Linux/iptables

Tables	Chains
FILTER	INPUT FORWARD OUTPUT
NAT	PREROUTING OUTPUT POSTROUTING
MANGLE (modify ip headers)	PREROUTING INPUT FORWARD OUTPUT POSTROUTING
RAW (connection tracking)	PREROUTING OUTPUT
(SECURITY) (SELinux)	INPUT FORWARD OUTPUT

Targets	valid in	note
REJECT	INPUT, FORWARD, OUTPUT	sends response back
DROP		no response
ACCEPT
RETURN
MASQUERADE	POSTROUTING
REDIRECT	NAT:PREROUTING + NAT:OUTPUT

Basics

iptables
         -A (append - add rule at end)             -i (input interface)          -j (target)
         -C (check)                                -o (output interface)
         -D (delete - remove rule)                 -s (source address)    
         -F (flush - remove all rules)             -d (destination address)
         -I (insert - add at position)              
         -L (list - show all rules in chain)       -p      (protocol (tcp/udp))
         -N (new chain)                            --dport (destination port)
         -X (delete chain)                         --sport (source port)

         -t (table to manipulate (default: filter)

         -n (numeric output of addresses and ports)

View state

iptables-save                  # show everything

iptables --list-rules          # list filter rules (default: filter)
iptables --list-rules -t nat   # list nat rules

Iptables + Docker

All of Docker's iptables rules are added to the DOCKER chain. Do not manipulate this chain manually. If you need to add rules which load before Docker's rules, add them to the DOCKER-USER chain. These rules are applied before any rules Docker creates automatically.
Other rules added to the FORWARD chain, either manually, or by another iptables-based firewall, are evaluated after the DOCKER-USER and DOCKER chains. This means that if you publish a port through Docker, this port gets published no matter what rules your firewall has configured. If you want rules to apply even when a port gets published through Docker, you must add these rules to the DOCKER-USER chain.

Random examples

iptables -A INPUT -p tcp --dport 22 -j ACCEPT

openvpn

iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE
iptables -t nat -A POSTROUTING -o tun0 -j MASQUERADE

wireguard

iptables -A FORWARD -i wg0 -j ACCEPT
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

iptables -D FORWARD -i wg0 -j ACCEPT
iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE

raspi wifi to ethernet

iptables --table nat --append POSTROUTING --out-interface wlan0 -j MASQUERADE
iptables --append FORWARD --in-interface eth0 -j ACCEPT

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i eth0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i wlan0 -o eth0 -j ACCEPT

Links