import socket

s = socket.socket()
host = socket.gethostname()
port = 12345
s.bind((host, port))
s.listen()

while True:
    (connection, addr) = s.accept()
    print('Verbindung von ', addr)
    req = connection.recv(1024)
    msg = req.decode('utf-8')
    if msg != 'STOP':
        msg = msg.upper()
        connection.send(msg.encode('utf-8'))
    else:
        connection.close()