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docs: add a pull backup / push restore how-to, fixes #1552
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@ -13,3 +13,4 @@ This chapter details deployment strategies for the following scenarios.
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deployment/hosting-repositories
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deployment/automated-local
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deployment/image-backup
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deployment/pull-backup
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@ -0,0 +1,176 @@
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.. include:: ../global.rst.inc
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.. highlight:: none
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Backing up in pull mode
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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 bsdflags. 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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