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Why not just block SSH access from the public internet and use a VPN? Trivially easy to setup and more secure than knocking. All it takes is me somehow being a

Why not just block SSH access from the public internet and use a VPN? Trivially easy to setup and more secure than knocking. All it takes is me somehow being able to listen in on your traffic - not even decrypt it - and now I know the knock sequence. I know that you have SSH listening on that server. I know you are actively doing something on it. vs. a VPN where... all I know is you are communicating over a VPN. With DPI I might be able to determine what type of traffic you're sending, but not where it is ultimately going.

rsync 6y ago
Again, this is where all port knocking debates devolve to ... Port knocking is not the christ child that will wash away all of our sins ... and therefore is not worth implementing. You're right! It doesn't add that much. But it's non-zero and has almost zero cost. It's very elegant, in my mind, and it makes me very happy.
- cthalupa 6y ago
  I guess my point is largely: I can set up a VPN in a roughly similar timeframe to setting up port knocking, and it has roughly similar overhead for end user, but the VPN gives me significantly more security while also solving the same issue port knocking does. In that case, why not just set up a VPN instead of port knocking?
  - justinsaccount 6y ago
    Which vpn though? https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=openvpn https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=openvpn https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=pulse+secure https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=pulse+secur... VPN doesn't magically fix all problems. wireguard is good so far, but the kernel implementation is in C, so who knows.
    - cthalupa 6y ago
      No, a VPN isn't magic in and of itself. And yes, I would suggest wireguard for the simplicity and performance these days. I do agree with the author of the original article that security should come in layers Once something is secured with SSH and a VPN, you've got that many more actual layers - you now need a CVE that allows access or credential leak for both the VPN and SSH. (And many of those CVEs don't necessarily allow a random attacker to arbitrarily gain access) https://news.ycombinator.com/item?id=24446919 https://news.ycombinator.com/item?id=24446919 has my list of what the bare minimum SSH protections should be for anything where you are storing customer/user data in my opinion, as well as additional best practices that I have employed.
  - rsync 6y ago
    I will again agree with you that the VPN is a more robust and more complete protection. You are correct. I think the reason I continue to prefer (and evangelize) port knocking is that the intersection of (modest) security gain and simplicity/robustness hits a sweet spot for me. Again, 10+ years in production on many hosts, worldwide, and never so much as a blip. IF knockd were to fail, it would fail in a very boring way. VPNs, on the other hand, are far more complex and they fail in fascinating ways. I am a sysop turned sysadmin - this is my lifes work. I prefer simple, unixy tools that fail in boring ways :)
    - Kalium 6y ago
      Not to be That Guy, but what I'm reading is that you have the wonderful opportunity before you to learn about VPNs until they fail in boring ways!
      - rsync 6y ago
        That's not how that works. Simple systems tend to fail in boring ways. Complex systems tend to fail in interesting ways. Learning more about a complex system, while rewarding in many ways, will not change that identity.
        Kalium 6y ago
        It's been my experience that a system's complexity (basic or complex) is less an intrinsic trait and more a matter of subjective perception, familiarity, and experience. Your experience is clearly different.
  - foxtrottbravo 6y ago
    My daily bread and butter is VPNs, but I must admit that I think there may be a truth here. While I fully agree that portknocking doesn’t provide the same layer of protection or flexibility a VPN does - but with the original article in mind: if your reason for deploying a VPN is because you fear to expose unknown bugs in sshd to the Internet the same could be said about every vpn solution. Therefore portknocking is / (would be*) indeed more elegant because - it makes no promises to be secure (as in as secure as a VPN) - one could argue: if you use it you know that portknocking is just an additional security layer - and maybe don’t get lazy as in a VPN - a misconfiguration or bug or an attacker might expose sshd on your hosts - a misconfigured VPN at least in a somewhat sizeable deployment can lead to countless attack surfaces Having said that, that only will work if the rest of the sshd security is in check and your password isn’t hunter2
    - cthalupa 6y ago
      I think get where you're coming from here, but I don't fully agree. >f your reason for deploying a VPN is because you fear to expose unknown bugs in sshd to the Internet the same could be said about every vpn solution. Yeah. You might have a VPN zero day - but then you still have to get into the other SSH servers. Two zero days simultaneously active for openssh and your VPN solution? Pretty unlikely, especially public ones. Someone burning two private zero days on you means you're an incredibly high value target and neither of these would suffice as your sole defense to begin with. The rest of your argument, if I'm understanding it correctly, is that you think people will get more lax with securing SSH on a box only reachable via VPN than if it was reachable by port knocking? It's possible, but I don't know that the evidence really shows that - lots of comments on this article are along the lines of "i set up port knocking and I've never even seen a malicious ssh connection attempt since then!" - no details of the rest of the security measures they've got in place. And yeah, going from 'I set up a VPN to connect to my web servers via ssh' to 'I have VPN access to a whole network with all sorts of things running on it' is a big step up, but I don't think it's really in the boundaries of this discussion. Port knocking was never going to be a replacement for a larger VPN deployment, and when you're opening up network access to a wider range of things then how you approach things definitely needs to change.
- justinsaccount 6y ago
  Like you said.. "worry less" Can a theoretical attacker intercept a port knocking sequence? maybe. Would a script kiddie running a new ssh 0day against the entire internet be able to do this? no.
- tptacek 6y ago
  It doesn't add enough to compensate for its costs, which are commensurate with those of VPNs, which provide drastically more return on the investment. But VPNs don't have a cheering section, because they're so obviously useful that nobody has any incentive to make that banal observation. "Port knocking" is idiosyncratic and widely looked down on by security engineering teams, so there's a contrarian impulse that makes them seem worth discussing.
  - beefarino 6y ago
    I'm struggling to walk away with a crystallized view of why port-knocking is bad, though. I do agree, nobody should be going to sleep at night, relying solely on obscurity as their source of protection. But these commenters are offering it as an additional layer of indirection. They're not touting it as _the_ solution, full stop. At the most basic level, would you refute the claim that port knocking or alternate ports are adding additional friction for an attacker, or no? Myself, I would prefer to run a simple, (hopefully) set-and-forget daemon on my server if it really did add an extra layer of obscurity to my secured SSH service. I guess I just fail to see why it's one against the other.
    - cthalupa 6y ago
      There's several components to this. Foremost, there is an opportunity cost to setting it up. The time you spend setting up port knocking could be spent setting up another form of security. I believe it is a sound argument to say that a VPN provides more security at a similar level effort. No public SSH means an attack cannot know SSH is running on the server from a port scan because it simply isn't listening. It allows you to reduce the attack surface - you can add more and more servers that you need to SSH into, but you are only allowing public access via your VPN - so you have fewer potential ingress points, and can ratchet up your security and auditing commensurately. And if your VPN concentrator is owned, you should have been setting things up so that they did not implicitly trust someone just because they were on the VPN, so you still have all of your usual measures of security in place. In that case, there's just not much point. You could also enable port knocking, but I don't think it provides much benefit. That brings us to the next part. Port knocking is a "weird" thing. It's idiosyncratic and not standardly used. Documenting it and understanding it is additional overhead, and it's something you have to manage and worry about on every server that's using it. Additionally, both standard and SPA implementations are vulnerable to man in the middle attacks, though most SPA based implementations will require an active MITM in that blocks the initial packet rather than just replaying a knock sequence. So extra complexity, less secure, and an oddity on the network that you have to have documented and explain to new team members, etc. If you're a single person managing a single server, well, honestly you're probably fine just turning off password auth. And you can feel free to do port knocking and whatever else. It probably doesn't matter.

rsync 6y ago

Again, this is where all port knocking debates devolve to ... Port knocking is not the christ child that will wash away all of our sins ... and therefore is not worth implementing. You're right! It doesn't add that much. But it's non-zero and has almost zero cost. It's very elegant, in my mind, and it makes me very happy.

cthalupa 6y ago
I guess my point is largely: I can set up a VPN in a roughly similar timeframe to setting up port knocking, and it has roughly similar overhead for end user, but the VPN gives me significantly more security while also solving the same issue port knocking does. In that case, why not just set up a VPN instead of port knocking?
- justinsaccount 6y ago
  Which vpn though? https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=openvpn https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=openvpn https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=pulse+secure https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=pulse+secur... VPN doesn't magically fix all problems. wireguard is good so far, but the kernel implementation is in C, so who knows.
  - cthalupa 6y ago
    No, a VPN isn't magic in and of itself. And yes, I would suggest wireguard for the simplicity and performance these days. I do agree with the author of the original article that security should come in layers Once something is secured with SSH and a VPN, you've got that many more actual layers - you now need a CVE that allows access or credential leak for both the VPN and SSH. (And many of those CVEs don't necessarily allow a random attacker to arbitrarily gain access) https://news.ycombinator.com/item?id=24446919 https://news.ycombinator.com/item?id=24446919 has my list of what the bare minimum SSH protections should be for anything where you are storing customer/user data in my opinion, as well as additional best practices that I have employed.
- rsync 6y ago
  I will again agree with you that the VPN is a more robust and more complete protection. You are correct. I think the reason I continue to prefer (and evangelize) port knocking is that the intersection of (modest) security gain and simplicity/robustness hits a sweet spot for me. Again, 10+ years in production on many hosts, worldwide, and never so much as a blip. IF knockd were to fail, it would fail in a very boring way. VPNs, on the other hand, are far more complex and they fail in fascinating ways. I am a sysop turned sysadmin - this is my lifes work. I prefer simple, unixy tools that fail in boring ways :)
  - Kalium 6y ago
    Not to be That Guy, but what I'm reading is that you have the wonderful opportunity before you to learn about VPNs until they fail in boring ways!
    - rsync 6y ago
      That's not how that works. Simple systems tend to fail in boring ways. Complex systems tend to fail in interesting ways. Learning more about a complex system, while rewarding in many ways, will not change that identity.
      - Kalium 6y ago
        It's been my experience that a system's complexity (basic or complex) is less an intrinsic trait and more a matter of subjective perception, familiarity, and experience. Your experience is clearly different.
- foxtrottbravo 6y ago
  My daily bread and butter is VPNs, but I must admit that I think there may be a truth here. While I fully agree that portknocking doesn’t provide the same layer of protection or flexibility a VPN does - but with the original article in mind: if your reason for deploying a VPN is because you fear to expose unknown bugs in sshd to the Internet the same could be said about every vpn solution. Therefore portknocking is / (would be*) indeed more elegant because - it makes no promises to be secure (as in as secure as a VPN) - one could argue: if you use it you know that portknocking is just an additional security layer - and maybe don’t get lazy as in a VPN - a misconfiguration or bug or an attacker might expose sshd on your hosts - a misconfigured VPN at least in a somewhat sizeable deployment can lead to countless attack surfaces Having said that, that only will work if the rest of the sshd security is in check and your password isn’t hunter2
  - cthalupa 6y ago
    I think get where you're coming from here, but I don't fully agree. >f your reason for deploying a VPN is because you fear to expose unknown bugs in sshd to the Internet the same could be said about every vpn solution. Yeah. You might have a VPN zero day - but then you still have to get into the other SSH servers. Two zero days simultaneously active for openssh and your VPN solution? Pretty unlikely, especially public ones. Someone burning two private zero days on you means you're an incredibly high value target and neither of these would suffice as your sole defense to begin with. The rest of your argument, if I'm understanding it correctly, is that you think people will get more lax with securing SSH on a box only reachable via VPN than if it was reachable by port knocking? It's possible, but I don't know that the evidence really shows that - lots of comments on this article are along the lines of "i set up port knocking and I've never even seen a malicious ssh connection attempt since then!" - no details of the rest of the security measures they've got in place. And yeah, going from 'I set up a VPN to connect to my web servers via ssh' to 'I have VPN access to a whole network with all sorts of things running on it' is a big step up, but I don't think it's really in the boundaries of this discussion. Port knocking was never going to be a replacement for a larger VPN deployment, and when you're opening up network access to a wider range of things then how you approach things definitely needs to change.
justinsaccount 6y ago
Like you said.. "worry less" Can a theoretical attacker intercept a port knocking sequence? maybe. Would a script kiddie running a new ssh 0day against the entire internet be able to do this? no.
tptacek 6y ago
It doesn't add enough to compensate for its costs, which are commensurate with those of VPNs, which provide drastically more return on the investment. But VPNs don't have a cheering section, because they're so obviously useful that nobody has any incentive to make that banal observation. "Port knocking" is idiosyncratic and widely looked down on by security engineering teams, so there's a contrarian impulse that makes them seem worth discussing.
- beefarino 6y ago
  I'm struggling to walk away with a crystallized view of why port-knocking is bad, though. I do agree, nobody should be going to sleep at night, relying solely on obscurity as their source of protection. But these commenters are offering it as an additional layer of indirection. They're not touting it as _the_ solution, full stop. At the most basic level, would you refute the claim that port knocking or alternate ports are adding additional friction for an attacker, or no? Myself, I would prefer to run a simple, (hopefully) set-and-forget daemon on my server if it really did add an extra layer of obscurity to my secured SSH service. I guess I just fail to see why it's one against the other.
  - cthalupa 6y ago
    There's several components to this. Foremost, there is an opportunity cost to setting it up. The time you spend setting up port knocking could be spent setting up another form of security. I believe it is a sound argument to say that a VPN provides more security at a similar level effort. No public SSH means an attack cannot know SSH is running on the server from a port scan because it simply isn't listening. It allows you to reduce the attack surface - you can add more and more servers that you need to SSH into, but you are only allowing public access via your VPN - so you have fewer potential ingress points, and can ratchet up your security and auditing commensurately. And if your VPN concentrator is owned, you should have been setting things up so that they did not implicitly trust someone just because they were on the VPN, so you still have all of your usual measures of security in place. In that case, there's just not much point. You could also enable port knocking, but I don't think it provides much benefit. That brings us to the next part. Port knocking is a "weird" thing. It's idiosyncratic and not standardly used. Documenting it and understanding it is additional overhead, and it's something you have to manage and worry about on every server that's using it. Additionally, both standard and SPA implementations are vulnerable to man in the middle attacks, though most SPA based implementations will require an active MITM in that blocks the initial packet rather than just replaying a knock sequence. So extra complexity, less secure, and an oddity on the network that you have to have documented and explain to new team members, etc. If you're a single person managing a single server, well, honestly you're probably fine just turning off password auth. And you can feel free to do port knocking and whatever else. It probably doesn't matter.