[Help]: Time Machine backup fails with authentication error even though SMB mount/write works

[julianrios]

Describe Your Issue or Question

Describe Your Issue or Question

I’m trying to use mbentley/timemachine on Unraid as a Time Machine destination for macOS.

The container starts successfully, advertises the TimeMachine share, and I can manually mount the SMB share from macOS using the same credentials. I can also create and delete files on the share manually.

However, Time Machine fails immediately. tmutil startbackup exits with Total copied: 0.00 MB, and no .sparsebundle is created.

The macOS backup logs show:

NAConnectToServerSync failed with error: 80 (Authentication error) for alternate url: smb://timemachine@172.16.0.2/TimeMachine
Authentication error (80) - the correct user or password info may not exist in the System.keychain or the server may no longer allow access for this user.
Backup failed: BACKUP_FAILED_AUTHENTICATION_ERROR (29)

Expected Behavior

Time Machine should mount the SMB share, create a sparsebundle, and start the initial backup.

Steps to Reproduce

1. Run the container on Unraid using a custom Docker network br0.
2. Mount backup storage directly from /mnt/disk1/timemachine to avoid Unraid FUSE /mnt/user.
3. Add the destination on macOS using:

sudo tmutil setdestination -p "smb://timemachine@172.16.0.2/TimeMachine"

4. Start a backup:

tmutil startbackup --block --auto

5. Backup immediately exits with 0 bytes copied.

How You're Launching the Container

docker run -d \
  --name='TimeMachine' \
  --net='br0' \
  --pids-limit 2048 \
  -e TZ="America/New_York" \
  -e HOST_OS="Unraid" \
  -e HOST_HOSTNAME="void" \
  -e HOST_CONTAINERNAME="TimeMachine" \
  -e 'VOLUME_SIZE_LIMIT'='500G' \
  -e 'TM_USERNAME'='timemachine' \
  -e 'PASSWORD'='REDACTED' \
  -e 'ADVERTISED_HOSTNAME'='timemachine' \
  -e 'CUSTOM_SMB_CONF'='false' \
  -e 'CUSTOM_USER'='false' \
  -e 'DEBUG_LEVEL'='1' \
  -e 'MIMIC_MODEL'='TimeCapsule8,119' \
  -e 'EXTERNAL_CONF'='' \
  -e 'HIDE_SHARES'='no' \
  -e 'TM_GROUPNAME'='timemachine' \
  -e 'TM_UID'='1000' \
  -e 'SET_PERMISSIONS'='false' \
  -e 'SMB_INHERIT_PERMISSIONS'='no' \
  -e 'SMB_NFS_ACES'='no' \
  -e 'SMB_METADATA'='stream' \
  -e 'SMB_PORT'='445' \
  -e 'SMB_VFS_OBJECTS'='fruit streams_xattr' \
  -e 'WORKGROUP'='WORKGROUP' \
  -e 'TM_GID'='1000' \
  -e 'SHARE_NAME'='TimeMachine' \
  -v '/mnt/disk1/timemachine/':'/opt/timemachine':'rw' \
  --hostname timemachine \
  'mbentley/timemachine'

Container Logs

INFO: running test for xattr support on your time machine persistent storage location...
INFO: xattr test successful - your persistent data store supports xattrs
INFO: Detected filesystem for /opt/timemachine is xfs
INFO: entrypoint complete; executing 's6-svscan /etc/s6'
dbus-daemon[42]: [system] org.freedesktop.DBus.Error.AccessDenied: Failed to set fd limit to 65536: Operation not permitted
nmbd version 4.22.8 started.
smbd version 4.22.8 started.
Server startup complete. Host name is timemachine.local.
Service "timemachine" (/etc/avahi/services/smbd.service) successfully established.

Time Machine client logs

NAConnectToServerSync failed with error: 80 (Authentication error) for alternate url: smb://timemachine@172.16.0.2/TimeMachine
Authentication error (80) - the correct user or password info may not exist in the System.keychain or the server may no longer allow access for this user.
Backup failed: BACKUP_FAILED_AUTHENTICATION_ERROR (29)

Time Machine client diagnostics logs using The Time Machine Mechanic

No response

Additional host information

No response

Additional Context

Manual SMB mount from the Mac works with the same credentials:

mkdir -p ~/tmtest
mount_smbfs "//timemachine:REDACTED@172.16.0.2/TimeMachine" ~/tmtest
touch ~/tmtest/testfile
ls -la ~/tmtest
rm ~/tmtest/testfile
umount ~/tmtest/

The write test succeeds.

I also tried deleting the Time Machine destination and clearing the System keychain entry:

sudo tmutil removedestination <destination-id>
sudo security delete-internet-password -s 172.16.0.2 /Library/Keychains/System.keychain
sudo tmutil setdestination -p "smb://timemachine@172.16.0.2/TimeMachine"

Still fails with the same Time Machine auth error.

Host is Unraid. The Docker path was originally /mnt/user/timemachine, which showed fuse.shfs, but I changed it to /mnt/disk1/timemachine, and the container now detects xfs.

[mbentley]

Hmm, it might be worth bumping DEBUG_LEVEL up to 3 to see if we get any more details in the smbd logs as to why it's failing.

Then when you try again, see if you can get anything useful using log show --predicate 'subsystem == "com.apple.TimeMachine"' --last 5m --info --debug.

[julianrios]

Thanks for helping out with this I decided to go with another approach for backups appreciate your insight. I'm okay to close this issue out.