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86 lines
2.7 KiB
ReStructuredText
86 lines
2.7 KiB
ReStructuredText
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=================
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Libvirt SSH proxy
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=================
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Sometimes it's necessary to run some commands inside a guest. While libvirt
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already provides a `NSS module <nss.html>`__ that can translate guest name to
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IP address it has some limitations (e.g. guest has to have a network interface
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plugged into a libvirt-managed network). To resolve some of these limitations,
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libvirt offers a SSH proxy. It consists of a SSH client config file
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(``/etc/ssh/ssh_config.d/30-libvirt-ssh-proxy.conf``) and a small binary. Both
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are automatically installed by ``libvirt-ssh-proxy`` package which is dragged
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in by ``libvirt-client``, ``libvirt-daemon-qemu`` and/or ``daemon-kvm`` RPM
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packages. After running either of:
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::
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ssh user@qemu:system/virtualMachine
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ssh user@qemu:session/virtualMachine
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the configuration file instructs SSH client to start the binary helper which
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finds a VSOCK device inside the ``virtualMachine`` and establishes a connection
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to it.
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For now, only QEMU domains are implemented and the lookup of the
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``virtualMachine`` is done under ``qemu:///system`` URI for ``qemu:system`` or
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under ``qemu:///session`` URI for ``qemu:session``.
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For convenience, there's also another alternative:
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::
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ssh user@qemu/virtualMachine
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where the ``virtualMachine`` is looked up under ``qemu:///system`` first,
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possibly followed by ``qemu:///session`` (for cases where ssh client runs as
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non-root, since there's no ``qemu:///session`` for root).
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Accepted values for ``virtualMachine`` are: domain name (as reported by e.g.
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`virsh list`), domain UUID and finally domain ID.
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Guest OS requirements
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---------------------
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It is obvious that the SSH daemon inside the guest needs to be configured to
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listen for incoming connections on a VSOCK. There are couple of ways to achieve
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this:
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* Run systemd-v256 or newer inside the guest.
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In this release, systemd started to deploy ``systemd-ssh-generator`` which
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should configure socket activation for SSHD automagically.
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* Set up socket activation for VSOCK.
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We can take an inspiration in the unit file generated by
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``systemd-ssh-generator``:
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::
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[Unit]
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Description=OpenSSH Server Socket (systemd-ssh-generator, AF_VSOCK)
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Documentation=man:systemd-ssh-generator(8)
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Wants=ssh-access.target
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Before=ssh-access.target
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[Socket]
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ListenStream=vsock::22
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Accept=yes
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PollLimitIntervalSec=30s
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PollLimitBurst=50
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* Run a service that forwards VSOCK <=> SSHD communication
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For instance:
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::
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socat VSOCK-LISTEN:22,reuseaddr,fork TCP:localhost:22
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Libvirt domain XML configuration
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--------------------------------
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Since the SSH proxy uses a VSOCK to communicate with the SSH daemon running
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inside the guest, it is a must to configure VSOCK in the `domain XML
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<formatdomain.html#vsock>`__.
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