Can anyone recommend a decent book on linux firewalls, iptables and the like? Every time I wade into this I feel I'm missing to much base knowledge to make good decisions.
It includes a further reading list: 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.
I don't know much about Linux networking. But recently I dove into it a little to let my laptop connect to my desktop via Ethernet (when present), but use Wifi for everything else.
I got it working-ish using "nmtui". But I'm left pretty confused about the relationship between all the network-related tools / services / files.
E.g., is "nmtui" just a convenience wrapper around thinks like iptables and resolved? Does it work around them? Which tools are mean to be used together, vs. which ones are redundant / incompatible? And then there's systemd as well.
Any suggestion for something explaining how all these tools and service fit together (or don't)?
systemd is an init system with a lot of bells and whistles. One of the whistles is systemd-networkd, which is also a complete networking solution, but it also delegates some work to systemd-resolved. It also doesn't take care of firewalling.
If you want to get a better understanding on how networks are set up, then you should rather look for more low-level tools like iproute2 (which provides the "ip" tool) and dhclient/dhcpd. For wireless you should look at wpa_supplicant (which is afaik used by Network Manager under the hood). And then there is of course iptables which is used to manipulate the flow of packages through the network. It should be noted that eBPF is intended to replace iptables but knowledge of iptables will still prove useful for many years to come.