systemd-ssh-proxy - Man Page

Host unix/* vsock/* vsock-mux/*
    ProxyCommand /usr/lib/systemd/systemd-ssh-proxy %h %p
    ProxyUseFdpass yes

/usr/lib/systemd/systemd-ssh-proxy [ADDRESS] [PORT]

systemd-ssh-proxy is a small "proxy" plugin for the ssh(1) tool that allows connecting to AF_UNIX and AF_VSOCK sockets. It implements the interface defined by ssh's ProxyCommand configuration option. It's supposed to be used with an ssh_config(5) configuration fragment like the following:

Host unix/* vsock/* vsock-mux/*
    ProxyCommand /usr/lib/systemd/systemd-ssh-proxy %h %p
    ProxyUseFdpass yes
    CheckHostIP no

Host .host
    ProxyCommand /usr/lib/systemd/systemd-ssh-proxy unix/run/ssh-unix-local/socket %p
    ProxyUseFdpass yes
    CheckHostIP no

A configuration fragment along these lines is by default installed into /etc/ssh/ssh_config.d/20-systemd-ssh-proxy.conf.

With this in place, SSH connections to host string "unix/" followed by an absolute AF_UNIX file system path to a socket will be directed to the specified socket, which must be of type SOCK_STREAM. Similar, SSH connections to "vsock/" followed by an AF_VSOCK CID will result in an SSH connection made to that CID. "vsock-mux/" followed by an absolute AF_UNIX file system path to a socket is similar but for cloud-hypervisor/firecracker which don't allow direct AF_VSOCK communication between the host and guests, and provide their own multiplexer over AF_UNIX sockets. See cloud-hypervisor VSOCK support^[1] and Using the Firecracker Virtio-vsock Device^[2].

Moreover connecting to ".host" will connect to the local host via SSH, without involving networking.

This tool is supposed to be used together with systemd-ssh-generator(8) which when run inside a VM or container will bind SSH to suitable addresses. systemd-ssh-generator is supposed to run in the container or VM guest, and systemd-ssh-proxy is run on the host, in order to connect to the container or VM guest.

On success, 0 is returned, a non-zero failure code otherwise.

Example 1. Talk to a local VM with CID 4711

ssh vsock/4711

Example 2. Talk to a VM guest hosted with cloud-hypervisor/firecracker

ssh vsock-mux/run/vm-1234.sock

Example 3. Talk to the local host via ssh

ssh .host

or equivalent:

ssh unix/run/ssh-unix-local/socket

systemd(1), systemd-ssh-generator(8), vsock(7), unix(7), ssh(1), sshd(8)

1. cloud-hypervisor VSOCK support
   https://github.com/cloud-hypervisor/cloud-hypervisor/blob/main/docs/vsock.md
2. Using the Firecracker Virtio-vsock Device
   https://github.com/firecracker-microvm/firecracker/blob/main/docs/vsock.md

systemd.directives(7), systemd.index(7), systemd-ssh-generator(8).