Time Machine backup failure to Linux/Samba target - Missing kown server marker
One of my Macs refused to start Time Machine backups to a Linux/Samba target, even though the same SMB share opened manually without issues, the same credentials worked from other Macs, and manual SMB mounting worked fine from the affected machine. Time Machine threw an “invalid username/password” error. That was misleading - the actual problem was macOS not recognizing the SMB host as a known server in the NetAuthSysAgent / backupd system context.
The interactive mount worked fine:
NAOpenSessionAsync reports sessionStatus: 0
NAMountAsync reports mountStatus: 0
Then backupd tried to run the backup and things fell apart:
NAConnectToServerSync failed with error: 80
Authentication error (80)
BACKUP_FAILED_AUTHENTICATION_ERROR (29)
NetAuthSysAgent
GetKnownServerMarker: enter for host "192.168.88.99"
isKnownServer 0
There are no user credentials to add to the MechType session
OpenSession failed 80
Solution
Add the server marker manually to the root server markers plist:
PL='/private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist'
sudo cp "$PL" "$PL.bak.$(date +%Y%m%d-%H%M%S)"
sudo /usr/libexec/PlistBuddy -c 'Print' "$PL"
sudo /usr/libexec/PlistBuddy -c 'Add :"192.168.88.99" bool true' "$PL" 2>/dev/null || \
sudo /usr/libexec/PlistBuddy -c 'Set :"192.168.88.99" true' "$PL"
sudo plutil -p "$PL"
Then remove and re-add the Time Machine destination. The old one had stale state from the failed authentication attempts — clean registration fixed it.
Key commands
Stream Time Machine and backup logs:
log stream --info --predicate 'process == "backupd" OR process == "backupd-helper" OR process == "smbd" OR subsystem CONTAINS "TimeMachine"'
For NetAuthSysAgent issues:
log show --last 10m --style compact --predicate 'process == "NetAuthSysAgent" OR process == "backupd" OR eventMessage CONTAINS[c] "NetAuth"'
Inspect the server marker plist:
sudo plutil -p '/private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist'
This wasn’t a Samba password problem. backupd / NetAuthSysAgent in system context simply didn’t recognize the SMB host as known (isKnownServer 0), so it withheld the saved credentials during automatic mount. Adding the host to the root server markers plist and re-registering the Time Machine destination resolved it.