Skip to main content


There is certainly some confusion with sockets, as my flatmate put it; sockets connect stuff together right?

Wikipedia puts it a little clearer with:
sockets are inter-process communication endpoints
That sounds pretty boring but I promise you can do some fairly interesting things with them. Sockets are a core part of the operating system, and a socket API is used to direct how the operating system should use sockets.

So the hello world of sockets would have to be an echo server. Let's create a basic Python3 socket server that will listen on IPv6 and repeat what it hears.

The Python Howto guide for sockets starts with this:
Sockets are used nearly everywhere, but are one of the most severely misunderstood technologies around.
On the server side, you follow these steps:
  1. Create a socket (possibly after querying the system for information)
  2. Bind the socket, which assigns the socket to an address
  3. Listen prepares it for incoming connections
  4. Accept an incoming connection from a client, giving you a new dedicated socket to communicate with that client.
Then the server can call send and recv on this new socket. Much like this:

#!/usr/bin/env python3
# Echo consumer_server program
import socket
import threading

host = "localhost"               # Symbolic name meaning all available interfaces
random_port = 50007              # Arbitrary non-privileged port

def echo(conn, addr):
    while True:
        data = conn.recv(1024)
        if not data: break
        print('Server heard "{}"'.format(data.decode()))

def consumer_server(address_family, socket_type, protocol, canonname, sa):
        s = socket.socket(address_family, socket_type, protocol)
    except OSError as msg:
        print('could not open socket')
    # Tell the OS it can reuse a socket if it wants
    s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        # tells the OS that we want it to queue up to 5 connect requests
    except OSError as msg:
        raise SystemExit('Socket binding/listening failed...')

    while True:
            conn, addr = s.accept()
            print('Connected by', addr)
            threading.Thread(target=echo, args=(conn, addr)).start()
        except KeyboardInterrupt:
            raise SystemExit()

address_info = socket.getaddrinfo(host, random_port, 
                         socket.AF_UNSPEC, socket.SOCK_STREAM, 
                         0, socket.AI_PASSIVE)[-1]
Hopefully this server program is easy enough to understand, it is based very closely on the socket example in the Python documentation. My small alterations cause the server to listen only on IPv6, and will spin off a new thread when a client connects, and the server keeps listening for more connections. Part two of "Hello World in Sockets" is the client which follows a sequence of:
  1. Creating a socket (also using system information from getaddrinfo)
  2. connecting to the socket
Then the client can send, sendall, and recv bytes over the socket. Simple!

# Echo client program
import socket

HOST = 'localhost'    # The remote host
PORT = 50007          # The same port as used by the server

af, socktype, proto, _, sa = socket.getaddrinfo(HOST, 

    s = socket.socket(af, socktype, proto)
except socket.error:
    raise SystemExit('Error: Could not open socket :-/')

def send_and_receive(outgoing_data):
    incoming_data = s.recv(1024)
    print('Received', incoming_data.decode())

for data in [b'Hello, world', '汉语/漢語 Hànyǔ'.encode()]:


That's the simple low level stuff out of the way! I'd like to point out that Python does have a higher level socket api that could easily implement our echo server, have a look at the TCP Socket examples that use the socketserver module. Now the stuff I'm really interested in, using sockets with Bluetooth and CAN!

Popular posts from this blog

Driveby contribution to Python Cryptography

While at PyConAU 2016 I attended the Monday sprints and spent some time looking at a proposed feature I hoped would soon be part of cryptography. As most readers of this blog will know, cryptography is a very respected project within the Python ecosystem and it was an interesting experience to see how such a prominent open source project handles contributions and reviews.

The feature in question is the Diffie-Hellman Key Exchange algorithm used in many cryptography applications. Diffie-Helman Key Exchange is a way of generating a shared secret between two parties where the secret can't be determined by an eavesdropper observing the communication. DHE is extremely common - it is one of the primary methods used to provide "perfect forward secrecy" every time you initiate a TLS connection to an HTTPS website. Mathematically it is extremely elegant and the inventors were the recipients of the 2015 Turing award.

I wanted to write about this particular contribution because man…

My setup for downloading & streaming movies and tv

I recently signed up for Netflix and am retiring my headless home media pc. This blog will have to serve as its obituary. The box spent about half of its life running FreeNAS, and half running Archlinux. I’ll briefly talk about my experience with FreeNAS, the migration, and then I’ll get to the robust setup I ended up with.

The machine itself cost around $1000 in 2014. Powered by an AMD A4-7300 3.8GHz cpu with 8GB of memory. A SilverStone DS380 case is both functional, quiet and looks great. The hard drives have been updated over the last two years until it had a full compliment of 6 WD Green 4TiB drives - all spinning bits of metal though.

Initially I had the BSD based FreeNAS operating system installed. I had a single hard drive in its own ZFS pool for TV and Movies, and a second ZFS pool comprised of 5 hard drives for documents and photos.

FreeNAS is straight forward to use and setup, provided you only want to do things supported out of the box or by plugins. Each plugin is install…

Markdown Editor Component for Angular2

Thought I'd share a component I've been hacking on for angular2: a syntax highlighted markdown editor with rendered preview.

The code including a basic example is available on github. Because Angular2 hasn't yet been released this is really just me kicking the tyres.

This component relies on two libraries:

- marked for rendering markdown as html
- and ace editor for editing markdown
Basic Usage Example Add to your html template:
<markdown-editor (save)="updatedText($event)" [initial-text]="markdownContent"></markdown-editor> Remember to include the Markdowndirective in your @Component annotation:
@Component({ selector:'about', directives: [CORE_DIRECTIVES, Markdown] }) Another Example You can also control the component with external ui:
<button (click)="md.editMode = true">Custom Edit Button</button><markdown-editor [initial-text]="myMarkdownText" [show-edit-but…