Starting with macOS Tahoe, the AFS protocol for Time Machine is no longer accepted. One must use Samba with modern macOS for a homemade Time Machine.

At the time of writing, Tahoe is the latest macOS.

1. Install samba. Make a group smbgroup and add a new user intended to only be used with the Samba connection:

   • sudo groupadd smbgroup
   • sudo useradd -M -s /usr/sbin/nologin timemachine
   • sudo usermod -aG smbgroup timemachine

2. Set that new user's Samba password with smbpasswd and enable the account:

   • sudo smbpasswd -a timemachine
   • sudo smbpasswd -e timemachine

3. Set up ACL support

   • sudo apt install acl (or equivalent for your package manager if not using a Debian-based distro)

4. Create the Time Machine directory. I keep it in /srv as it's the idiomatic place to keep it on Linux.

   • sudo mkdir -p /srv/timemachine
   • sudo chown timemachine:smbgroup /srv/timemachine
   • sudo chmod 2770 /srv/timemachine
   • We use 2770 rather than 770 because the 2 (setgid bit) ensures new files are owned by smbgroup.

5. Add ACLs

   • sudo setfacl -m group:smbgroup:rwx /srv/timemachine
   • sudo setfacl -d -m group:smbgroup:rwx /srv/timemachine

There are several guides on how to configure smb.conf correctly for Time Machine. I will share my config below, just in case it helps. This is on Debian 13 and only serves macOS clients on Sequoia and Tahoe. I don't use Avahi/Zeroconf because the server remains connected around the clock without any issues.

[global]
   server role = standalone server
   workgroup = WORKGROUP
   netbios name = fileserver
   security = user

   log file = /var/log/samba/log.%m
   max log size = 1000

   server min protocol = SMB3
   client signing = required

   obey pam restrictions = yes
   pam password change = yes
   unix password sync = yes

   inherit acls = yes
   map to guest = Bad User

   vfs objects = catia fruit streams_xattr
   fruit:metadata = stream
   fruit:resource = stream

[TimeMachine]
   path = /srv/time-machine
   browsable = yes
   read only = no
   guest ok = no
   valid users = @smbgroup
   vfs objects = catia fruit streams_xattr
   fruit:time machine = yes
   fruit:model = TimeCapsule
   fruit:metadata = stream
   create mask = 0660
   directory mask = 0770

Pro Tip: If making a fresh partition for Time Machine use, XFS is recommended more so than EXT4, because XFS is not susceptible to issues that could arise with an EXT4 FS, especially on larger drives. Basically, each file creates an inode for its metadata, but you have a limited number of inodes in ext4, whereas this isn't a problem that exists in XFS. This matters because Time Machine creates many very small files, so it would be possible to run out of inodes. This is what -T news exists for (the term is from news servers would would have many small text files). Formatting ext4 with -T news gives you more inodes if XFS isn't an option, so if you must use ext4, it would be recommended to format it with the news flag.

Thought I'd post some additional info for people. My currently working smb.conf is as follows. Adding fruit:metadata = stream worked for only a short while for me, so below are the results of hours of additional testing.

Note this is on Debian 10 (Buster) so things like password change command will be different on other distributions. Also note that the order of modules in the setting vfs objects = catia fruit streams_xattr is significant and important.

# NOTE: Whenever you modify this file you should run the command
# "testparm" to check that you have not made any basic syntactic 
# errors. 
[global]
   workgroup = WORKGROUP
   min protocol = SMB2


   log file = /var/log/samba/log.%m
   max log size = 5000
   logging = file
   security = USER
   
   panic action = /usr/share/samba/panic-action %d

   server role = standalone server
   obey pam restrictions = yes
   unix password sync = yes

   passwd program = /usr/bin/passwd %u
   passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
   pam password change = yes
   username map = /etc/samba/users.map
   map to guest = bad user
   guest account = XXXYOURGUESTACCOUNT

# Time Machine settings
   vfs objects = catia fruit streams_xattr
   fruit:model = RackMac
   fruit:advertise_fullsync = true
   fruit:metadata = stream
   fruit:veto_appledouble = no
#default is yes, not necessary to specify fruit:posix_rename = no
#default is yes, not necessary to specify fruit:zero_file_id = yes
   fruit:wipe_intentionally_left_blank_rfork = yes
   fruit:delete_empty_adfiles = yes
   ea support = yes


#default is yes, not necessary to specify   fruit:aapl = yes

   # Make share visible to Windows
#Disabled for a Mac-Only network
#   lanman auth = no
#   ntlm auth = yes
#   wins support = yes
#   local master = yes
#   preferred master = yes

   # Allow symlinks
#   follow symlinks = yes
#   wide links = yes
#   unix extensions = no

#======================= Share Definitions =======================
[SHARED DRIVE]

   path = /PATH/TO/YOUR/SHARED/FOLDER
   valid users = @YOURGUESTUSERGROUP
   writable = yes
   durable handles = yes
   kernel oplocks = no
   kernel share modes = no
   posix locking = no
   vfs objects = catia fruit streams_xattr
#default is yes, not necessary to specify   ea support = yes
#default is yes, not necessary to specify browseable = yes
   read only = no
   inherit acls = yes
   fruit:time machine = yes
   fruit:metadata = stream
   fruit:locking = netatalk
   guest ok = yes

[Time Machine]
   path = /PATH/TO/YOUR/SHARED/FOLDER
   valid users = @YOURGUESTUSERGROUP
   writable = yes
   durable handles = yes
   kernel oplocks = no
   kernel share modes = no
   posix locking = no
   vfs objects = catia fruit streams_xattr
#default is yes, not necessary to specify   ea support = yes
   browseable = no
   read only = no
   inherit acls = yes
   fruit:time machine = yes
   fruit:metadata = stream
   fruit:locking = netatalk
   fruit:time machine max size = 1.9T

**EDIT: Turns out Samba 4.9 on Debian 10 DOES automatically advertise TimeMachine destinations, as long as there are no Samba-related advertisements in /etc/avahi/services. My system wasn't advertising the TimeMachine share because I also had a manual smb share setup for avahi. When I disabled BOTH, Samba properly advertised both.

*** OLD SEE ABOVE Also note that The Samba build with Debian 10 currently does not have built-in support for advertising Time Machine shares. Normally specifying fruit:time machine = yes will trigger Samba to advertise a Time Machine destination via Avahi, but this specific support is not built with the Samba included in Debian 10. You'll need to configure a .service definition in /etc/avahi/services/. I created time_machine_advert.service in that folder. You'll need to restart avahi to be sure it gets applied with sudo systemctl restart avahi. I have two Time Machine destinations. Multiples can be advertised within a single file by simply duplicating the txt record and incrementing dkX, i.e. dk1, dk2 etc.

<?xml version="1.0" standalone='no'?><!--*-nxml-*-->
<!DOCTYPE service-group SYSTEM "avahi-service.dtd">
<service-group>
 <name replace-wildcards="yes">%h</name>
 <service>
   <type>_adisk._tcp</type>
   <txt-record>sys=waMa=0,adVF=0x100</txt-record>
   <txt-record>dk0=adVN=SAMBA SHARE NAME EXACTLY,adVF=0x82</txt-record>
   <txt-record>dk1=adVN=SAMBA SHARE NAME #2 EXACTLY,adVF=0x82</txt-record>
 </service>
  <service>
    <type>_smb._tcp</type>
    <port>445</port>
  </service>
</service-group>

There are some additional useful sites for Samba configuration with regards to Time Machine here:

Samba team's official guide to Time Machine configuration, although this alone didn't get a working config for me: https://wiki.samba.org/index.php/Configure_Samba_to_Work_Better_with_Mac_OS_X

smb.conf reference: https://www.samba.org/samba/docs/current/man-html/smb.conf.5.html

vfs_fruit (the Apple compatibility module for Samba) reference https://www.samba.org/samba/docs/current/man-html/vfs_fruit.8.html

Ken Murphy's smb.conf on GitHub that finally pushed me over the edge and got my setup working: https://github.com/KenMurphy/SambaConfigs/blob/master/smb.conf

A general Samba on Debian guide: https://www.antoneliasson.se/journal/time-machine-compatible-samba-on-debian-buster/