A classic on *nix networking fundamentals: https://beej.us/guide/bgnet/ https://beej.us/guide/bgnet/ It includes a further reading list: https://beej.us/guide/

A classic on *nix networking fundamentals: https://beej.us/guide/bgnet/ https://beej.us/guide/bgnet/ It includes a further reading list: https://beej.us/guide/bgnet/html/split/more-references.html#more-references https://beej.us/guide/bgnet/html/split/more-references.html#... These don't cover iptables and other firewalls themselves, but they give you enough knowledge that you can read the iptables manpage and other manuals and understand them.