Backing up in pull mode

Assuming you have a pull backup system set up with borg, where a backup server pulls the data from the target via SSHFS. In this mode, the backup client’s file system is mounted remotely on the backup server. Pull mode is even possible if the SSH connection must be established by the client via a remote tunnel. Other network file systems like NFS or SMB could be used as well, but SSHFS is very simple to set up and probably the most secure one.

There are some restrictions caused by SSHFS. For example, unless you define UID and GID mappings when mounting via sshfs, owners and groups of the mounted file system will probably change, and you may not have access to those files if BorgBackup is not run with root privileges.

SSHFS is a FUSE file system and uses the SFTP protocol, so there may be also other unsupported features that the actual implementations of ssfs, libfuse and sftp on the backup server do not support, like file name encodings, ACLs, xattrs or flags. So there is no guarantee that you are able to restore a system completely in every aspect from such a backup.

Warning

To mount the client’s root file system you will need root access to the client. This contradicts to the usual threat model of BorgBackup, where clients don’t need to trust the backup server (data is encrypted). In pull mode the server (when logged in as root) could cause unlimited damage to the client. Therefore, pull mode should be used only from servers you do fully trust!

Creating a backup

Generally, in a pull backup situation there is no direct way for borg to know the client’s original UID:GID name mapping of files, because Borg would use /etc/passwd and /etc/group of the backup server to map the names. To derive the right names, Borg needs to have access to the client’s passwd and group files and use them in the backup process.

The solution to this problem is chrooting into an sshfs mounted directory. In this example the whole client root file system is mounted. We use the stand-alone BorgBackup executable and copy it into the mounted file system to make Borg available after entering chroot; this can be skipped if Borg is already installed on the client.

# Mount client root file system.
mkdir /tmp/sshfs
sshfs [email protected]:/ /tmp/sshfs
# Mount BorgBackup repository inside it.
mkdir /tmp/sshfs/borgrepo
mount --bind /path/to/repo /tmp/sshfs/borgrepo
# Make borg executable available.
cp /usr/local/bin/borg /tmp/sshfs/usr/local/bin/borg
# Mount important system directories and enter chroot.
cd /tmp/sshfs
for i in dev proc sys; do mount --bind /$i $i; done
chroot /tmp/sshfs

Now we are on the backup system but inside a chroot with the client’s root file system. We have a copy of Borg binary in /usr/local/bin and the repository in /borgrepo. Borg will back up the client’s user/group names, and we can create the backup, retaining the original paths, excluding the repository:

borg create --exclude /borgrepo --files-cache ctime,size /borgrepo::archive /

For the sake of simplicity only /borgrepo is excluded here. You may want to set up an exclude file with additional files and folders to be excluded. Also note that we have to modify Borg’s file change detection behaviour – SSHFS cannot guarantee stable inode numbers, so we have to supply the --files-cache option.

Finally, we need to exit chroot, unmount all the stuff and clean up:

exit # exit chroot
rm /tmp/sshfs/usr/local/bin/borg
cd /tmp/sshfs
for i in dev proc sys borgrepo; do umount ./$i; done
rmdir borgrepo
cd ~
umount /tmp/sshfs
rmdir /tmp/sshfs

Thanks to secuser on IRC for this how-to!

Restore methods

The counterpart of a pull backup is a push restore. Depending on the type of restore – full restore or partial restore – there are different methods to make sure the correct IDs are restored.

Partial restore

In case of a partial restore, using the archived UIDs/GIDs might lead to wrong results if the name-to-ID mapping on the target system has changed compared to backup time (might be the case e.g. for a fresh OS install).

The workaround again is chrooting into an sshfs mounted directory, so Borg is able to map the user/group names of the backup files to the actual IDs on the client. This example is similar to the backup above – only the Borg command is different:

# Mount client root file system.
mkdir /tmp/sshfs
sshfs [email protected]:/ /tmp/sshfs
# Mount BorgBackup repository inside it.
mkdir /tmp/sshfs/borgrepo
mount --bind /path/to/repo /tmp/sshfs/borgrepo
# Make borg executable available.
cp /usr/local/bin/borg /tmp/sshfs/usr/local/bin/borg
# Mount important system directories and enter chroot.
cd /tmp/sshfs
for i in dev proc sys; do mount --bind /$i $i; done
chroot /tmp/sshfs

Now we can run

borg extract /borgrepo::archive PATH

to partially restore whatever we like. Finally, do the clean-up:

exit # exit chroot
rm /tmp/sshfs/usr/local/bin/borg
cd /tmp/sshfs
for i in dev proc sys borgrepo; do umount ./$i; done
rmdir borgrepo
cd ~
umount /tmp/sshfs
rmdir /tmp/sshfs

Full restore

When doing a full restore, we restore all files (including the ones containing the ID-to-name mapping, /etc/passwd and /etc/group). Everything will be consistent automatically if we restore the numeric IDs stored in the archive. So there is no need for a chroot environment; we just mount the client file system and extract a backup, utilizing the --numeric-owner option:

sshfs [email protected]:/ /mnt/sshfs
cd /mnt/sshfs
borg extract --numeric-owner /path/to/repo::archive
cd ~
umount /mnt/sshfs

Simple (lossy) full restore

Using borg export-tar it is possible to stream a backup to the client and directly extract it without the need of mounting with SSHFS:

borg export-tar /path/to/repo::archive - | ssh [email protected] 'tar -C / -x'

Note that in this scenario the tar format is the limiting factor – it cannot restore all the advanced features that BorgBackup supports. See borg export-tar for limitations.