
conn.close()
while True:
conn, addr = s.accept()
print("Connected to:", addr)
start_new_thread(threaded_client, (conn,))
client code
import socket
class Network:
def __init__(self):
self.client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.server = "192.168.0.4"
self.port = 5555
self.addr = (self.server, self.port)
self.id = self.connect()
print(self.id)
def connect(self):
try:
self.client.connect(self.addr)
return self.client.recv(2048).decode()
except:
pass
def send(self, data):
try:
self.client.send(str.encode(data))
return self.client.recv(2048).decode()
except socket.error as e:
print(e)
from network import Network
n = Network()
while True:
n.send("sending stuff") #this works/sends properly

You forgot to use print() to display data from server
while True:
print( n.send("sending stuff") )
BTW: in server you send the same data two times - with conn.send() and conn.sendall()