systemd-ssh-proxy - Man Page

Host unix/* unix%* vsock/* vsock%* vsock-mux/* 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/* unix%* vsock/* vsock%* vsock-mux/* 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 do not 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]​. Note that "%" can be used as a separator instead of "/" to be compatible with tools like "scp" and "rsync"​.

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

Example ​4. ​Copy local 'foo' file to a local VM with CID 1348

scp foo vsock%1348:

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