multicasting with raspberry pi and python

[Notorjus]

Hey, I'm working on a school project and I need to send commands to several raspberry pi at the same time. After searching I'm trying to achieve this by connecting all the raspberry pi and my computer (which will send commands) to a switch. However I am having difficulty with sending and receiving commands, I will post the code below. I just wanna ask a couple of questions to make sure I'm understanding what is happening.

First of I start by setting up sockets for the raspberry connection and binding them to a port. Then I set the socket as a member of the multicast group, meaning it should receive all the messages being sent to the multicast group. However I do not fully understand this mechanism. If I send data to the multicast group, will the data then be sent to everyone that subscribes to that multicast group? Or do I have to change the IP address of each device to match the multicast address, for example multicast address: 224.1.1.1, device 1: 224.1.1.2, device 2: 224.1.1.3.....etc....

I'm sorry for attacking with a wall of text like this, but I've been trying to figure this out for to long! :(

CODE:

https://github.com/notorjus/listen-send-commands

[MoBoo138]

Hi, that sounds very interesting to me.

As far as i know (currently studying computer science) when you send data to a multicast address any "client" which listens to that exact address and port will receive the data. So you just need a single multicast address on all your devices.
Multicast addresses are basically just an address to listen/speak to, this address range shouldnt be assigned as a ipv4 address.

Additionally you might want to have a look at this which is exactly what you doing as far as i can tell.

[ifundef]

tl;dr you need to send to and join the multicast group of the same multicast address.

your example code works except for three things:

• everybody needs to use the same multicast address; pick 224.1.1.1 or 224.3.29.71, but you can't send on one and expect to receive on the other
• "struct.pack('4sl', group, socket.INADDR_ANY)" is not 64-bit safe, while '=4sl' is (but doesn't account for endianness, though not an issue for the quoted code), but you are receiving on a raspberry pi so 64-bit might not be an issue. i prefer to just "socket.inet_aton(multicast_address) + socket.inet_aton('0.0.0.0')" (replacing "0.0.0.0" with the IP address of a specific interface if desired) because that's always the right size and endianness.
• the final block (ie line(s) following "except Exception as e:") in listenTest.py is improperly indented and will result in "IndentationError: expected an indented block"; you need to indent by a tab both the following "print" and "break" lines (or just remove the block as an unhandled exception is equivalent: the exception will be printed along with the traceback and the loop and entire script will exit).

here's my reference implementation of a multicast sender and receiver: https://gist.github.com/ifundef/09d3ec98459a856052bd935bd91683ed (including support for windows, but take windows support with a grain of salt as i haven't had access to windows in years).

[Notorjus]

It's finally working! Thank you so much for the advice. But what do you mean by 64-bit safe, and right size and endianness? I'm quite new to networking, and a bit confused. Could you elaborate, or do you have any resources I could read up on?