Tech/Basics/Firewall

IPTABLES

iptables is a userspace wrapper to configure filtering in the Linux Kernel filter stack

Default accept

iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT

Default deny

iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP

Permit from one source

iptables -A INPUT -s sourceaddress -i interface -j ACCEPT

NFT

nft is a replacement and upgrade for iptables that does more