Linux trasparent proxy - Iptable rules

help
1

In this tutorial:
sudo iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8080
sudo iptables -t nat -A PREROUTING -p tcp --dport 443 -j REDIRECT --to-port 8080

How can I find port number in my system to replace those and what are the factors that decides the port numbers used in those iptable rules?

2

80 and 443 are the standard ports used in the browsers for almost all sites.
8080 is the default mitmproxy port, you can configure it passing -p argument.