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?
Firewall for incoming traffic :
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If you a home user with your computer or laptop inside a LAN you would not really need a firewall, unless you start to use applications which expose its ports to 0.0.0.0 rather than 127.0.0.1 (I believe Redis server software did this a few years ago) and do not trust other users or devices (smart home devices, phones, tablets, modems, switches and so on) inside your LAN.
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If you are running a server with just a few services, for example ssh, smtp, https, some hosting company people I knew argue that no firewall is needed. I am not sure, my knowledge is lacking.
Application firewalls, watching also outgoing traffic :
If you compare Linux with some other Operating System you will see that on Linux for years an application firewall was non existing. But there is a choice now : opensnitch This can be useful if you run desktop applications that you do not fully trust, or want more control.
If you a home user with your computer or laptop inside a LAN you would not really need a firewall, unless you start to use applications which expose its ports to 0.0.0.0 rather than 127.0.0.1
Interestingly, on one of my devices, running
# ss -utpnlshows quite a number of Spotify, and Steam sockets listening on 0.0.0.0. I looked up some of the ports, and, for example, one of the steam ones was a socket for Remote Play.But there is a choice now : opensnitch
This is really cool! Thank you so much for this recommendation! This pretty much solves what was bugging me about outgoing connections in a layer 3/4 firewall like nftables.
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I personally use a firewall for containing the local services I am running on my non-server PC, ex. Tiny Tiny RSS. If I am only using Tiny Tiny RSS locally, it’s just potentially dangerous to make this service visible and accessible for every client in my local network, which in my case, isn’t populated by my own personal devices, as I live in a dormitory. Other than that, you can block the well-known ports of commonly exploited protocols such as UPnP. That’s not because someone will “break into your device” with UPnP, but rather as a matter of digital autonomy, to control the mode of network communication done by the software on your device.
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?
Depends on the setup. For most people at home their router also does firewalling and NAT, and that is enough.
Even in corporate it is not uncommon for a firewall to be the gateway, or transparent in between, with maybe more internally too. There are just more routers inside and out, but those routers are real network routers in the traditional sense.
For most people at home their router also does firewalling and NAT, and that is enough.
It is important to note (as was pointed out by others in this thread) that one must also consider threats emanating from within the LAN, as well: Do you have guests that you allow onto your network with potentially un-vetted devices? Do you have other network-capable devices connected to your network that you cannot guarantee their security? Can you guarantee that there are no unintended services with potential security vulnerabilities listening to ports on your device? If so, it is worth considering, at the very least, a packet filtering firewall, e.g. nftables, and if you cant trust the services running on your device, perhaps also an application layer firewall like OpenSnitch.
My setup is pretty basic, only thing I have is a media server accessed locally, and a pi running pihole and pivpn that has a port forwarded on my router for remote access. The pi has password login disabled, and the port forward is set to the static IP set for the pi with my router. The router has the firewall set, but nothing on any other machine. Do I need more?
What service do you have forwarded? Do you have any devices on your lan you don’t 100% trust?
I have a similar set up only forwarding a wire guard vpn port. I live alone and fully trust every device on my LAN, so I let my router take care of the firewall and dont have any firewalls on the devices on my lan.
Some will still argue this is bad practice but I really have no desire to toggle firewall rules every time I want to expose a port while I’m developing/testing software. If someone cracks wireguard then I don’t think they will risk exposing the industry halting 0 day to run a crypto miner on my raspberry pi.
IOT and friends get the guest wifi.
wire guard vpn port
This is the only thing forwarded. As for devices the worst offender would be my Roku TV but I’m not sure how much of a security threat that actually would be. More of a privacy threat, hence running pihole.
but I’m not sure how much of a security threat that actually would be. More of a privacy threat, hence running pihole.
It is important to note that being unaware of something’s level of security is not an argument that it is more secure, or not worthy of scrutiny.
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.
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.
A “typical” home user, whom I assume is less knowledgeable about technology, is probably the person who would benefit the most from strict firewalls installed on their device. Such an individual assumedly doesn’t have the prerequisite knowledge, or awareness required to adequately gauge the threats on their network.
Anything you do to your PC that causes you genuine discomfort will more than likely be your own fault rather than an explicit vulnerability.
Would this not be adequate rationale for having contingencies, i.e. firewalls? A risk/threat needn’t only be an external malicious actor. One’s own mistakes could certainly be interpreted as a potential threat, and are, therefore, worthy of mitigation.
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.
Well, no, not necessarily. It’s important to understand what the purpose of the firewall is. If a device can potentially become an attack vector, it’s important to take precautions against that – you’d want to secure other devices on the network in the off chance that it does become compromised, or secure that very device to limit the potential damage that it could inflict.
A “typical” home user, whom I assume is less knowledgeable about technology, is probably the person who would benefit the most from strict firewalls installed on their device. Such an individual assumedly doesn’t have the prerequisite knowledge, or awareness required to adequately gauge the threats on their network.
They also would not realistically be doing anything that would cause open ports on their machine to serve data to some external application. It’s not like someone can just “hack” your computer by picking a random port and weaseling their way in. They have to have some exploitable mechanism on the machine that serves data in a way that’s insecure.
Would this not be adequate rationale for having contingencies, i.e. firewalls? A risk/threat needn’t only be an external malicious actor. One’s own mistakes could certainly be interpreted as a potential threat, and are, therefore, worthy of mitigation.
I am assuming that there’s a hierarchy of needs in terms of maintaining any Linux system. Whenever you learn how to use something (and you would have to learn how to use a firewall), you are sacrificing time and energy that would be spent learning something else. Knowing how your package manager works, or how to use systemctl, or understanding your file system structure, or any number of pieces of fundamental Linux knowledge is, for a less technically sophisticated user, going to do comparatively more to guarantee the longevity and health of their system than learning how to use a firewall, which is something capable of severely negatively impacting your user experience if you misconfigure it. In other words: don’t mess around with a firewall if you don’t know what you’re doing. Use your time learning other things first if you’re a not technically sophisticated user. I also don’t exactly know what “mistakes” you’d be mitigating by installing a firewall if you aren’t binding processes to those ports (something a novice user should not be doing anyway).
Well, no, not necessarily. It’s important to understand what the purpose of the firewall is. If a device can potentially become an attack vector, it’s important to take precautions against that – you’d want to secure other devices on the network in the off chance that it does become compromised, or secure that very device to limit the potential damage that it could inflict.
You just wrote that “One’s own mistakes could certainly be interpreted as a potential threat, and are, therefore, worthy of mitigation.” The best way of mitigating mistakes is by not making them in the first place, or creating a scenario in which you could potentially make them. Prevention is always better than cure. You should never open ports on your local network. Ever. I don’t care if you have firewalls on everything down to your smart thermostat - if you need to expose locally hosted services you should be maintaining a cloud VM or similar cloud based service that forwards connections to the desired service on your internal network via a VPN like Tailscale. Or, even better, just put Tailscale’s service on whatever machine you’re using that needs access to your personal network. And, yes, if you’re doing things like that, you would also want robust firewall protections everywhere. But the firewall simply isn’t ever “enough.”
Anyway, just my 2 cents. The more you know and do, the greater steps you should take to protect yourself. For someone who knows very little, the most important thing that can help them is knowing more, and there is a hierarchy of learning that will take them from “knowing little” to “knowing much,” but they shouldn’t/don’t need to concern themselves with certain mechanisms before they know enough to reliably use them or mitigate their own mistakes. That said, if you are a new user, you’re probably installing a linux distro that already comes with its own preconfigured firewall that’s already running and you just don’t know about it. In which case, moot point. If you’re not, though, I’m assuming your goal is learning linux stuff, in which case, I’ve gone into that.
They also would not realistically be doing anything that would cause open ports on their machine to serve data to some external application.
They may not explicitly do it, no, but I could certainly see the possibility of the software that they use having such a vulnerability, or even a malicious bit of software inadvertently being installed on their device.
In other words: don’t mess around with a firewall if you don’t know what you’re doing. Use your time learning other things first if you’re a not technically sophisticated user. I also don’t exactly know what “mistakes” you’d be mitigating by installing a firewall if you aren’t binding processes to those ports (something a novice user should not be doing anyway).
This sort of skirts around answering the question.
The best way of mitigating mistakes is by not making them in the first place
But mistakes will be made all the same.
Prevention is always better than cure.
This is exactly the point that I am trying to make. Having contingencies in place on the off chance that something doesn’t go as expected could certainly be interpreted as “prevention”.
You should never open ports on your local network. Ever.
What would be the rationale for this statement?
if you need to expose locally hosted services you should be maintaining a cloud VM or similar cloud based service that forwards connections to the desired service on your internal network via a VPN like Tailscale.
I’m not sure that I understand what issue that this would solve. Would the malicious connections not still be forwarded through the VPN to the service? I am quite lacking in knowledge on Tailscale, and how related infrastructure is used in production, so please pardon my ignorance.
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.
(friendly wrapper on iptables)
iptables is deprecated, so it’s better to label it as a wrapper for nftables.
As i see it, the term “firewall” was originally the neat name for an overall security concept for your systems privacy/integrity/security. Thus physical security is (or can be) as well part of a firewall concept as maybe training of users. The keys of your server rooms door could be part of that concept too.
In general you only “need” to secure something that actually is there, you won’t build a safe into the wall and hide it with an old painting without something to put in it or - could be part of the concept - an alarmsensor that triggers when that old painting is moved, thus creating sort of a honeypot.
if and what types of security you want is up to you (so don’t blame others if you made bad decisions).
but as a general rule out of practice i would say it is wise to always have two layers of defence. and always try to prepare for one “error” at a time and try to solve it quickly then.
example: if you want an rsync server on an internet facing machine to only be accessible for some subnets, i would suggest you add iptables rules as tight as possible and also configure the service to reject access from all other than the wanted addresses. also consider monitoring both, maybe using two different approaches: monitor the config to be as defined as well as setup an access-check from one of the unwanted, excluded addresses that fires an alarm when access becomes possible.
this would not only prevent those unwanted access from happening but also prevent accidental opening or breaking of config from happen unnoticed.
here the same, if you want monitoring is also up to you and your concept of security, as is with redundancy.
In general i would suggest to setup an ip filtering “firewall” if you have ip forwarding activated for some reason. a rather tight filtering would maybe only allow what you really need, while DROPping all other requests, but sometimes icmp comes in handy, so maybe you want ping or MTU discovery to actually work. always depends on what you have and how strong you want to protect it from what with what effort. a generic ip filter to only allow outgoing connections on a single workstation may be a good idea as second layer of “defence” in case your router has hidden vendor backdoors that either the vendor sold or someone else simply discovered. Disallowing all that might-be-usable-for-some-users-default-on-protocols like avahi & co in some distros would probably help a bit then.
so there is no generic fault-proof rule of thumb…
to number 5.: what sort of “not trusting” the software? might, has or “will” have: a. security flaws in code b. insecurity by design c. backdoors by gov, vendor or distributor d. spy functionality e. annoying ads as soon as it has internet connection f. all of the above (now guess the likely vendors for this one)
for c d and e one might also want to filter some outgoing connection…
one could also use an ip filtering firewall to keep logs small by disallowing those who obviously have intentions you dislike (fail2ban i.e.)
so maybe create a concept first and ask how to achieve the desired precautions then. or just start with your idea of the firewall and dig into some of the appearing rabbit holes afterwards ;-)
regards
for c d and e one might also want to filter some outgoing connection…
Is there any way to reliably do this in practice? There’s no way of really knowing what outgoing source ports are being used, as they are chosen at random when the connection is made, and if the device is to be practically used at all, some outgoing destination ports must be allowed as well e.g. DNS, HTTP, HTTPS, etc. What other methods are there to filter malicious connections originating from the device using a packet filtering firewall? There is the option of using a layer 7 firewall like OpenSnitch, but, for the purpose of this post, I’m mostly curious about packet filtering firewalls.
one could also use an ip filtering firewall to keep logs small by disallowing those who obviously have intentions you dislike (fail2ban i.e.)
This is a fair point! I hadn’t considered that.
you do not need to know the source ports for filtering outgoing connections.
(i usually use “shorewall” as a nice and handy wrapper around iptables and a “reject everything else policy” when i configured everything as i wanted. so i only occasionally use iptables directly, if my examples dont work, i simply might be wrong with the exact syntax)
something like:
iptables -I OUTPUT -p tcp --dport 22 -j REJECT
should prevent all new tcp connection TO ssh ports on other servers when initiated locally (the forward chain is again another story)
so … one could run an http/s proxy under a specific user account, block all outgoing connections except those of that proxy (i.e. squid) then every program that wants to connect somewhere using direct ip connections would have to use that proxy.
better try this first on a VM on your workstation, not your server in a datacenter:
iptables -I OUTPUT -j REJECT iptables -I OUTPUT -p tcp -m owner --owner squiduser -j ACCEPT
“-I” inserts at the beginning, so that the second -I actually becomes the first rule in that chain allowing tcp for the linux user named “squiduser” while the very next would be the reject everything rule.
here i also assume “squiduser” exists, and hope i recall the syntax for owner match correctly.
then create user accounts within squid for all applications (that support using proxies) with precise acl’s to where (the fqdn’s) these squid-users are allowed to connect to.
there are possibilities to intercept regular tcp/http connections and “force” them to go through the http proxy, but if it comes to https and not-already-known domains the programs would connect to, things become way more complicated (search for “ssl interception”) like the client program/system needs to trust “your own” CA first.
so the concept is to disallow everything by iptables, then allow more finegrained by http proxy where the proxy users would have to authenticate first. this way your weather desktop applet may connect to w.foreca.st if configured, but not e.vili.sh as that would not be included in its users acl.
this setup, would not prevent everything applications could do to connect to the outside world: a local configured email server could probably be abused or even DNS would still be available to evil applications to “transmit” data to their home servers, but thats a different story and abuse of your resolver or forwarder, not the tcp stack then. there exists a library to tunnel tcp streams through dns requests and their answers, a bit creepy, but possible and already prepaired. and only using a http-only proxy does not prevent tcp streams like ssh, i think a simple tcp-through-http-proxy-tunnel software was called “corckscrew” or similar and would go straight through a http proxy but would need the other ond of the tunnel software to be up and running.
much could be abused by malicious software if they get executed on your computer, but in general preventing simple outgoing connections is possible and more or less easy depending on what you want to achieve
should prevent all new tcp connection TO ssh ports on other servers when initiated locally (the forward chain is again another story)
But the point that I was trying to make was that that would then also block you from using SSH. If you want to connect to any external service, you need to open a port for it, and if there’s an open port, then there’s a opening for unintended escape.
so … one could run an http/s proxy under a specific user account, block all outgoing connections except those of that proxy (i.e. squid) then every program that wants to connect somewhere using direct ip connections would have to use that proxy.
I don’t fully understand what this is trying to accomplish.
But the point that I was trying to make was that that would then also block you from using SSH. If you want to connect to any external service, you need to open a port for it, and if there’s an open port, then there’s a opening for unintended escape.
now i have the feeling as if there might be a misunderstanding of what “ports” are and what an “open” port actually is. Or i just dont get what you want. i am not on your server/workstation thus i cannot even try to connect TO an external service “from” your machine. i can do so from MY machine to other machines as i like and if those allow me, but you cannot do anything against that unless that other machine happens to be actually yours (or you own a router that happens to be on my path to where i connect to)
lets try something. your machine A has ssh service running my machine B has ssh and another machine C has ssh.
users on the machines are a b c , the machine letters but in small. what should be possible and what not? like: “a can connect to B using ssh” “a can not connect to C using ssh (forbidden by A)” “a can not connect to C using ssh (forbidden by C)” […]
so what is your scenario? what do you want to prevent?
I don’t fully understand what this is trying to accomplish.
accomplish control (allow/block/report) over who or what on my machine can connect to the outside world (using http/s) and to exactly where, but independant of ip addresses but using domains to allow or deny on a per user/application + domain combonation while not having to update ip based rules that could quickly outdate anyway.
now i have the feeling as if there might be a misunderstanding of what “ports” are and what an “open” port actually is. Or i just dont get what you want. i am not on your server/workstation thus i cannot even try to connect TO an external service “from” your machine.
This is most likely a result of my original post being too vague – which is, of course, entirely my fault. I was intending it to refer to a firewall running on a specific device. For example, a desktop computer with a firewall, which is behind a NAT router.
so what is your scenario? what do you want to prevent?
What is your example in response to? Or perhaps I don’t understand what it is attempting to clarify. I don’t necessarily have any confusion regarding setting up rules for known and discrete connections like SSH.
accomplish control (allow/block/report) over who or what on my machine can connect to the outside world (using http/s) and to exactly where, but independant of ip addresses but using domains to allow or deny on a per user/application + domain combonation while not having to update ip based rules that could quickly outdate anyway.
Are you referring to an application layer firewall like, for example, OpenSnitch?
This is most likely a result of my original post being too vague – which is, of course, entirely my fault.
Never mind, and i got distracted and carried away a bit from your question by the course the messages had taken
What is your example in response to?
i thought it could possibly help clarifying something, sort of it did i guess.
Are you referring to an application layer firewall like, for example, OpenSnitch?
no, i do not conside a proxy like squid to be an “application level firewall” (but i fon’t know opensnitch however), i would just limit outbound connections to some fqdn’s per authenticated client and ensure the connection only goes to where the fqdns actually point to. like an atracker could create a weather applet that “needs” https access to f.oreca.st, but implements a backdoor that silently connects to a static ip using https. with such a proxy, f.oreca.st would be available to the applet, but the other ip not as it is not included in the acl, neither as fqdn nor as an ip. if you like to say this is an application layer firewall ok, but i dont think so, its just a proxy with acls to me that only checks for allowed destination and if the response has some http headers (like 200 ok) but not really more. yet it can make it harder for some attackers to gain the control they are after ;-)
so here are some reasons for having a firewall on a computer, i did not read in the thread (could have missed them) i have already written this but then lost the text again before it was saved :( so here a compact version:
- having a second layer of defence, to prevent some of the direct impact of i.e. supply chain attacks like “upgrading” to an malicously manipulated version.
- control things tightly and report strange behaviour as an early warning sign ‘if’ something happens, no matter if attacks or bugs.
- learn how to tighten security and know better what to do in case you need it some day.
- sleep more comfortable when knowing what you have done or prevented
- compliance to some laws or customers buzzword matching whishes
- the fun to do because you can
- getting in touch with real life side quests, that you would never be aware of if you did not actively practiced by hardening your system.
one side quest example i stumbled upon: imagine an attacker has ccompromised the vendor of a software you use on your machine. this software connects to some port eventually, but pings the target first before doing so (whatever! you say). from time to time the ping does not go to the correct 11.22.33.44 of the service (weather app maybe) but to 0.11.22.33 looks like a bug you say, never mind.
could be something different. pinging an IP that does not exist ensures that the connection tracking of your router keeps the entry until it expires, opening a time window that is much easier to hit even if clocks are a bit out of sync.
also as the attacker knows the IP that gets pinged (but its an outbound connection to an unreachable IP you say what could go wrong?)
lets assume the attacker knows the external IP of your router by other means (i.e. you’ve send an email to the attacker and your freemail provider hands over your external router address to him inside of an email received header, or the manipulated software updates an dyndns address, or the attacker just guesses your router has an address of your providers dial up range, no matter what.)
so the attacker knows when and from where (or what range) you will ping an unreachable IP address in exact what timeframe (the software running from cron, or in user space and pings at exact timeframes to the “buggy” IP address) Then within that timeframe the attacker sends you an icmp unreachable packet to your routers external address, and puts the known buggy IP in the payload as the address that is unreachable. the router machtes the payload of the package, recognizes it is related to the known connection tracking entry and forwards the icmp unreachable to your workstation which in turn gives your application the information that the IP address of the attacker informs you that the buggy IP 0.11.22.33 cannot be reached by him. as the source IP of that packet is the IP of the attacker, that software can then open a TCP connection to that IP on port 443 and follow the instructions the attacker sends to it. Sure the attacker needs that backdoor already to exist and run on your workstation, and to know or guess your external IP address, but the actual behaviour of the software looks like normal, a bit buggy maybe, but there are exactly no informations within the software where the command and control server would be, only that it would respond to the icmp unreachable packet it would eventually receive. all connections are outgoing, but the attacker “connects” to his backdoor on your workstation through your NAT “Firewall” as if it did not exist while hiding the backdoor behind an occasional ping to an address that does not respond, either because the IP does not exist, or because it cannot respond due to DDos attack on the 100% sane IP that actually belongs to the service the App legitimately connects to or to a maintenance window, the provider of the manipulated software officially announces. the attacker just needs the IP to not respond or slooowly to increase the timeframe of connecting to his backdoor on your workstation before your router deletes the connectiin tracking entry of that unlucky ping.
if you don’t understand how that example works, that is absolutely normal and i might be bad in explaining too. thinking out of the box around corners that only sometimes are corners to think around and only under very specific circumstances that could happen by chance, or could be directly or indirectly under control of the attacker while only revealing the attackers location in the exact moment of connection is not an easy task and can really destroy the feeling of achievable security (aka believe to have some “control”) but this is not a common attack vector, only maybe an advanced one.
sometimes side quests can be more “informative” than the main course ;-) so i would put that (“learn more”, not the example above) as the main good reason to install a firewall and other security measures on your pc even if you’ld think you’re okay without it.
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.
If nothing else, the constant login attempts from bots will fill up your logs.
Yeah, this is defintely a scenario that I hadn’t considerd.
You always need a firewall, no other answer’s.
Why do you think windows and most linix distributions come packaged with one?
You always need a firewall, no other answer’s.
Okay, but why? That’s kind of the point of why I made this post, as is stated in the post’s body.
To keep your system secure no matter what, you open up only the ports you absolutely need.
People will always make a mistake while configuring software, a firewall is there to make sure that error is caught. With more advanced firewall’ you can even make sure only certain app’s have access to the internet to make sure only what you absolutely need toconnect to the internet does.
In general it’s for security, but can also be privacy related depending on how deep you want to get into it.
EDIT: It isnt about not trusting other devices on your netork,or software you run, or whether you are runni g a server. It’s about general security of your system.
With more advanced firewall’ you can even make sure only certain app’s have access to the internet to make sure only what you absolutely need toconnect to the internet does.
This sounds very interesting. This would have to be some forme of additional layer 7 firewall, right (As in it would have to interract with system processes, rather than filtering by network packet at layers 3, and 4)? Does this type of firewall have a specific name, or do you perhaps have some examples? I don’t think it would be possible with something like nftables, but I could certainly be wrong.
I honestly only know of a windows one called simplewall.
I used to use it to outright block windows telemetry, microsoft services, apps, …
It also helped me to save a lot of bandwith in regards to windows and all the stuff that comes preinstalked with it.
I havent searched for one for linux, mostly because 90% of apps I run are cli tools that don’t require internet connection, but I’m sure there is probably one that exists.
OpenSnitch was recommended to me in this comment. I’ve set it up, and it seems to be working quite well. While doing some research on the topic, I also came across Portmaster, but, while it does look nice, some of it’s features are locked behind a paywall, so I’m not interested – OpenSnitch works just fine!
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.
Defaults are robust enough
Would you mind defining what “defaults” are?
Defaults are the default settings of your firewall (netfilter in linux).
Is netfilter not just the API through which you can make firewall rules (e.g. nftables) for the networking stack?
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.
This example feels mildly contrived, as it is probably unlikely that one would have an email server running on a mobile device, but I understand your point.
I have another firewall setting for an untrusted LAN
This sounds interesting. Is it possible to implement this with a packet filtering firewall (e.g. nftables)?
Probably
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.
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 firewalls, 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.
Perhaps I don’t understand your point. If I understand your point in the sense that there are also issues with firewalls and that one always has attack vectors against usable systems, I fully agree with your remark. My point is simply, as a rule of thump a firewall usually mitigates a lot of attack vectors (see my remark about LIMIT for ssh ports elsewhere). Especially for client systems having a firewall which blocks all incoming traffic by default is IMHO high payoff for almost no effort.
My point is simply, as a rule of thump a firewall usually mitigates a lot of attack vectors
The only quibble that I would have with your statement is that I would say that it’s better to word it as it “mitigates a lot of potential attack vectors”, but, other than that, I completely agree with what you said.
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…
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).
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.
Heartbleed could steal your passwords even if you ran ostensibly trustworthy software.
Heartbleed is independent of a firewall though – it’s a protocol vulnerability that was patched into a specific library – this feels somewhat like a strawman argument.
So unless you harden the software and prove it’s completely exploit-free, then you can’t trust it.
The type of “firewall” that I am referring to operates at layer 3/4. From what I understand, you seem to be describing exploits closer to the application layer.
I’m not saying there would be a Heartbleed 2.0 that you need a firewall against
I’m saying unless you read the code you’re running, including the firmware and the kernel, how can you trust there isn’t a remote execution exploit?
At work I showed a trivial remote execution using an upload form. If we didn’t run php, it wouldn’t happen. If the folder had proper
.htaccess, it wouldn’t happen. If we didn’t trust the uploader’s MIME type, it wouldn’t happen.There’s something to be said about defense in depth. Even if you have some kind of a bug or exploit, the firewall just blocking everything might save you.
I’m saying unless you read the code you’re running, including the firmware and the kernel, how can you trust there isn’t a remote execution exploit?
A packet filtering firewall isn’t able to protect against server, or protocol exploits directly. Sure, if you know that connections originating from a specific IP are malicious, then you can drop connections originating from that IP, but it will not be able to direclty protect against application layer exploits.
There do exist application layer firewalls (an example of which was pointed out to me here (opensnitch)), but those are out of the scope of this post.
#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?
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!
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.
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
I looked up a few of the ports, and yeah an example of one of them was Steam Remote Play.
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.
_ /\ _
put a fresh-install of MS-Windows on a machine, & connected it to the internet.
What version of Windows? Connected how? Through a NAT, or was it through a DMZ connection, or netiher? Was Windows’ firewall enabled?
It took SEVERAL MINUTES for it to be broken-into, & corrupted, botnetted.
This is highly dependent on the setup, ofc. I can’t really comment without more knowledge of the experiment.
haven’t done it in years: they’ll not pay-for good anti-virus
Idk, nowadays, 3rd party anti-virus software on Windows doesn’t have too much user – Windows Defender is pretty dang good. If anything, a lot of them are borderline scams, or worse.
get AIDS, then, & don’t use anti-AIDS drugs, & see how “healthy” you are, 2 years in.
You don’t catch AIDS. HIV is the virus which causes AIDS to develop over time, if untreated. I’m not sure what you mean by anti-AIDS drugs. You could potentially be referring to anti-retroviral medication, or other related medication used to treat HIV, but, again that’s treating HIV to prevent the development of AIDS. You could also be referring to PrEP, but, once again, that is for protection against contracting the virus, not the collection of symptoms from a chronic HIV infection which is referred to as AIDS.
Tarpit was a wonderful-looking invention
This is interesting, I hadn’t heard of this!
Linux’s netfilter/iptables
Just a side note: iptables is deprecated – it has been succeeded by nftables.
EDIT: “when do I need to wear a seatbelt?”
is essentially the same category of question.
Fair point!
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.
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.
In the world of Windows XP before SP2, your system would be taken over by internet worms within minutes of connecting to the internet. If you had an Internet connection while running setup, it would happen before you even booted the computer into the OS for the first time.
Things have gotten better, but vulerabilities are still discovered all the time. A big point of a firewall is to have a device guaranteed to have very little attack surface in between devices that are more unknown quantities. Then they can add additional features, like recognizing when someone is trying to take advantage of a vulnerability in the webserver on port 80 and blocking it.
A big point of a firewall is to have a device guaranteed to have very little attack surface in between devices that are more unknown quantities.
Are you referring to a NAT?
Then they can add additional features, like recognizing when someone is trying to take advantage of a vulnerability in the webserver on port 80 and blocking it.
It seems that you are using more of a general interperetation of the term “firewall” rather than something more specific like a packet filtering firewall (which is more of the focus of my post). Am I correct In my interperetation?
No, I was referring to a firewall, how many ports are open on one versus a random user’s device?
My response is general to any firewall as you did not specify. They go all the way into deep packet inspection and intrusion detection (blocking exploitation of your webserver). NGFWs have extensive capabilities beyond packet filtering.
No, I was referring to a firewall
A NAT is a type of firewall.
how many ports are open on one versus a random user’s device?
I don’t understand the wording of this question.
NGFWs have extensive capabilities beyond packet filtering.
Interesting. Do you have any recommendations for software, or further reading on the topic?
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.
You most likely don’t need on device firewall if your in your home network behind a router that has a firewall.
Under what circumstance(s) would one need a device firewall? If I were to guess, I would say that it is when the internet facing device doesn’t contain a firewall within it (e.g. some enterprise-grade router), so a dedicated firewall device must exist behind it.
