How to properly close everything and open only the necessary (incoming / outgoing) iptables and debian?

Question

bash

Hello.
Can you please tell me how to properly close all incoming and outgoing ports on Debian using iptables, except for the 5-6 necessary ones?
That is, we close everything (outgoing and incoming), except, for example, 21,22,25,80,81,443. Ports should also work both incoming and outgoing.

Answer 1

# Разрешит локалный траффик
iptables -A INPUT -i lo -j ACCEPT

# Разрешит пинг
iptables -A INPUT -p icmp -j ACCEPT

# разрешит порты 21,22,25,80,81,443
iptables -A INPUT -p tcp -m multiport --port 21,22,25,80,81,443 -j ACCEPT

# Разрешит сессии
iptables -A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT 

# ну и запретит все остальное
iptables -A INPUT -j DROP