Do you need to have a firewall on a linux desktop?

Hi. I wanted to know if it's needed to install a firewall on a linux desktop/laptop. Why yes or why no?

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If your computer is connected to a network, I don't see any downside of enabling a firewall. It's a good security layer to have and costs basically no resources to keep running.
- I guess a downside is having to fiddle with it, allowing stuff you want to get through. Sometimes it blocks stuff you don't want blocked
  
  I'd rather have to open up stuff my self then have an uninvited visitor doing it without me knowing about it.

There is no reason not having a firewall

Yes, and hopefully you will have one preinstalled, blocking all incoming connections.
An outbound firewall like Opensnitch or Portmaster is also nice. But here I would say often you dont need one. Balena Etcher was the only App loading Ads, at all. Firefox and Thunderbird can be hardened. The rest is okay and doesnt phone home, Flatpak permissions ard also great.
- Balena Etcher shouldn't be loading ads. Where did you install it from?
  
  Official website. They just advertise their own products which I consider Adware

Linux comes with a default firewall it's called IPtables/NFtables, Just make sure that it's on. Example I Ubuntu Sudo ufw enable.
- Actually it's Netfilter. IPTables is just a frontend.
- Linux is a kernel. The actual tooling varies and isn't always preinstalled

Why not? It's comically easy to set up one.
- It’s also comically useless to have a desktop firewall application installed when you’re already behind some sort of firewall solution like a router not forwarding most incoming traffic.
  
  It depends. Sure, maybe somewhat redundant for a home desktop that just stays at home on a network you control, but for a laptop it is absolutely essential.
  You may also want a firewall to defend against other devices within your local network. Let's say you have IoT devices, many of which are poorly secured and maintained by their manufacturers, or you live with family members or guests who don't practice or even know about proper computing hygiene and are bringing in devices onto your local WiFi.
  
  There's incoming and then there's outgoing traffic. Software firewalls can forbid processes that may be advertised as "offline only" from reaching out; typically a hardware firewall doesn't care about this kind of thing.
  
  Some people like hosting some servers on their desktop as well, and doesn't want others on their local network to access them. With firewalls, you can allow specific IP address to reach those servers.
  
  That's fine right up until something on your network, even the ISP modem-firewall-router-switch itself, gets compromised.

Ironically enough if you do not know what a firewall actually does (and saying it "protects" against "stuff is NOT enough) IMHO you do need one. That being said unless you know what you are doing, better leave it to the default one with default settings.

On a laptop absolutely. My firewall on my laptop doesn't let me discriminate between networks so I'm always worried someone will try to attack me on public WiFi for the few ports I want open
On a desktop on a network you trust less important but still no firewall means if another device on your network gets compromised you're screwed
- Keep in mind that a firewall may not be nessasary if you don't have any network services running.
  
  Yes, except that whether you do is not at all obvious.
  Do you use KDE Connect? If yes, you run a network service. Syncthing, a bittorrent client? Those too, and many others. I know this is a Linux community, but as a comparison windows has a bunch of network services running by default.
  By saying "network services", I think menu of us would just think about a web server or a file server, conclude that they don't run any of those, and don't bother with it.
  When in doubt, sudo netstat -lutpn, and look for the listening ports to see what software listens for informing requests.
  
  I'd still have one, eventually something is going to slip past a new configuration or dependency that listens.
- For laptop, what kind of attack would we be protecting ourselves from? I get the relevance of antivirus, VPN, and device encryption, but what about firewall?
  
  Any vulnerabilities in any of your software that can talk to a network, ssh, browser, the operating system itsself could be exploited if your firewall is down
  If you're using username and password and have ssh enabled, for example anyone on your network could attempt to log in to your machine
  
  If you have the ability to take a look at either SANS website, and see their articles, or have your system show you all the automatic attacks hitting your machine, then maybe you will understand...
  Botnets are coded to hammer-away at all possible internet-addresses, trying to break-in & highjack more machines, to include in the established criminal-machine that the botnet is...
  SANS said, a decade or 2 ago, that it took, on average, something like 6 or 4 minutes for a new MS-Windows machine to be owned by some attack from the internet.
  I've had linux machines cracked/owned, and wiped 'em to get 'em clean.
  Having no immune-system is BAD.
  Linux botnets, apple operating-system botnets, they exist.
  I don't think there is any operating-system that is connected to the internet that doesn't have attacks coded to crack it.
  I just looked at SANS.org, and they have totally changed, so they are now .. more a moneymaking-machine wanting B2B biz?
  Here, though, are some cheat-sheets they made:
  https://www.sans.org/posters/?msc=main-nav
  They used to tell us the top-20 most effective protections for particular threats, identifying how prevalent the threats were, etc...
  No idea who does that nowadays...

Do you need a lock on the door of your place?
No, but it’s a damn good idea to protect whatever is in it.
- That's a bad comparison. Without a lock you can just open the door from the outside.
  
  But… Is it?

Yes, because while I trust my device, I do not trust the Chromecast or the WAP to not be an asshole and fuck with things.
- You shouldn't trust your device. Modern software is insanely complicated, even netbsd had an rce.
  
  Trust may be the worng word as Windows will be Windows but I know that no one is going to zero day my devices because I am not that high value of a target.

I'd say if you plan on using it any public places, or if you don't have full control over all the computers/technology in your home network, it's a good idea to run one. It's a nice last resort should someone on the same lan have a piece of malicious software.

Its always a good idea. I have a firewall on my laptop since I travel and connect to many different networks.
I don't run any services that would be broadcasting open ports but its always better to be safe
- Hell, even if you don't travel. You never know when something nasty is going to get loose on your network. Especially since most folks have some IoT things, and don't bother to properly isolate them.

No, you don't. But it's also not necessarily a bad idea if it's not going to create issues for you.
Normally, a software package that wants to talk over IP and only wants to let local software communicate with it can listen only on loopback addresses (like 127.0.0.1). But I have definitely seen software packages that have defaulted to listening to the world (gpsd used to do this by default, for example, which would let anyone in the world who could talk to your machine see precisely where you were). Having a firewall makes the default to be secure -- you know that the default is not to be reachable, regardless of what some software author thinks is a good idea.
Most home systems these days are generally behind a NATting router, which effectively firewalls them against the outside world (though maybe IPv6 will change this), so they can't be directly reached from the outside anyway, unless a user has set up port forwarding on the router, the systems are effectively firewalled anyway, unless an attacker can get inside the network somehow.
It's one more thing that you're going to have to remember to deal with if you're installing software and troubleshooting network problems. You install software package X and it isn't reachable, you're going to have to figure out how to diagnose problems. As long as this isn't a problem for you...shrugs
I don't personally run firewalls on my desktops. But I have also, over the years, occasionally checked netstat -ntap and discovered that a service that I thought only listened locally was listening to the world, gpsd probably being the most-flagrant example.
If I were not behind a router, or if I were forwarding all ports to my system, I would be firewalling my desktop systems.
On a dedicated server, I'd be less worried, because I'm not normally installing tons of random software on the thing. If you aren't going to firewall it, though, be sure that you've checked to see what is listening on the server.
- There is no reason to not leverage a firewall. Plenty of browser based malware can breach your home router and call back out to allow an attacker to explore your network. Windows has included a firewall for over a decade and it's fine. You can with a single command enable Linux's built in firewall and be glad you did the bare minimum of cyber security.
  
  Windows network layer is a desaster. It is so bad, you could currupt the system with Ping in the past.
  I would not say, you should not have a firewall on a desktop, but saying it's a good idea bc/ windows makes me laugh.
- I strongly disagree.
  For example on systems based on Debian (running myself) which simply enables networked services by default running a firewall is a total no brainer. Same is true for literally every Linux distribution with dependency management.
  Especially if someone asks, it is a strong indicator this person should run a firewall.

You most likely already have one installed, but not enabled. It doesn't harm anything (maybe you need to allow traffic to ssh or other configuration, but after that you're all set) and it's a layer of protection, specially if you need to move between networks (public wifi etc).
- On Mint the firewall is enabled by default. Setting up firewall rules is pretty easy though, so it's worth taking five minutes to read up on it.

No - If you are aware which programs open incoming ports. E.g. check with netstat -tulpe
Yes - If you want to make sure e.g. TCP port 22 or a webserver on 443 etc is only available from IP/Adapter xy. Or if you want to mess with filtering outgoing connections.
- ss -tulp

Yes, because you can forget what services are running and maybe they can be explited.
An example can be Syncthing which setting are done via web browser at port 8384. If you do not have a firewall, everyone on the same network would be able to change Syncthing settings and then sync your directories to their devices.
- Is syncthing listening on all addresses, or only loopback? A firewall would block it, sure, but that would also be bad design.
  
  Syncthing only listens on loopback by default unless you modify the config.
- Good point, but syncthing only listens on localhost as others have said too.
  However it still is a network service that can have vulnerabilities, besides many others like KDE Connect (which may be a target as one of its purposes is remote control and monitoring) or a bittorrent client

A sane firewall configuration should have no/minimal impact on a desktop focused OS.
On the other hand, sometimes programs are really badly made and expect stupid things like there being no firewall.
You should have one yes, but to each their own.
I manage a bunch of windows computers and regularly make adding firewall rules part of install scripts, good example: Dreamweaver.

Very few people do, you probably don't need to worry about it

If you are only at home you don't need it because the router already has a firewall. But if you're is using public WiFi definitely use it.
- It depends on how much do you trust your router.
  Some home routers have poor security: unfrequent updates, http (not https) web consoles, single factor authentication (password only, without username for instance).
  Enabling your firewall is the bare minimum, costs nothing and it's a good security practice.
  
  It depends on how much do you trust your router
  Your router and every single device connected to your local network.
  
  Sure that is true but if you're getting it from the ISP and it's that bad, you need to change ISP.
  Plus, ALWAYS get into the router and set a new password. Always. And go over all the settings to be sure. As you say, a shit ISP may have lax security.

I still don't know what a firewall actually does lol. I just install nftables and ufw and hope it somehow improves my security. I just don't know how.
- Every computer has a bunch of ports (1-65535 if I recall correctly), each of which is a unique entity to which a single service can bind. In layman's terms, a port is a door that one service is able to answer when someone knocks. By convention, some ports have a specific associated service (80 = HTTP, 443 = HTTPS, 22 = SSH), but there are a lot that you can just use as you deem appropriate.
  If you want a service (e.g. a web server) to be accessible, you have to run a service that binds to a known port (e.g. 80), and a client has to reach out to your server on that same port. A firewall sits between your service(s) and any potential clients, much like those steel security screen doors. If that's closed, nobody gets through on that port, even if a service is bound to that port and is listening for a connection.
  As a general rule of thumb, you want your firewall to block as much traffic as possible without breaking something (I.e. blocking one of your public-facing services). If you don't run any services on your computer (web services, media servers, etc.), you can probably get away with blocking all inbound traffic. without any discernable impact.
- Programs that want to receive or send data across the network do so using a port. A firewall (in the sense of a firewall on your computer) is basically a program that sits between the rest of your programs and your network connection and determines what programs should be allowed out to the network and what incoming connections should be allowed to talk to your programs.
  This is "increasing security" by making sure you don't let incoming connections talk to whatever happens to be running on your computer that might be listening for network traffic.
  I can provide a more ELI5 explanation if you would like it.

Eh, the router acts like one. I have no open ports. On a laptop you use on the go I would use a firewall tho.

i dont even undedtsand what a firewall is, i yhinm it blocks incouming / ourgojng connectikns lol

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