this post was submitted on 10 Nov 2023
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Have I successfully blocked ssh logins to root? (lemmy.ml)
submitted 8 months ago by cyberwolfie@lemmy.ml to c/linux@lemmy.ml
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I have a server where I believe I have disabled root login via ssh. I think it is done correctly, as I cannot login with root myself via ssh, but I would've thought that it would be reflected in /var/log/auth.log. Instead, it shows up as failed password entry. Is this intended?

What I've done is to uncomment the PermitRootLogin no line in /etc/ssh/sshd_config. Rest of the config file is left at default.

Bonus question: All login attempts by ssh seems to go over some random port (even my own successful logins). Why is this?

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[–] Markaos@lemmy.one 5 points 8 months ago* (last edited 8 months ago) (1 children)

Ssh listens on port 22, as soon as a connection is made the host moves the connection to another port to free up 22 for other new connections.

There's no limit on the number of concurrent connections on a single port, and SSH runs completely on the one port it is configured to use. Otherwise allowing just the port 22 in firewall wouldn't be enough to have a functional SSH connection with default settings.

You can verify that quite easily for example by spinning up three barebone Debian VMs connected to a single virtual network, configuring the firewall on the "server" VM to drop everything other than port 22 and then connecting from both client VMs - it will work just fine.

Maybe you're confusing it with the fact that only one process can listen on a given port at a time? But that's only for establishing new connections. Existing connections can be passed off to another running process or a child process just fine, and that's how SSH handles separation between connections.

Edit: oh, you're talking about the high port OP is wondering about. That's just the source port, which is chosen randomly by the client OS when making a connection. Using port 22 (or any other port below 1025) as a source port would require root privileges on the client and would also conflict with the SSH server that could be running there. Still, it has nothing to do with SSH "moving connections over"

[–] cyberwolfie@lemmy.ml 2 points 8 months ago* (last edited 8 months ago)

Edit: oh, you’re talking about the high port OP is wondering about. That’s just the source port, which is chosen randomly by the client OS when making a connection. Using port 22 (or any other port below 1025) as a source port would require root privileges on the client and would also conflict with the SSH server that could be running there. Still, it has nothing to do with SSH “moving connections over”

Ah, I see, so the port numbers shown in auth.log are all client side ports. I guess I thought that the listening port would be in the log and assumed that the port listed there would be it, but when I read the lines again, it clearly says "from ip.ad.dr.ess port 12345"