Python code to create bash script that block all Steam servers.

generate_block_steam_script.py

#!/usr/bin/env python
"""
Python code to create bash script that block all Steam servers.

You must have installed ufw:
$ sudo apt install ufw

Then you can run this script and generated bash script:
$ python3 generate_block_steam_script.py > steam_block.sh
$ chmod +x steam_block.sh && sudo bash steam_block.sh

This block only direct Valve addresses.
You also may want to block other ports used by steam application.
This can be easy done via GUFW program, by adding preconfigured rule for Steam.
"""
import ipaddress
import logging
import re
from urllib.request import urlopen

logger = logging.getLogger(__name__)


def genSteamIpAddress() -> ipaddress.IPv4Network | ipaddress.IPv6Network:
    """Get steam IP addresses.

    Based on https://support.steampowered.com/kb_article.php?ref=8571-GLVN-8711
    """
    body = urlopen('https://bgp.he.net/AS32590#_prefixes').read().decode('utf-8')

    ipv4 = re.findall(r'<a href=.*?>([\d./]+)</a>', body)
    yield from map(ipaddress.ip_network, ipv4)

    ipv6 = re.findall(r'<a href=.*?>([\da-f:/]+)</a>', body)
    yield from map(ipaddress.ip_network, ipv6)


def createUfwScript():
    content = """
#!/bin/bash

ufw reset
ufw default allow incoming
ufw default allow outgoing
"""
    for ip in genSteamIpAddress():
        content += f'ufw deny from {ip} to any\n'
        content += f'ufw deny out to {ip}\n'

    print(content)


if __name__ == '__main__':
    logging.basicConfig(level=logging.DEBUG)
    createUfwScript()