“Setup a firewall” - on my server or on the router? Or both?
Both. Access from the internet to your devices is protected from your Router. You should only ever open ports to things you want to access outside of your house. For example, a website on TCP 443 is a good thing to allow. Database access on TCP 3306 is NOT something you want to access from the internet.
Internal to your network, you can open up the Database port on TCP 3306 if another computer in your network needs access to it. Don’t leave it open for no reason.
And since my router is provided by my ISP which has its settings exposed through their online portal (which I hate the thought of), how does that factor in?
Is it a private IP Address you use to access (IE 192.168.0.1 or is it some other thing?) If it’s a private IP address, that’s standard and is no problem. I’ve never heard of logging into something like a public website to open ports on your router.
What use is a router firewall if someone gains access to this portal and can configure at will?
If someone has access to your Internet Providers firwall/router combo device thingy, then it’s game over. they can open any ports, do anything they want to it. However, this is unlikely. Make sure to keep your Router up to date and only open ports that you know what they’re for 100% and what they do. NEVER allow access to the admin panel from the internet (WAN port). Though, if you need to use their public website (which I doubt) then it’s moot and you cant do anything.
Can I set up the router in bridge mode and incorporate my own router, and thus have complete local control of my network?
Yes, if they support it. This is what I do and that’s exactly what’s it’s for.
Couldn’t someone simply deactivate this in the online portal if they gained access there?
Sure but this would give them access to your Providers firewall, which you have your own firewall plugged into so it doesn’t matter. You will still be protected by your self provided firewall, some things will stop working (you’ll be double NATed so public services might not work) and it’ll be a clue someone changed something on your Providers firewall.
And if I open ports in the firewall for a specific application, what risks am I running outside of exploits in the applications themselves? For example, I have opened a port in the router settings for torrenting Linux ISOs (for a specific local IP) - could traffic through the same port be used to compromise the network in other ways? etc. etc.
Yeah, so if you open no ports. you block everything… The only exposure (of people trying to connect to you) is the firewall/router you have sitting there watching what goes on. Keeping this up to date is VERY important and they have pretty good history of not being hacked so I’d say you’re safe. For each port you open you add a service that people from anywhere in the world can talk to. So, if you open port TCP 443 and have Nginx or a website answering requests, you now need to make sure this is as secure as possible. ANYONE ANYWHERE can talk to it. If a exploit is found in Nginx/your website then it can be used to get access into that computer, From there, they are on your computer and can see anything that computer has access to. If you have 2 ports open. IE website + Torrenting, Now you have two things you MUST keep up to date because anyone can talk to them and exploit them to get into your computer. This is why you MUST know what is open to the internet and what it’s doing. The more things you have the more options you give hackers to get in.
Suddenly I have fifteen questions. So when trying to research the answer to these questions, I often get slapped with five concepts I either barely have grasp of or don’t know at all in one sentence that tries to explain what is going on. It’s not that it is impossible to learn this way, but it tends to quickly become overwhelming, and I run into explanations of concepts I don’t have enough prerequisites to learn properly yet. Which is why I am trying to get a coherent introduction to all the topics in a sensible, curated way to beef up my understanding of it, so that the research process becomes easier.
I would do what they said. Start small and at the first point of contact. Understand what is plugged into the internet (ISP Router) and all the settings on it. Understand what NAT means, understand how to open ports, etc. Then start small, Set up a computer and make sure you know what ports are open. If you want a website set it up so it works internally, test it, maybe port scan your server (with nmap or something) to see what’s open and understand what they’re for. Close the ports you don’t need open then you can consider opening it from the internet.
the most important thing is keeping it up to date and only open what you need access to.
A side point is, if it’s just you that needs access to it, consider a VPN (wireguard) or overlay network (tailscale) so you only need to open one port and that will give you access to everything you need in your network.