systemd/man/systemd-ssh-proxy.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<refentry id="systemd-ssh-proxy"
    xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>systemd-ssh-proxy</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>systemd-ssh-proxy</refentrytitle>
    <manvolnum>1</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>systemd-ssh-proxy</refname>
    <refpurpose>SSH client plugin for connecting to <constant>AF_VSOCK</constant> and
    <constant>AF_UNIX</constant> sockets</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <programlisting>
Host unix/* vsock/* vsock-mux/*
    ProxyCommand /usr/lib/systemd/systemd-ssh-proxy %h %p
    ProxyUseFdpass yes
</programlisting>
    <cmdsynopsis>
      <command>/usr/lib/systemd/systemd-ssh-proxy</command> <arg>ADDRESS</arg> <arg>PORT</arg>
    </cmdsynopsis>
  </refsynopsisdiv>


  <refsect1>
    <title>Description</title>

    <para><command>systemd-ssh-proxy</command> is a small "proxy" plugin for the <citerefentry
    project="man-pages"><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    tool that allows connecting to <constant>AF_UNIX</constant> and <constant>AF_VSOCK</constant> sockets. It
    implements the interface defined by <filename>ssh</filename>'s <varname>ProxyCommand</varname>
    configuration option. It's supposed to be used with an <citerefentry
    project="man-pages"><refentrytitle>ssh_config</refentrytitle><manvolnum>5</manvolnum></citerefentry>
    configuration fragment like the following:</para>

    <programlisting>
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
</programlisting>

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

    <para>With this in place, SSH connections to host string <literal>unix/</literal> followed by an absolute
    <constant>AF_UNIX</constant> file system path to a socket will be directed to the specified socket, which
    must be of type <constant>SOCK_STREAM</constant>. Similar, SSH connections to <literal>vsock/</literal>
    followed by an <constant>AF_VSOCK</constant> CID will result in an SSH connection made to that
    CID. <literal>vsock-mux/</literal> followed by an absolute <constant>AF_UNIX</constant> file system
    path to a socket is similar but for cloud-hypervisor/firecracker which don't allow
    direct <constant>AF_VSOCK</constant> communication between the host and guests, and provide their own
    multiplexer over <constant>AF_UNIX</constant> sockets. See
    <ulink url="https://github.com/cloud-hypervisor/cloud-hypervisor/blob/main/docs/vsock.md">cloud-hypervisor VSOCK support</ulink>
    and <ulink url="https://github.com/firecracker-microvm/firecracker/blob/main/docs/vsock.md">Using the Firecracker Virtio-vsock Device</ulink>.</para>

    <para>Moreover connecting to <literal>.host</literal> will connect to the local host via SSH, without
    involving networking.</para>

    <para>This tool is supposed to be used together with
    <citerefentry><refentrytitle>systemd-ssh-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    which when run inside a VM or container will bind SSH to suitable
    addresses. <command>systemd-ssh-generator</command> is supposed to run in the container of VM guest, and
    <command>systemd-ssh-proxy</command> is run on the host, in order to connect to the container or VM
    guest.</para>
  </refsect1>

  <refsect1>
    <title>Exit status</title>

    <para>On success, 0 is returned, a non-zero failure code
    otherwise.</para>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <example>
      <title>Talk to a local VM with CID 4711</title>

      <programlisting>ssh vsock/4711</programlisting>
  </example>

    <example>
      <title>Talk to a VM guest hosted with cloud-hypervisor/firecracker</title>

      <programlisting>ssh vsock-mux/run/vm-1234.sock</programlisting>
    </example>

    <example>
      <title>Talk to the local host via ssh</title>

      <programlisting>ssh .host</programlisting>

      <para>or equivalent:</para>

      <programlisting>ssh unix/run/ssh-unix-local/socket</programlisting>
    </example>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para><simplelist type="inline">
      <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd-ssh-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
      <member><citerefentry project="man-pages"><refentrytitle>vsock</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry project="man-pages"><refentrytitle>unix</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry project="man-pages"><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry project="man-pages"><refentrytitle>sshd</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
    </simplelist></para>
  </refsect1>
</refentry>