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Comment by JimDabell | Hacker News Reader

Moving the port reduces security. Port 22 is a privileged port. Standard users can’t listen on port 22. If you move the port to 2222 or wherever, then if an attacker with local access can get sshd to crash, they can run their own sshd instead. If you left the port as the default, they wouldn’t be able to do that without chaining it with a privilege escalation attack. But because you changed the port, you disabled this security feature and it could become a privilege escalation attack.

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Only if they already have local access. And the sshd is not listening on the port (if it’s on, you can’t just bind it)

And they still can’t show the host certificate, so your client would tell you the certificate changed.

Unless, of course, they have a local root exploit — but then port 22 is just as unsafe.

I fail to see how changing a port makes anything less safe.

Its pretty trivial to disable the ability of users to listen on certain ports higher than 1024 with a firewall config.

A firewall config can block listening? What would that firewall config be? The firewall can block packets by owner uid. But I am not sure who is the owner in legitimate sshd case. Root or the user logged in?

SELinux can do it, probably other LSMs, too.

you cant block the actual port bind, but you can block any packets from reaching it, so the difference is mostly semantics

Yes, but that port needs to remain "open" for the legitimate sshd traffic. Can you see a difference in ownership as the firewall sees it between sshd and some user daemon? Sshd drops root partially when login succeeds.

that has nothing to do with sshd's listening socket, which remains owned by root

Sure the listening one remains owned by root. But the connected one? If you limit packets from/to e.g. 2222 to uid 0, will legitimate ssh traffic work? I don't say it won't, genuinely unsure. Haven't tried and today is a holiday. Maybe tomorrow :)

In practice, people won’t do that. Case in point: the article doesn’t mention this mitigation at all. It introduces an additional attack vector and tells you it’s safer.

I am not sure it reduces security. You have a valid point. I adds an attack vector. However "only" if you have the attacker in your system already.

Having all the log spam from failed attempts on 22 might the make the admin negligent on carefully following their logs at all. Having it pretty silent on a higher port usually will make you notice occasional scanning. At least I have noticed it and made me tighten the firewall a bit.

There is no perfect security.

Your point about privileged ports is valid. So maybe change to a different but still privileged port?

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