zrepl/docs/configuration/transports.rst

.. highlight:: bash

.. _transport:

Transports
==========

The zrepl RPC layer uses **transports** to establish a single, bidirectional data stream between an active and passive job.
On the passive (serving) side, the transport also provides the **client identity** to the upper layers:
this string is used for access control and separation of filesystem sub-trees in :ref:`sink jobs <job-sink>`.
Transports are specified in the ``connect`` or ``serve`` section of a job definition.

.. ATTENTION::

    The **client identities must be valid ZFS dataset path components**
    because the :ref:`sink job <job-sink>` uses ``${root_fs}/${client_identity}`` to determine the client's subtree.

.. _transport-tcp:

``tcp`` Transport
-----------------

The ``tcp`` transport uses plain TCP, which means that the data is **not encrypted** on the wire.
Clients are identified by their IPv4 or IPv6 addresses, and the client identity is established through a mapping on the server.

This transport may also be used in conjunction with network-layer encryption and/or VPN tunnels to provide encryption on the wire.
To make the IP-based client authentication effective, such solutions should provide authenticated IP addresses.
Some options to consider:

* `WireGuard <https://www.wireguard.com/>`_: Linux-focussed, in-kernel TLS
* `OpenVPN <https://openvpn.net/>`_: Cross-platform VPN, uses tun on \*nix
* `IPSec <https://en.wikipedia.org/wiki/IPsec>`_: Properly standardized, in-kernel network-layer VPN
* `spiped <http://www.tarsnap.com/spiped.html>`_: think of it as an encrypted pipe between two servers
* SSH

  * `sshuttle <https://sshuttle.readthedocs.io/en/stable/overview.html>`_: VPN-like solution, but using SSH
  * `SSH port forwarding <https://help.ubuntu.com/community/SSH/OpenSSH/PortForwarding>`_: Systemd user unit & make it start before the zrepl service.

Serve
~~~~~

::

    jobs:
    - type: sink
      serve:
        type: tcp
        listen: ":8888"
        clients: {
          "192.168.122.123" : "mysql01"
          "192.168.122.123" : "mx01"
        }
      ...

Connect
~~~~~~~

::

    jobs:
     - type: push
       connect:
         type: tcp
         address: "10.23.42.23:8888"
         dial_timeout: # optional, default 10s
       ...

.. _transport-tcp+tlsclientauth:

``tls`` Transport
-----------------

The ``tls`` transport uses TCP + TLS with client authentication using client certificates.
The client identity is the common name (CN) presented in the client certificate.
It is recommended to set up a dedicated CA infrastructure for this transport, e.g. using OpenVPN's `EasyRSA <https://github.com/OpenVPN/easy-rsa>`_.

The implementation uses `Go's TLS library <https://golang.org/pkg/crypto/tls/>`_.
Since Go binaries are statically linked, you or your distribution need to recompile zrepl when vulnerabilities in that library are disclosed.

All file paths are resolved relative to the zrepl daemon's working directory.
Specify absolute paths if you are unsure what directory that is (or find out from your init system).

Serve
~~~~~

::

    jobs:
      - type: sink
        root_fs: "pool2/backup_laptops"
        serve:
          type: tls
          listen: ":8888"
          ca: /etc/zrepl/ca.crt
          cert: /etc/zrepl/prod.crt
          key: /etc/zrepl/prod.key
          client_cns:
            - "laptop1"
            - "homeserver"

The ``ca`` field specified the certificate authority used to validate client certificates.
The ``client_cns`` list specifies a list of accepted client common names (which are also the client identities for this transport).

Connect
~~~~~~~

::

    jobs:
    - type: pull
      connect:
        type: tls
        address: "server1.foo.bar:8888"
        ca: /etc/zrepl/ca.crt
        cert: /etc/zrepl/backupserver.crt
        key:  /etc/zrepl/backupserver.key
        server_cn: "server1"
        dial_timeout: # optional, default 10s

The ``ca`` field specifies the CA which signed the server's certificate (``serve.cert``).
The ``server_cn`` specifies the expected common name (CN) of the server's certificate.
It overrides the hostname specified in ``address``.
The connection fails if either do not match.

.. _transport-ssh+stdinserver:

``ssh+stdinserver`` Transport
-----------------------------

``ssh+stdinserver`` is inspired by `git shell <https://git-scm.com/docs/git-shell>`_ and `Borg Backup <https://borgbackup.readthedocs.io/en/stable/deployment.html>`_.
It is provided by the Go package ``github.com/problame/go-netssh``.

.. _transport-ssh+stdinserver-serve:

Serve
~~~~~

::

    jobs:
    - type: source
      serve:
        type: stdinserver
        client_identities:
        - "client1"
        - "client2"
      ...

First of all, note that ``type=stdinserver`` in this case:
Currently, only ``connect.type=ssh+stdinserver`` can connect to a ``serve.type=stdinserver``, but we want to keep that option open for future extensions.

The serving job opens a UNIX socket named after ``client_identity`` in the runtime directory.
In our example above, that is ``/var/run/zrepl/stdinserver/client1`` and ``/var/run/zrepl/stdinserver/client2``.

On the same machine, the ``zrepl stdinserver $client_identity`` command connects to ``/var/run/zrepl/stdinserver/$client_identity``.
It then passes its stdin and stdout file descriptors to the zrepl daemon via *cmsg(3)*.
zrepl daemon in turn combines them into an object implementing ``net.Conn``:
a ``Write()`` turns into a write to stdout, a ``Read()`` turns into a read from stdin.

Interactive use of the ``stdinserver`` subcommand does not make much sense.
However, we can force its execution when a user with a particular SSH pubkey connects via SSH.
This can be achieved with an entry in the ``authorized_keys`` file of the serving zrepl daemon.

::

    # for OpenSSH >= 7.2
    command="zrepl stdinserver CLIENT_IDENTITY",restrict CLIENT_SSH_KEY
    # for older OpenSSH versions
    command="zrepl stdinserver CLIENT_IDENTITY",no-port-forwarding,no-X11-forwarding,no-pty,no-agent-forwarding,no-user-rc CLIENT_SSH_KEY

* CLIENT_IDENTITY is substituted with an entry from ``client_identities`` in our example
* CLIENT_SSH_KEY is substituted with the public part of the SSH keypair specified in the ``connect.identity_file`` directive on the connecting host.

.. NOTE::

    You may need to adjust the ``PermitRootLogin`` option in ``/etc/ssh/sshd_config`` to ``forced-commands-only`` or higher for this to work.
    Refer to sshd_config(5) for details.

To recap, this is of how client authentication works with the ``ssh+stdinserver`` transport:

* Connections to the ``/var/run/zrepl/stdinserver/${client_identity}`` UNIX socket are blindly trusted by zrepl daemon.
  The connection client identity is the name of the socket, i.e. ``${client_identity}``.
* Thus, the runtime directory must be private to the zrepl user (this is checked by zrepl daemon)
* The admin of the host with the serving zrepl daemon controls the ``authorized_keys`` file.
* Thus, the administrator controls the mapping ``PUBKEY -> CLIENT_IDENTITY``.

.. _transport-ssh+stdinserver-connect:

Connect
~~~~~~~

::

    jobs:
    - type: pull
      connect:
        type: ssh+stdinserver
        host: prod.example.com
        user: root
        port: 22
        identity_file: /etc/zrepl/ssh/identity
        options: # optional, default [], `-o` arguments passed to ssh
        - "Compression=on"
        dial_timeout: # optional, default 10s, max time.Duration until initial handshake is completed

The connecting zrepl daemon

#. Creates a pipe
#. Forks
#. In the forked process

   #. Replaces forked stdin and stdout with the corresponding pipe ends
   #. Executes the ``ssh`` binary found in ``$PATH``.

      #. The identity file (``-i``) is set to ``$identity_file``.
      #. The remote user, host and port correspond to those configured.
      #. Further options can be specified using the ``options`` field, which appends each entry in the list to the command line using ``-o $entry``.

#. Wraps the pipe ends in a ``net.Conn`` and returns it to the RPC layer.

As discussed in the section above, the connecting zrepl daemon expects that ``zrepl stdinserver $client_identity`` is  executed automatically via an ``authorized_keys`` file entry.

The ``known_hosts`` file used by the ssh command must contain an entry for ``connect.host`` prior to starting zrepl.
Thus, run the following on the pulling host's command line (substituting ``connect.host``):

::

    ssh -i /etc/zrepl/ssh/identity root@prod.example.com

.. NOTE::

    The environment variables of the underlying SSH process are cleared. ``$SSH_AUTH_SOCK`` will not be available.
    It is suggested to create a separate, unencrypted SSH key solely for that purpose.


.. _transport-local:

``local`` Transport
-------------------

The local transport can be used to implement :ref:`local replication <replication-local>`, i.e., push replication between a push and sink job defined in the same configuration file.

The ``listener_name`` is analogous to a hostname and must match between ``serve`` and ``connect``.
The ``client_identity`` is used by the sink as documented above.

::

    jobs:
    - type: sink
      serve:
        type: local
        listener_name: localsink
      ...

    - type: push
      connect:
        type: local
        listener_name: localsink
        client_identity: local_backup
      ...