systemd-ssh-generator

systemd

Generator for binding a socket-activated SSH server to local

AF_VSOCK

and

AF_UNIX

sockets

/usr/lib/systemd/system-generators/systemd-ssh-generator

Description

systemd-ssh-generator binds a socket-activated SSH server to local AF_VSOCK and AF_UNIX sockets under certain conditions. It only has an effect if the sshd(8) binary is installed. Specifically, it does the following:

If invoked in a VM with AF_VSOCK support, a socket-activated SSH per-connection service is bound to AF_VSOCK port 22.
If invoked in a container environment with a writable directory /run/host/unix-export/ pre-mounted it binds SSH to an AF_UNIX socket /run/host/unix-export/ssh. The assumption is that this directory is bind mounted to the host side as well, and can be used to connect to the container from there. See Container Interface for more information about this interface.
A local AF_UNIX socket /run/ssh-unix-local/socket is also bound, unconditionally. This may be used for SSH communication from the host to itself, without involving networking, for example to traverse security boundaries safely and with secure authentication.
Additional AF_UNIX and AF_VSOCK sockets are optionally bound, based on the systemd.ssh_listen= kernel command line option or the ssh.listen system credential (see below).

See systemd-ssh-proxy(1) for details on how to connect to these sockets via the ssh client.

The ssh.authorized_keys.root credential can be used to allow specific public keys to log in over SSH. See systemd.system-credentials(7) for more information.

The generator will use a packaged sshd@.service service template file if one exists, and otherwise generate a suitable service template file.

systemd-ssh-generator implements systemd.generator(7).

Kernel Command Line

systemd-ssh-generator understands the following kernel-command-line(7) parameters:

systemd.ssh_auto=
This option takes an optional boolean argument, and defaults to yes. If enabled, the automatic binding to the AF_VSOCK and AF_UNIX sockets listed above is done. If disable, this is not done, except for those explicitly requested via systemd.ssh_listen= on the kernel command line or via the ssh.listen system credential.

systemd.ssh_listen=
This option configures an additional socket to bind SSH to. It may be used multiple times to bind multiple sockets. The syntax should follow the one of ListenStream=, see systemd.socket(5) for details. This functionality supports all socket families systemd(1) supports, including AF_INET and AF_INET6.

Credentials

systemd-ssh-generator supports the system credentials logic. The following credentials are used when passed in:

ssh.listen
This credential should be a text file, with each line referencing one additional socket to bind SSH to. The syntax should follow the one of ListenStream=, see systemd.socket(5) for details. This functionality supports all socket families systemd supports, including AF_INET and AF_INET6.

ssh.ephemeral-authorized_keys-all
Provides additional public keys, given in the customary authorized_keys format, for all users, for incoming connections via the generated AF_VSOCK and AF_UNIX socket units.

The intended use of this is for a host system (in either VM or container configurations) to generate a keypair and inject the public key into the guest, using the private key to connect to any user account on the guest via ssh, without further authentication.