Raspberry Pi : Configuring a Time Capsule/Backintime server

By | 7th October 2021

In this post, I am setting up a Time Capsule and Backintime server. I am using a Raspberry Pi that has Ubuntu installed, with a USB disk that has been configured into a ZFS pool.

Setting up backup users

You are going to have to create users for each of the services/users that will be connecting to the server. You want to keep files and access as isolated as possible. As in a given user shouldn’t have any visibility or notion of other users’ backups. We are also creating accounts that can’t login into the system for Time Machine, only authenticate.

Check if there is an entry for nologin in:

$ cat /etc/shells

If there is no entry add it:

# vim /etc/shells
# /etc/shells: valid login shells

Create a generic user for the backups, or dedicated accounts for each user to increase security:

Generic user example:

# useradd -s /usr/sbin/nologin timemachine
# passwd timemachine

Dedicated user example:

# useradd -s /usr/sbin/nologin timemachine_john
# passwd timemachine_john

Note that useradd doesn’t create a home

If required, the default shell can be changed with:

# usermod -s /usr/sbin/nologin timemachine_john

Setting up backup user groups

If more than one system is going to be backed up it is advisable to use different accounts for each.

It is possible to isolate users by assigning them individual datasets, but that might create storage silos.

An alternative is to create individual users that belong to the same backup group. The backup group can access the backintime dataset, but not each other’s data.

Create the group.

# addgroup backupusers

Assign main group and secondary group (the secondary group would be the shared one).

# usermod -g timemachine_john -G backupusers timemachine_john

Although not required, you could force the UID and GID to be a specific one.

# usermod -u 1012 timemachine
# groupmod -g 1012 timemachine

Time Capsule

Install netatalk

Install netatalk from the repositories.

# apt install netatalk

Allow access to all the appropriate accounts to the directory where the backups are going to be written to:

# chown :timemachine_john /backups/timecapsule/
# chmod 775 /backups/timecapsule/

Edit the settings of the netatalk service so that that share can be seen with the name of your choice and work as a Time Capsule server.

# vim /etc/netatalk/AppleVolumes.default

Enter the following:

/backups/timecapsule "pi-capsule" options:tm

Note that you can give the capsule a name with spaces above.

Restart the service:

# systemctl restart netatalk

Check that netatalk has been installed correctly:

# afpd -V

afpd 3.1.12 - Apple Filing Protocol (AFP) daemon of Netatalk
afpd has been compiled with support for these features:

          AFP versions: 2.2 3.0 3.1 3.2 3.3 3.4 
         CNID backends: dbd last tdb 
      Zeroconf support: Avahi
  TCP wrappers support: Yes
         Quota support: Yes
   Admin group support: Yes
    Valid shell checks: Yes
      cracklib support: No
            EA support: ad | sys
           ACL support: Yes
          LDAP support: Yes
         D-Bus support: Yes
     Spotlight support: Yes
         DTrace probes: Yes

              afp.conf: /etc/netatalk/afp.conf
           extmap.conf: /etc/netatalk/extmap.conf
       state directory: /var/lib/netatalk/
    afp_signature.conf: /var/lib/netatalk/afp_signature.conf
      afp_voluuid.conf: /var/lib/netatalk/afp_voluuid.conf
       UAM search path: /usr/lib/netatalk//
  Server messages path: /var/lib/netatalk/msg/

Configure netatalk

# vim /etc/nsswitch.conf

Change this line:

hosts:          files mdns4_minimal [NOTFOUND=return] dns

to this:

hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4 mdns

Note that if you are running Netatalk 3.1.11 or above it is not necessary any more to create the /etc/avahi/services/afpd.service. Using this file will cause an error.

If you are running an older version go ahead, otherwise jump to the next section.

Create /etc/avahi/services/afpd.service as root

# vim /etc/avahi/services/afpd.service

and fill it up with:

<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
        <name replace-wildcards="yes">%h</name>

Configure the AFP service

Edit the configuration file.

# vim /etc/netatalk/afp.conf
; Global server settings
mimic model = TimeCapsule6,106

path = /backups/timecapsule
time machine = yes

Check configuration and reload if needed:

# systemctl status avahi-daemon	

[restart if necessary]
# systemctl restart netatalk

[Make the service automatically start]
# systemctl enable netatalk.service

If you go to your Mac’s Time Machine preferences the new volume will be available and you can start using it.

netatalk troubleshooting

Some notes of things to check from the server side (Time Capsule server):


Backintime setup

Configuring Backintime

Prepare users

If you have disabled passwords and are only using keys, you will need to temporarily change the security settings to allow Backintime to exchange keys.

On the remote system/Pi/server:

# vim /etc/ssh/sshd_config
PasswordAuthentication yes
# systemctl restart ssh

Backintime uses SSH, so the user accounts need to be allowed to login. Therefore the default login shell needs to reflect this.

If not created already, assign the user a home directory. Finally, allow the user to read and write the folder containing the backups.

# usermod -s /usr/bin/bash backintime_john

# mkdir /home/backintime_john

# chown backintime_tuxedo:backintime_john /home/backintime_john/

# usermod -d /home/backintime_john/ backintime_john

# chown :backupusers /backups/backintime/

# chmod 775 /backups/backintime/

Permissions for some of the subfolders might be required in multi-user configuration after the first backup:

# chown :backupusers /backups/backintime*/backintime

# chmod 770 /backups/backintime*/backintime/system1/
# chmod 770 /backups/backintime*/backintime/laptop2/

Prepare keys

To simplify things these are the roles:

[Local system]
The client machine that is running Backintime and that you want to backup your data from.

[Remote system]
The SSH server that has the storage where your backup is going to be stored.

From the local system account you want to run backintime (either your user or root, depending on how you run Backintime) SSH into the remote system. In my case, a Raspberry Pi.

# ssh backintime_john@pi-capsule.local

After logging in check the host key.

$ ssh-keygen -l -f /etc/ssh/ssh_host_ecdsa_key.pub
256 SHA256:KjzU6aGqH6tXri/K87xz3H+cP35PMT7n+Ob6MIaBZb0 root@pi-capsule (ECDSA)

You can then log out from the remote machine.

From the local account, you want to run Backintime from generate a new SSH key pair.

# ssh-keygen

And then copy the public key to the Pi.

# ssh-copy-id -i ~/.ssh/id_rsa.pub backintime_david@pi-capsule
ECDSA key fingerprint is SHA256:KjzU6aGqH6tXri/K87xz3H+cP35PMT7n+Ob6MIaBZb0.
Number of key(s) added: 1

Note that the fingerprint is the same as the one displayed in the previous step.

Configure Backintime profile

You can now configure the SSH profile from Backintime and make the first run.

In the General tab:

Mode:               SSH

SSH Settings
Host:   pi-capsule
Port:   22
User:   backintime_david
Path:   /backups/backintime_david
Cipher:     [Leave as default]
Private Key:/root/.ssh/id_rsa

SSH private key:[empty in most cases]
Enable Cache Password

Host:       tuxedo
User:       root
Profile:    2

[Select appropriate setting after testing]

Exclude (example)

Older than 10 years
If free space is less than 50GiB
If free inodes is less than 2%

Smart remove
Run in background on remote Host
Keep last
14 days (7 days)
21 days (14 days)
8 weeks (6 weeks)
36 months (14 months)

Don't remove named snapshots
Enable notifications
Backup replaced files on restore
Continue on errors (keep incomplete snapshots)
Log level: Changes & Errors

After the first run has completed you can check which is the best performing cipher from the CLI.

# backintime benchmark-cipher --profile-id 2

After a few rounds, aes192-ctr came out as the best performing cipher for me.

Secure SSH

If you changed the SSH configuration at the beginning, after setting everything up, remember to secure SSH again on the server/remote system.

# vim /etc/ssh/sshd_config
PasswordAuthentication no
# systemctl restart ssh

Restoring restrictions to backup users

The login account is required for Backintime to be able to run rsync. It is worth doing a bit more research on how to harden/limit these accounts.


Some examples of some issues and some troubleshooting steps you can apply.

Time Capsule can’t be reached / firewall settings

Make sure the server is allowing AFP connections from the Mac client.

# ufw allow proto tcp from CLIENT_IP to PI_CAPSULE_IP port 548

Time Capsule – Configuring Time Machine backups via the network on a macOS VM

The destination needs to be configured manually.

Mount the AFP/Time Capsule mount via the Finder.

In the CLI configure the destination:

# tmutil setdestination -a /Volumes/pi-capsule

The backups can then be started from the GUI.

You can get information about the current configured destinations via the CLI.

# tmutil destinationinfo
Name            : pi-capsule
Kind            : Network
Mount Point     : /Volumes/pi-capsule
ID              : 7B648734-9BFC-417F-B5A1-F31B8AD52F4B

Time Capsule – Checking backup status

# tmutil currentphase
# tmutil status

ZFS stalling on a Raspberry Pi

Check the recordsize property. Reduce it to the default 128 kiB.

Reduce ARC size to reduce the amount of memory consumed/reserved for ZFS.

Understanding rsync logs

The logs indicate the type of change rsync is seeing. A reference is available here:

XYcstpoguax  path/to/file
||||||||||╰- x: The extended attribute information changed
|||||||||╰-- a: The ACL information changed
||||||||╰--- u: The u slot is reserved for future use
|||||||╰---- g: Group is different
||||||╰----- o: Owner is different
|||||╰------ p: Permission are different
||||╰------- t: Modification time is different
|||╰-------- s: Size is different
||╰--------- c: Different checksum (for regular files), or
||              changed value (for symlinks, devices, and special files)
|╰---------- the file type:
|            f: for a file,
|            d: for a directory,
|            L: for a symlink,
|            D: for a device,
|            S: for a special file (e.g. named sockets and fifos)
╰----------- the type of update being done::
             <: file is being transferred to the remote host (sent)
             >: file is being transferred to the local host (received)
             c: local change/creation for the item, such as:
                - the creation of a directory
                - the changing of a symlink,
                - etc.
             h: the item is a hard link to another item (requires 
             .: the item is not being updated (though it might have
                attributes that are being modified)
             *: means that the rest of the itemized-output area contains
                a message (e.g. "deleting")

Some example output:

>f+++++++++ some/dir/new-file.txt
.f....og..x some/dir/existing-file-with-changed-owner-and-group.txt
.f........x some/dir/existing-file-with-changed-unnamed-attribute.txt
>f...p....x some/dir/existing-file-with-changed-permissions.txt
>f..t..g..x some/dir/existing-file-with-changed-time-and-group.txt
>f.s......x some/dir/existing-file-with-changed-size.txt
>f.st.....x some/dir/existing-file-with-changed-size-and-time-stamp.txt 
cd+++++++++ some/dir/new-directory/
.d....og... some/dir/existing-directory-with-changed-owner-and-group/
.d..t...... some/dir/existing-directory-with-different-time-stamp/ 
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