Standalone userland SSH server

November 9, 2021

I guess I have pretty unusual software needs sometimes. I’ve been wanting to spawn a one-off SSH server on one of my computers so that I can rsync something to it from another machine.

I didn’t want to enable SSH connections systemwide on this host. Ideally, I wanted to start a server from my unprivileged user, that would only allow access to that particular user, and only to the specific SSH key of my other machine.

Turns out with a simple sshd_config, this is possible!

Edit: this is now automated in a easy to use Git repository, go check it out!

First, let’s make a directory to contain our server files.

mkdir standalone-sshd
cd standalone-sshd

In there, we generate our host RSA key (the -N '' part specifies an empty passphrase).

ssh-keygen -f ssh_host_rsa_key -N ''

Then, add your public key to a authorized_keys file in this same directory (same format as a regular ~/.ssh/authorized_keys), and add the following configuration in a sshd_config file.

Port 2222
HostKey /path/to/standalone-sshd/ssh_host_rsa_key
PidFile /path/to/standalone-sshd/

# Don't allow interactive authentication
KbdInteractiveAuthentication no

# Same as above but for older SSH versions
ChallengeResponseAuthentication no

# Don't allow password authentication
PasswordAuthentication no

# Only allow my own user
AllowUsers val

# Only allow my own key
AuthorizedKeysFile /path/to/standalone-sshd/authorized_keys

Tweak the port, user, and other settings to your liking, but that should give you a good base!

With that, you can run the server with the following command (the -D option starts the server in the foreground instead of the default daemon mode).

/usr/sbin/sshd -f sshd_config -D

Alternative with password

Alternatively, if you want to enable password authentication (with your user’s Unix login password), you can get away with an even simpler config:

Port 2222
HostKey /path/to/standalone-sshd/ssh_host_rsa_key
PidFile /path/to/standalone-sshd/

# PAM is necessary for password authentication on Debian-based systems
UsePAM yes

# Allow interactive authentication (default value)
#KbdInteractiveAuthentication yes

# Same as above but for older SSH versions (default value)
#ChallengeResponseAuthentication yes

# Allow password authentication (default value)
#PasswordAuthentication yes

# Only allow my own user
AllowUsers val

I included but commented out the settings that are necessary but whose default value is already what we want (essentially, password authentication is enabled by default).

We only need UsePAM yes on Debian-based systems for password authentication to work. As pointed out in this answer, contrary to what the sshd_config(5) man page says (“If UsePAM is enabled, you will not be able to run sshd(8) as a non-root user”), it’s not actually a problem when running in userland, and it’s even required if we want to support password authentication.

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