mirror of https://github.com/borgbackup/borg.git
[DOCS] Add socat pull mode (#5150)
docs: describe socat pull mode, fixes #900 also: fix sphinx deprecation warning borg/docs/conf.py:114: RemovedInSphinx40Warning: The app.add_stylesheet() is deprecated. Please use app.add_css_file() instead.
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@ -111,7 +111,7 @@ def set_rst_settings(app):
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def setup(app):
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app.add_stylesheet('css/borg.css')
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app.add_css_file('css/borg.css')
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app.connect('builder-inited', set_rst_settings)
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# Theme options are theme-specific and customize the look and feel of a theme
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@ -1,9 +1,23 @@
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.. include:: ../global.rst.inc
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.. highlight:: none
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=======================
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Backing up in pull mode
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=======================
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Typically the borg client connects to a backup server using SSH as a transport
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when initiating a backup. This is referred to as push mode.
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If you however require the backup server to initiate the connection or prefer
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it to initiate the backup run, one of the following workarounds is required to
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allow such a pull mode setup.
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A common use case for pull mode is to backup a remote server to a local personal
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computer.
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SSHFS
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=====
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Assuming you have a pull backup system set up with borg, where a backup server
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pulls the data from the target via SSHFS. In this mode, the backup client's file
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system is mounted remotely on the backup server. Pull mode is even possible if
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@ -174,3 +188,116 @@ directly extract it without the need of mounting with SSHFS:
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Note that in this scenario the tar format is the limiting factor – it cannot
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restore all the advanced features that BorgBackup supports. See
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:ref:`borg_export-tar` for limitations.
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socat
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=====
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In this setup a SSH connection from the backup server to the client is
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established that uses SSH reverse port forwarding to transparently
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tunnel data between UNIX domain sockets on the client and server and the socat
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tool to connect these with the borg client and server processes, respectively.
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The program socat has to be available on the backup server and on the client
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to be backed up.
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When **pushing** a backup the borg client (holding the data to be backed up)
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connects to the backup server via ssh, starts ``borg serve`` on the backup
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server and communicates via standard input and output (transported via SSH)
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with the process on the backup server.
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With the help of socat this process can be reversed. The backup server will
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create a connection to the client (holding the data to be backed up) and will
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**pull** the data.
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In the following example *borg-server* connects to *borg-client* to pull a backup.
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To provide a secure setup sockets should be stored in ``/run/borg``, only
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accessible to the users that run the backup process. So on both systems,
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*borg-server* and *borg-client* the folder ``/run/borg`` has to be created::
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sudo mkdir -m 0700 /run/borg
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On *borg-server* the socket file is opened by the user running the ``borg
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serve`` process writing to the repository
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so the user has to have read and write permissions on ``/run/borg``::
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borg-server:~$ sudo chown borgs /run/borg
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On *borg-client* the socket file is created by ssh, so the user used to connect
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to *borg-client* has to have read and write permissions on ``/run/borg``::
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borg-client:~$ sudo chown borgc /run/borg
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On *borg-server*, we have to start the command ``borg serve`` and make its
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standard input and output available to a unix socket::
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borg-server:~$ socat UNIX-LISTEN:/run/borg/reponame.sock,fork EXEC:"borg serve --append-only --restrict-to-path /path/to/repo"
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Socat will wait until a connection is opened. Then socat will execute the
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command given, redirecting Standard Input and Output to the unix socket. The
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optional arguments for ``borg serve`` are not necessary but a sane default.
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.. note::
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When used in production you may also use systemd socket-based activation
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instead of socat on the server side. You would wrap the ``borg serve`` command
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in a `service unit`_ and configure a matching `socket unit`_
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to start the service whenever a client connects to the socket.
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.. _service unit: https://www.freedesktop.org/software/systemd/man/systemd.service.html
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.. _socket unit: https://www.freedesktop.org/software/systemd/man/systemd.socket.html
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Now we need a way to access the unix socket on *borg-client* (holding the
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data to be backed up), as we created the unix socket on *borg-server*
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Opening a SSH connection from the *borg-server* to the *borg-client* with reverse port
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forwarding can do this for us::
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borg-server:~$ ssh -R /run/borg/reponame.sock:/run/borg/reponame.sock borgc@borg-client
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.. note::
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As the default value of OpenSSH for ``StreamLocalBindUnlink`` is ``no``, the
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socket file created by sshd is not removed. Trying to connect a second time,
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will print a short warning, and the forwarding does **not** take place::
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Warning: remote port forwarding failed for listen path /run/borg/reponame.sock
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When you are done, you have to manually remove the socket file, otherwise
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you may see an error like this when trying to execute borg commands::
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Remote: YYYY/MM/DD HH:MM:SS socat[XXX] E connect(5, AF=1 "/run/borg/reponame.sock", 13): Connection refused
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Connection closed by remote host. Is borg working on the server?
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When a process opens the socket on *borg-client*, SSH will forward all
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data to the socket on *borg-server*.
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The next step is to tell borg on *borg-client* to use the unix socket to communicate with the
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``borg serve`` command on *borg-server* via the socat socket instead of SSH::
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borg-client:~$ export BORG_RSH="sh -c 'exec socat STDIO UNIX-CONNECT:/run/borg/reponame.sock'"
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The default value for ``BORG_RSH`` is ``ssh``. By default Borg uses SSH to create
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the connection to the backup server. Therefore Borg parses the repo URL
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and adds the server name (and other arguments) to the SSH command. Those
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arguments can not be handled by socat. We wrap the command with ``sh`` to
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ignore all arguments intended for the SSH command.
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All Borg commands can now be executed on *borg-client*. For example to create a
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backup execute the ``borg create`` command::
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borg-client:~$ borg create ssh://borg-server/path/to/repo::archive /path_to_backup
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When automating backup creation, the
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interactive ssh session may seem inappropriate. An alternative way of creating
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a backup may be the following command::
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borg-server:~$ ssh \
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-R /run/borg/reponame.sock:/run/borg/reponame.sock \
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borgc@borg-client \
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borg create \
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--rsh "sh -c 'exec socat STDIO UNIX-CONNECT:/run/borg/reponame.sock'" \
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ssh://borg-server/path/to/repo::archive /path_to_backup \
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';' rm /run/borg/reponame.sock
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This command also automatically removes the socket file after the ``borg
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create`` command is done.
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