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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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ReStructuredText
303 lines
12 KiB
ReStructuredText
.. 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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the SSH connection must be established by the client via a remote tunnel. Other
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network file systems like NFS or SMB could be used as well, but SSHFS is very
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simple to set up and probably the most secure one.
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There are some restrictions caused by SSHFS. For example, unless you define UID
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and GID mappings when mounting via ``sshfs``, owners and groups of the mounted
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file system will probably change, and you may not have access to those files if
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BorgBackup is not run with root privileges.
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SSHFS is a FUSE file system and uses the SFTP protocol, so there may be also
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other unsupported features that the actual implementations of ssfs, libfuse and
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sftp on the backup server do not support, like file name encodings, ACLs, xattrs
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or flags. So there is no guarantee that you are able to restore a system
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completely in every aspect from such a backup.
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.. warning::
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To mount the client's root file system you will need root access to the
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client. This contradicts to the usual threat model of BorgBackup, where
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clients don't need to trust the backup server (data is encrypted). In pull
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mode the server (when logged in as root) could cause unlimited damage to the
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client. Therefore, pull mode should be used only from servers you do fully
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trust!
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Creating a backup
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-----------------
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Generally, in a pull backup situation there is no direct way for borg to know
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the client's original UID:GID name mapping of files, because Borg would use
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``/etc/passwd`` and ``/etc/group`` of the backup server to map the names. To
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derive the right names, Borg needs to have access to the client's passwd and
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group files and use them in the backup process.
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The solution to this problem is chrooting into an sshfs mounted directory. In
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this example the whole client root file system is mounted. We use the
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stand-alone BorgBackup executable and copy it into the mounted file system to
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make Borg available after entering chroot; this can be skipped if Borg is
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already installed on the client.
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::
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# Mount client root file system.
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mkdir /tmp/sshfs
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sshfs root@host:/ /tmp/sshfs
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# Mount BorgBackup repository inside it.
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mkdir /tmp/sshfs/borgrepo
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mount --bind /path/to/repo /tmp/sshfs/borgrepo
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# Make borg executable available.
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cp /usr/local/bin/borg /tmp/sshfs/usr/local/bin/borg
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# Mount important system directories and enter chroot.
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cd /tmp/sshfs
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for i in dev proc sys; do mount --bind /$i $i; done
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chroot /tmp/sshfs
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Now we are on the backup system but inside a chroot with the client's root file
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system. We have a copy of Borg binary in ``/usr/local/bin`` and the repository
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in ``/borgrepo``. Borg will back up the client's user/group names, and we can
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create the backup, retaining the original paths, excluding the repository:
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::
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borg create --exclude /borgrepo --files-cache ctime,size /borgrepo::archive /
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For the sake of simplicity only ``/borgrepo`` is excluded here. You may want to
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set up an exclude file with additional files and folders to be excluded. Also
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note that we have to modify Borg's file change detection behaviour – SSHFS
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cannot guarantee stable inode numbers, so we have to supply the
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``--files-cache`` option.
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Finally, we need to exit chroot, unmount all the stuff and clean up:
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::
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exit # exit chroot
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rm /tmp/sshfs/usr/local/bin/borg
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cd /tmp/sshfs
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for i in dev proc sys borgrepo; do umount ./$i; done
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rmdir borgrepo
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cd ~
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umount /tmp/sshfs
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rmdir /tmp/sshfs
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Thanks to secuser on IRC for this how-to!
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Restore methods
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---------------
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The counterpart of a pull backup is a push restore. Depending on the type of
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restore – full restore or partial restore – there are different methods to make
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sure the correct IDs are restored.
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Partial restore
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~~~~~~~~~~~~~~~
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In case of a partial restore, using the archived UIDs/GIDs might lead to wrong
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results if the name-to-ID mapping on the target system has changed compared to
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backup time (might be the case e.g. for a fresh OS install).
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The workaround again is chrooting into an sshfs mounted directory, so Borg is
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able to map the user/group names of the backup files to the actual IDs on the
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client. This example is similar to the backup above – only the Borg command is
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different:
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::
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# Mount client root file system.
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mkdir /tmp/sshfs
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sshfs root@host:/ /tmp/sshfs
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# Mount BorgBackup repository inside it.
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mkdir /tmp/sshfs/borgrepo
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mount --bind /path/to/repo /tmp/sshfs/borgrepo
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# Make borg executable available.
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cp /usr/local/bin/borg /tmp/sshfs/usr/local/bin/borg
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# Mount important system directories and enter chroot.
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cd /tmp/sshfs
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for i in dev proc sys; do mount --bind /$i $i; done
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chroot /tmp/sshfs
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Now we can run
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::
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borg extract /borgrepo::archive PATH
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to partially restore whatever we like. Finally, do the clean-up:
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::
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exit # exit chroot
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rm /tmp/sshfs/usr/local/bin/borg
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cd /tmp/sshfs
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for i in dev proc sys borgrepo; do umount ./$i; done
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rmdir borgrepo
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cd ~
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umount /tmp/sshfs
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rmdir /tmp/sshfs
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Full restore
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~~~~~~~~~~~~
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When doing a full restore, we restore all files (including the ones containing
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the ID-to-name mapping, ``/etc/passwd`` and ``/etc/group``). Everything will be
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consistent automatically if we restore the numeric IDs stored in the archive. So
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there is no need for a chroot environment; we just mount the client file system
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and extract a backup, utilizing the ``--numeric-owner`` option:
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::
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sshfs root@host:/ /mnt/sshfs
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cd /mnt/sshfs
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borg extract --numeric-owner /path/to/repo::archive
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cd ~
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umount /mnt/sshfs
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Simple (lossy) full restore
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Using ``borg export-tar`` it is possible to stream a backup to the client and
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directly extract it without the need of mounting with SSHFS:
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::
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borg export-tar /path/to/repo::archive - | ssh root@host 'tar -C / -x'
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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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