This post describes how to overcome the hetzner backup space restriction of not being able to set linux uid / gids and so on on the (sshfs) mounted backup space. It does not describe how to do incremental backups using rsync and hardlinks.

1. The problem

The www.hetzner.de backup space does not allow for rsync to change the linux uids. It however is often desirable to keep those file informations when doing backups. Rsync will print the following error when trying to copy files to the hetzner backup space mounted when via sshfs:

sending incremental file list
rsync: chown "/srv/backup/test" failed: Permission denied (13)

Please also notice that hetzner closes the sshfs connection upon multiple errors. sshfs mount will then automatically be unmounted. For running rsyncs, this results in the following error:

rsync: read errors mapping "/srv/backup/[...]": Input/output error (5)

2. The solution

Hetzner describes here that in order to take full advantage of rsync it is required to create an image file of the size of the booked backup space, loop mount this image file, create a filesystem on it and mount it for the backups. The following describes how to do that.
Of course this will cost some speed when doing backups. Compared to using just the normal backup space with rsync while not keeping the uids it seems negligible. Besides, you don’t have any choice.
Also note that the access speed to the hetzner backup space is also dependent on other hetzner customers currently using it, as described here.

1. Automatic sshfs for hetzner backup space

The following commands allow to automatically mount the hetzner backup space using sshfs, without having to enter a password every time.

mkdir -p /srv/backup/{backup,hetzner-backup-space}
apt-get install sshfs
sshfs u01234@u01234.your-backup.de:/ /srv/backup/hetzner-backup-space # Enter password
mkdir /srv/backup/hetzner-backup-space/.ssh
# If you did not yet create a ssh keypair for your user, do this now
ssh-keygen
# Export the public key in RFC4716 format, hetzner requires that
ssh-keygen -e -f ~/.ssh/id_rsa.pub | grep -v "Comment:" > ~/.ssh/id_rsa_rfc.pub
cat ~/.ssh/id_rsa_rfc.pub > /srv/backup/hetzner-backup-space/.ssh/authorized_keys
chmod 700 /srv/backup/hetzner-backup-space/.ssh
chmod 600 /srv/backup/hetzner-backup-space/.ssh/authorized_keys

2. Create a dd image for the actual backups

Now an empty empty dd image has to be created and a filesystem has to be written on it. Using dd without seek on the sshfs would take forever, as every zero from /dev/zero would have to be sent over the network to the backup space. Therefore I recommend to create a sparse image file (think thin provisioning) with dd like follows. This does not take longer than a few seconds. Its also possible to use `fallocate` for this.

dd if=/dev/zero of=/srv/backup/hetzner-backup-space/filesystem.img bs=1 seek=100G count=1
mkfs.ext4 /srv/backup/hetzner-backup-space/filesystem.img

If you need more space to backup your data you can just order more from hetzner as described here. To grow the image file use the following commands (note the change from 100G to 200G in the dd seek argument). This will not overwrite existing data on the image file.

dd if=/dev/zero of=/srv/backup/hetzner-backup-space/filesystem.img bs=1 seek=200G count=1
resize2fs /srv/backup/hetzner-backup-space/filesystem.img

3. Automatically mounting the backup space and the image

To automatically mount the hetzner backup space and the filesystem.img just created append the following lines to the /etc/fstab file. You can then also manually execute `mount -a` or `mount /srv/backup/hetzner-backup-space; mount /srv/backup/backup` to mount them during runtime.

u0815@u0815.your-backup.de:/ /srv/backup fuse.sshfs defaults,_netdev 0 0
/srv/backup/hetzner-backup-space/filesystem.img /srv/backup/backup ext4 defaults,loop 0 0

3. Storing backups encrypted

Note that this way it is also possible to encrypt your backups using cryptsetup. It is recommended to not store keyfiles for temporary usage on the hard disk, as deleted files can be recovered using data forensic tools. Permanently store it in a (preferably offline) safe location and only upload it to a virtual filesystem.

# 32 bytes are 256 bits
dd if=/dev/random of=/dev/shm/hetzner-backup.key bs=1 count=32

Save the file on a safe location now. Do not permanently store it on the server.

cryptsetup luksFormat --key-file=/dev/shm/hetzner-backup.key /dev/backup/hetzner-backup-space/filesystem.img
cryptsetup luksOpen --key-file=/dev/shm/hetzner-backup.key /dev/backup/hetzner-backup-space/filesystem.img backup_crypt

Remember to delete the keyfile on the server after usage

rm /dev/shm/hetzner-backup.key

Create a filesystem on the unencrypted mapping

mkfs.ext4 /dev/mapper/backup_crypt

The decrypted mapping has to be mounted by hand. The /etc/fstab (or /etc/crypttab) file makes no sense here, as the keyfile is not stored on the server and therefore requires human interaction to access the encrypted backup image.

mount -o loop /dev/mapper/backup_crypt /srv/backup/backup

When all backups are finished for today, you can also close the cryptsetup mapping again. This has to be done before umounting the backup directory!

cryptsetup luksClose /dev/mapper/backup_crypt

4. Additional information

http://wiki.hetzner.de/index.php/Backup_Space_SSH_Keys/en
http://wiki.hetzner.de/index.php/Backup/en#Backup_Space
https://wiki.archlinux.org/index.php/sparse_file#Resizing_the_sparse_file