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.