I’ve spent some time searching this question, but I have yet to find a satisfying answer. The majority of answers that I have seen state something along the lines of the following:
- “It’s just good security practice.”
- “You need it if you are running a server.”
- “You need it if you don’t trust the other devices on the network.”
- “You need it if you are not behind a NAT.”
- “You need it if you don’t trust the software running on your computer.”
The only answer that makes any sense to me is #5. #1 leaves a lot to be desired, as it advocates for doing something without thinking about why you’re doing it – it is essentially a non-answer. #2 is strange – why does it matter? If one is hosting a webserver on port 80, for example, they are going to poke a hole in their router’s NAT at port 80 to open that server’s port to the public. What difference does it make to then have another firewall that needs to be port forwarded? #3 is a strange one – what sort of malicious behaviour could even be done to a device with no firewall? If you have no applications listening on any port, then there’s nothing to access. #4 feels like an extension of #3 – only, in this case, it is most likely a larger group that the device is exposed to. #5 is the only one that makes some sense; if you install a program that you do not trust (you don’t know how it works), you don’t want it to be able to readily communicate with the outside world unless you explicitly grant it permission to do so. Such an unknown program could be the door to get into your device, or a spy on your device’s actions.
If anything, a firewall only seems to provide extra precautions against mistakes made by the user, rather than actively preventing bad actors from getting in. People seem to treat it as if it’s acting like the front door to a house, but this analogy doesn’t make much sense to me – without a house (a service listening on a port), what good is a door?
It doesn’t. If you’re running a laptop with a local web server for development, you wouldn’t want other devices in i.e. the coffee shop WiFi to be able to connect to your (likely insecure) local web server, would you?
Who is “they”? What about all the other ports?
Imagine a family member visits you and wants internet access in their Windows laptop, so you give them the WiFi password. Do you want that possibly malware infected thing poking around at ports other than 80 running on your server?
Obviously you shouldn’t have insecure things listening there in the fist place but you don’t always get to choose whether some thing you’re hosting is currently secure or not or may not care too much because it’s just on the local network and you didn’t expose it to the internet.
This is what defense in depth is about; making it less likely for something to happen or the attack less potent even if your primary protections have failed.
Mostly addressed by the above but also note that you likely do have applications listening on ports you didn’t know about. Take a look at
sudo ss -utpnl.It’s rather the other way around; you don’t want the outside world to be able to talk to untrusted software on your computer. To be a classical “door”, the application must be able to listen to connections.
OTOH, smarter malware can of course be something like a door by requesting intrusion by itself, so outbound filtering is also something you should do with untrusted applications.
I’d rather liken it to a razor fence around your house, protecting you from thieves even getting near it. Your windows are likely safe from intrusion but they’re known to be fragile. Razor fence can also be cut through but not everyone will have the skill or patience to do so.
If it turned out your window could easily be opened from the outside, you’d rather have razor fence in front until you can replace the window, would you?