When do I actually need a firewall?

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:

  1. “It’s just good security practice.”
  2. “You need it if you are running a server.”
  3. “You need it if you don’t trust the other devices on the network.”
  4. “You need it if you are not behind a NAT.”
  5. “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?

lemmyvore,

You need to understand the mindset behind running a firewall, and that mindset is that you define with mathematical precision what’s possible within the network connectivity of a device, you leave nothing to chance or circumstance, because doing so would be sloppy.

Provided you want to subscribe to this mindset, and that the circumstances of that device warrant it, and that you have the networking knowledge to pull it off, you should in theory start with a DENY policy on everything and open up specific ports for specific users and related connections only. But it’s not trivial and if you’re a beginner it’s best done directly on the server console, because you WILL break your SSH connection doing this. And of course maybe not persist the firewall rules permanently until you’ve learned more and can verify you can get in.

Now obviously this is an extreme mindset and yes you should use it in a professional setting. As a hobbyist? Up to you. In theory you don’t need a firewall if your server only exposes the services you want to expose and you were gonna expose them through the firewall anyway. In practice, keeping track on what’s running on a box and what’s using what connections can be a bit harder than that.

If you’re a beginner my recommendation is to use a dedicated router running OpenWRT with LUCI, which comes with a sensible firewall out of the box, an easy to use UI, and other goodies like an easy to use DNS+DHCP server combo and the ability to install plugins for DoH, DDNS etc.

GravitySpoiled, (edited )

I’ve got two services on my computer. One is for email, I want that this port to be open to the public WAN and one is for immich which hosts all my private pictures, I don’t want this port to be public but reachable on LAN. In my router I open the port for email but not for immich. Emal can communicate on LAN and WAN and immich only on LAN. On a foreign, untrusted LAN, like an airport I don’t want other people being able to sniff my immich traffic which is why I have another firewall setting for an untrusted LAN.

iopq,

Even if you do trust the software running on your computer, did you actually fuzz it for vulnerabilities? Heartbleed could steal your passwords even if you ran ostensibly trustworthy software.

So unless you harden the software and prove it’s completely exploit-free, then you can’t trust it.

bizdelnick,

You always need it and you actually use it. The smarter question is when you need to customize its settings. Defaults are robust enough, so unless you know what and why you need to change, you don’t.

TCB13, (edited )
@TCB13@lemmy.world avatar

#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.

Agreed. That’s mostly BS from people who make commissions from some vendor.

#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?

A Firewall might be more advanced than just NAT/poking a hole, it may do intrusion detection (whatever that means) and DDoS protection

#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.

Maybe you’ve a bunch of IoT devices in your network that are sold by a Chinese company or any IoT device (lol) and you don’t want them to be able to access the internet because they’ll establish connections to shady places and might be used to access your network and other devices inside it.

#5 is the only one that makes some sense;

Essentially the same answer and in #3

If we’re talking about your home setup and/or homelab just don’t get a hardware firewall, those are overpriced and won’t add much value. You’re better off by buying an OpenWRT compatible router and ditching your ISP router. OpenWRT does NAT and has a firewall that is easy to manage and setup whatever policies you might need to restrict specific devices. You’ll also be able to setup things such as DoH / DoT for your entire network, setup a quick Wireguard VPN to access your local services from the outside in a safe way and maybe use it to setup a couple of network shares. Much more value for most people, way cheaper.

Petter1, (edited )

You most likely don’t need on device firewall if your in your home network behind a router that has a firewall. If you‘d disable that firewall as well and one of your devices has e.g. SSH activated using username and password, than there is nothing stopping a “hacker” or “script kiddy” from penetrating/spamming your SSH port and brute force your password. The person than can take over your PC and can e.g. install software for his botnet or install keylogger or can overtake your browser session including all authentication cookies or many other bad stuff.

If you are using puplic WiFi, I’d recommend a good on device firewall, or better just use a VPN to get an encrypted tunnel to your home (where you would need to open a port for that tho) and go into the internet from there.

thanks_shakey_snake,

For me, it’s primarily #5: I want to know which apps are accessing the network and when, and have control over what I allow and what I don’t. I’ve caught lots of daemons for software that I hadn’t noticed was running and random telemetry activity that way, and it’s helped me sort-of sandbox software that IMO does not need access to the network.

Not much to say about the other reasons, other than #2 makes more sense in the context of working with other people: If your policy is “this is meant to be an HTTPS-only machine,” then you might want to enforce that at the firewall level to prevent some careless developer from serving the app on port 80 (HTTP), or exposing the database port while they’re throwing spaghetti at the wall wrestling with some bug. That careless developer could be future-you, of course. Then once you have a policy you like, it’s also easier to copy a firewall config around to multiple machines (which may be running different apps), instead of just making sure to get it consistently right on a server-by-server basis.

So… Necessary? Not for any reason I can think of. But useful, especially as systems and teams grow.

Atemu,
@Atemu@lemmy.ml avatar

#2 is strange – why does it matter?

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?

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?

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.

#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

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.

#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.

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.

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?

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?

Kalcifer,
@Kalcifer@sh.itjust.works avatar

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?

This is a fair point that I hadn’t considered for the mobile use-case.

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?

Fair point!

note that you likely do have applications listening on ports you didn’t know about. Take a look at sudo ss -utpnl.

Interesting! In my case I have a number of sockets from spotify, and steam listening on port 0.0.0.0. I would assume, that these are only available to connections from the LAN?

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.

It could also be malicious software that simply makes a request to a remote server – perhaps even siphoning your local data.

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?

Fair point!

Atemu,
@Atemu@lemmy.ml avatar

In my case I have a number of sockets from spotify, and steam listening on port 0.0.0.0. I would assume, that these are only available to connections from the LAN?

That’s exactly the kind of thing I meant :)

These are likely for things like in-house streaming, LAN game downloads and remote music playing, so you may even want to consider explicitly allowing them through the firewall but they’re also potential security holes of applications running under your user that you have largely no control over.

wolf, (edited )

Seriously, unless you are extremely specialized and know exactly what you are doing, IMHO the answer is: Always (and even being extremely specialized, I would still enable a firewall. :-P)

Operating systems nowadays are extremely complex with a lot of moving parts. There are security relevant bugs in your network stack and in all applications that you are running. There might be open ports on your computer you did not even think about, and unless you are monitoring 24/7 your local open ports, you don’t know what is open.

First of all, you can never trust other devices on a network. There is no way to know, if they are compromised. You can also never trust the software running on your own computer - just look at CVEs, even without malicious intentions your software is not secure and never will be.

As soon as you are part of a network, your computer is exposed, doesn’t matter if desktop/laptop, and especially for attacking Linux there is a lot of drive by attacks happening 24/7.

Your needs for firewalls mostly depend on your threat model, but just disabling accepting incoming requests is trivial and increases your security by a great margin. Further, setting a rate limit for failed connection attempts for open ports like SSH if you use this services, is another big improvement for security. (… and of course disabling password authentication, YADA YADA)

That said, obviously security has to be seen in context, the only snake oil that I know of are virus scanners, but that’s another story.

People, which claim you don’t need a firewall make at least one of the following wrong assumptions:

  • Your software is secure - demonstrably wrong, as proven by CVEs
  • You know exactly what is running/reachable on your computer - this might be correct for very small specialized embedded systems, even for them one still must always assume security relevant bugs in software/hardware/drivers

Security is a game, and no usable system can be absolutely secure. With firewalls, you can (hopefully) increase the price for successful attacks, and that is important.

bushvin,

You may also want to check up on regulations and laws of your country.

In Belgium, for instance, I am responsible for any and all attacks originating from my PC. If you were hacked and said hackers used your computer to stage an attack, the burden of proof is upon you. So instead of hiring very expensive people to trace the real source of an attack originating from your own PC, enabling a firewall just makes sense, besides making it harder on hackers…

Kalcifer,
@Kalcifer@sh.itjust.works avatar

That’s a strange law. That’s like saying one should be held responsible for a thief stealing their car and then running over someone with it (well, perhaps an argument could be made for that, but I would disagree with it).

Kalcifer,
@Kalcifer@sh.itjust.works avatar

Seriously, unless you are extremely specialized and know exactly what you are doing, IMHO the answer is: Always

In what capacity, though? I see potential issues with both server firewals, and client firewalls. Unless one wants their devices to be offline, there will always be at least one open port (for example, inbound on a server, and outbound on a client) which can be used as an attack vector.

conorab,

Other comments have hit this, but one reason is simply to be an extra layer. You won’t always know what software is listening for connections. There are obvious ones like web servers, but less obvious ones like Skype. By rejecting all incoming traffic by default and only allowing things explicitly, you avoid the scenario where you leave something listening by accident.

Paragone, (edited )

A couple of decades ago, iirc, SANS.org ( IF I’m remembering who it was who did it ) put a fresh-install of MS-Windows on a machine, & connected it to the internet.

It took SEVERAL MINUTES for it to be broken-into, & corrupted, botnetted.

The auto-attacks by botnets are continuous: hitting different ports, trying to break-in, automatically.

I’ve had linux desktops pwned from me.

the internet should be considered something like a mix of toxic & corrosive chemicals: “maybe” your hand will be fine, if you dip it in for a moment & immediately rinse it off ( for 3 hours ), but if you leave you limbs dwelling in the virulent slop, Bad Things™ are going to happen, sooner-or-later.

I used to de-infest Windows machines for my neighbours…

haven’t done it in years: they’ll not pay-for good anti-virus, they’ll not resist installing malware: therefore there is no point.

Let 'em rot.

I’ve got a life to work-on uncrippling, & too-little strength/time left.

“but I don’t need antivirus: i never get infected!!”

then how come I needed to de-infest it for you??

“but I don’t need an immune-system: pathogens are a hoax!!”

get AIDS, then, & don’t use anti-AIDS drugs, & see how “healthy” you are, 2 years in.

Same argument, different context-mapping.

Tarpit was a wonderful-looking invention, for Linux’s netfilter/iptables, years ago: don’t help botnets scan quickly & efficiently to help them find a way to break-in…

Anyways, just random thoughts from an old geek…

EDIT: “when do I need to wear a seatbelt?”

is essentially the same category of question.

_ /\ _

utopiah,

When you expose ports to the Internet. It’s honestly interesting to setup a Web server with the default page on it and see how quickly you get hits on it. You don’t need to register a DNS or be part of an index anywhere. If you open a port (and your router does forward it) then you WILL get scanned for vulnerabilities. It’s like going naked in the forest, you sure can do that but clothes help, even if it’s “just” again ivy or random critters. Now obviously the LONGER you run naked or leave a computer exposed, the most likely you are to get a bad bug.

Feathercrown,

Can confirm. As an example, I’m developing a game server that runs a raw socket connection over the Telnet port. Within 10 minutes of opening the port, I reliably get requests trying to use Telnet to enable command mode or login as admin. People are constantly scanning.

mvirts,

Ya. And sometimes hosting companies run active scans on customer machines. I get a crazy number of login attempts over ssh. I ❤️ fail2ban

atzanteol,

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.

You say that like that isn’t providing value. How many services are listening on a port on your system right now? Run ‘ss -ltpu’ and prepare to be surprised.

Security isn’t about “this will make you secure” it’s about layers of protection and probability. It’s a “good practice” because people make mistakes and having a second line of defense helps reduce the odds of a hack.

treadful,
@treadful@lemmy.zip avatar

Security isn’t about “this will make you secure” it’s about layers of protection and probability. It’s a “good practice” because people make mistakes and having a second line of defense helps reduce the odds of a hack.

AKA Defense In Depth and should be considered for any type of security.

c0mbatbag3l,
@c0mbatbag3l@lemmy.world avatar

In the military when learning ORM we called this the “swiss cheese” theory.

The more layers of sliced swiss cheese, the fewer holes that go all the way through.

MajorMajormajormajor,

It seems that the consensus from all the comments is that you do in fact need a firewall. So my question is how does that look exactly? A hardware firewall device directly between modem and router? I using the software firewall on the router enough? Or, additionally having software firewall installed on all capable devices on the network? A combination of the above?

possiblylinux127,

I use the firewall built into Proxmox with a device running openwrt

treadful,
@treadful@lemmy.zip avatar

Depends on your setup. I got a network-level firewall+router setup between my modem and my LAN. But also, got firewalld (friendly wrapper on iptables) on every Linux device I care about because I don’t want to unintentionally expose something to the network.

hm, guess maybe I should find something for Android and my Windows boxes.

rwhitisissle, (edited )

And like most things related to Linux on the internet, the consensus is generally incorrect. For a typical home user who isn’t opening ports or taking a development laptop to places with unsecure wifi networks, you don’t really need a firewall. It’s completely superflous. Anything you do to your PC that causes you genuine discomfort will more than likely be your own fault rather than an explicit vulnerability. And if you’re opening ports on your home network to do self-hosting, you’re already inviting trouble and a firewall is, in that scenario, a bandaid on a sucking chest wound you self-inflicted.

ShittyBeatlesFCPres,

I think it’s better to have one but you probably don’t need multiple layers. When I’m setting up servers nowadays, it’s typically in the cloud and AWS and the like typically have firewalls. So, I don’t really do much on those machines besides change ports to non-standard things. (Like the SSH port should be a random one instead of 22.)

But you should use one if you don’t have an ecosystem where ports can be blocked or forwarded. If nothing else, the constant login attempts from bots will fill up your logs. I disable password logins on web servers and if I don’t change the port, I get a zillion attempts to ssh using “admin” and some common password on port 22. No one gets in but it still requires more compute than just blocking port 22 and making your SSH port something else.

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