Summary page of the Netfilter related resources
iptables 101 and FAQ to get started quickly
iptables tutorial from frozentux
flowgraph of the packets in netfilter as
words of wisdom regarding iptables
sample iptables rule sets for IPv4 and IPv6
Things You Should Know About Netfilter
about asking questions the smart way
What have you tried?
Why automatic conntrack helper assignment is dangerous
IProute2 cheat sheet
No, nftables is not production ready.