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Websocket implementation in Python 3

Trying to create a web interface for an application running Python3. The application will require bidirectional streaming, which sounds like a good opportunity to peek at websites.

My first wish was to use something already existing, and the sample applications from mod-pywebsocket proved to be valuable. Unfortunately, their APIs seem to be easily extensible, and that is Python2.

Looking back at the blogosphere, many people wrote their own websocket server for earlier versions of the websocket protocol, most of them do not implement hashing the security key, so they do not work.

Reading RFC 6455 I decided to take a hit at it myself and came up with the following:

#!/usr/bin/env python3 """ A partial implementation of RFC 6455 http://tools.ietf.org/pdf/rfc6455.pdf Brian Thorne 2012 """ import socket import threading import time import base64 import hashlib def calculate_websocket_hash(key): magic_websocket_string = b"258EAFA5-E914-47DA-95CA-C5AB0DC85B11" result_string = key + magic_websocket_string sha1_digest = hashlib.sha1(result_string).digest() response_data = base64.encodestring(sha1_digest) response_string = response_data.decode('utf8') return response_string def is_bit_set(int_type, offset): mask = 1 << offset return not 0 == (int_type & mask) def set_bit(int_type, offset): return int_type | (1 << offset) def bytes_to_int(data): # note big-endian is the standard network byte order return int.from_bytes(data, byteorder='big') def pack(data): """pack bytes for sending to client""" frame_head = bytearray(2) # set final fragment frame_head[0] = set_bit(frame_head[0], 7) # set opcode 1 = text frame_head[0] = set_bit(frame_head[0], 0) # payload length assert len(data) < 126, "haven't implemented that yet" frame_head[1] = len(data) # add data frame = frame_head + data.encode('utf-8') print(list(hex(b) for b in frame)) return frame def receive(s): """receive data from client""" # read the first two bytes frame_head = s.recv(2) # very first bit indicates if this is the final fragment print("final fragment: ", is_bit_set(frame_head[0], 7)) # bits 4-7 are the opcode (0x01 -> text) print("opcode: ", frame_head[0] & 0x0f) # mask bit, from client will ALWAYS be 1 assert is_bit_set(frame_head[1], 7) # length of payload # 7 bits, or 7 bits + 16 bits, or 7 bits + 64 bits payload_length = frame_head[1] & 0x7F if payload_length == 126: raw = s.recv(2) payload_length = bytes_to_int(raw) elif payload_length == 127: raw = s.recv(8) payload_length = bytes_to_int(raw) print('Payload is {} bytes'.format(payload_length)) """masking key All frames sent from the client to the server are masked by a 32-bit nounce value that is contained within the frame """ masking_key = s.recv(4) print("mask: ", masking_key, bytes_to_int(masking_key)) # finally get the payload data: masked_data_in = s.recv(payload_length) data = bytearray(payload_length) # The ith byte is the XOR of byte i of the data with # masking_key[i % 4] for i, b in enumerate(masked_data_in): data[i] = b ^ masking_key[i%4] return data def handle(s): client_request = s.recv(4096) # get to the key for line in client_request.splitlines(): if b'Sec-WebSocket-Key:' in line: key = line.split(b': ')[1] break response_string = calculate_websocket_hash(key) header = '''HTTP/1.1 101 Switching Protocols\r Upgrade: websocket\r Connection: Upgrade\r Sec-WebSocket-Accept: {}\r \r '''.format(response_string) s.send(header.encode()) # this works print(receive(s)) # this doesn't s.send(pack('Hello')) s.close() s = socket.socket( socket.AF_INET, socket.SOCK_STREAM) s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) s.bind(('', 9876)) s.listen(1) while True: t,_ = s.accept() threading.Thread(target=handle, args = (t,)).start()

Using this basic test page (which works with mod-pywebsocket):

 <!DOCTYPE html> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Web Socket Example</title> <meta charset="UTF-8"> </head> <body> <div id="serveroutput"></div> <form id="form"> <input type="text" value="Hello World!" id="msg" /> <input type="submit" value="Send" onclick="sendMsg()" /> </form> <script> var form = document.getElementById('form'); var msg = document.getElementById('msg'); var output = document.getElementById('serveroutput'); var s = new WebSocket("ws://"+window.location.hostname+":9876"); s.onopen = function(e) { console.log("opened"); out('Connected.'); } s.onclose = function(e) { console.log("closed"); out('Connection closed.'); } s.onmessage = function(e) { console.log("got: " + e.data); out(e.data); } form.onsubmit = function(e) { e.preventDefault(); msg.value = ''; window.scrollTop = window.scrollHeight; } function sendMsg() { s.send(msg.value); } function out(text) { var el = document.createElement('p'); el.innerHTML = text; output.appendChild(el); } msg.focus(); </script> </body> </html>

This receives the data and clears it correctly, but I cannot get the transmission traffic to work.

As a test for writing "Hello" to a socket, the program above calculates the bytes that should be written to the socket, like:

 ['0x81', '0x5', '0x48', '0x65', '0x6c', '0x6c', '0x6f']

Which correspond to the hexadecimal values ​​given in Section 5.7 of the RFC. Unfortunately, the frame never appears in Chrome Developer Tools.

Any idea what I am missing? Or the current working Python3 web layout example?

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1 answer

When I try to talk to your Python code from Safari 6.0.1 on Lion, I get

 Unexpected LF in Value at ...

in the Javascript console. I also get an IndexError exception from Python code.

When I talk to your Python code from Chrome version 24.0.1290.1 ​​dev on Lion, I get no Javascript errors. In your javascript, the onopen() and onclose() methods are onopen() , but not onmessage() . The python code does not throw any exceptions and appears to be receiving a message and sending a response, namely the behavior you see.

Since Safari did not like the final LF in the header, I tried to remove it, i.e.

 header = '''HTTP/1.1 101 Switching Protocols\r Upgrade: websocket\r Connection: Upgrade\r Sec-WebSocket-Accept: {}\r '''.format(response_string)

When I make this change, Chrome can see your reply message i.e.

 got: Hello

displayed in javascript console.

Safari is still not working. Now I am raising another problem when I try to send a message.

 websocket.html:36 INVALID_STATE_ERR: DOM Exception 11: An attempt was made to use an object that is not, or is no longer, usable.

None of the javascript websocket event handlers fires, and I still see an IndexError exception from python.

Finally. Your Python code did not work with Chrome due to the extra LF in the header response. Something else is happening there, because the code that works with Chrome does not work with Safari.

Update

I developed the main problem, and now I have an example of working in Safari and Chrome.

base64.encodestring() always adds trailing \n to it. This is the LF source Safari complained about.

calling .strip() on the return value of calculate_websocket_hash and using the original header template works correctly in Safari and Chrome.

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