MacOS 26.4 has broken TimeMachine backups to NAS.

Justin Creveling wrote:

for anyone using TrueNAS Scale: update to at least 25.04. after editing plists and removing destinations and verifying passwords and verifying encryption keys and verifying i can connect to the server in finder etc. etc. etc., the only thing that i needed to do was update truenas to 25.04 from 24.10.

what complicated things for me and had me chasing my tail was that i moved my backup sparsebundle to a different truenas server, so i was also chasing red herrings with the file and permissions etc., in the end i had everything fine and the issue was the smb handshake between the new macos version of smb and the truenas version.

There is a Samba bug in this realm, fixed with Samba 4.22.7, though maybe not fixed in 4.23.* per the following:

https://bugzilla.samba.org/show_bug.cgi?id=15926

"I believe this is only fixed in the 4.22.x branch; it's still an issue with the latest 4.23 releases, including 4.23.5."

I have a UNAS PRO 4 that stopped backing up. My solution is as follows:

I just resolved this by deleting my backup instance in Time Machine Settings and then trashing this plist: /Library/Preferences/com.apple.TimeMachine.plist

I saw references to a machine that I no longer own and migrated from last year in that plist. The system recreated a fresh, clean file, and I was able to continue to back up my system by creating a new backup in TM Settings.

for anyone using TrueNAS Scale: update to at least 25.04. after editing plists and removing destinations and verifying passwords and verifying encryption keys and verifying i can connect to the server in finder etc. etc. etc., the only thing that i needed to do was update truenas to 25.04 from 24.10.

what complicated things for me and had me chasing my tail was that i moved my backup sparsebundle to a different truenas server, so i was also chasing red herrings with the file and permissions etc., in the end i had everything fine and the issue was the smb handshake between the new macos version of smb and the truenas version.

Hi!

Try this, you have to replace nas.localdomain with your local NAS name like synology.local or nas.local....:

sudo /usr/libexec/PlistBuddy -c 'Add :nas.localdomain bool true' '/private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist'

After this, backups should run again...

Try my given Advice!

It will work!

You have to replace nas.localdomain with your local NAS name like synology.local or nas.local in the following command. All in small letters!

Run it in Terminal.

sudo /usr/libexec/PlistBuddy -c 'Add :nas.localdomain bool true' '/private/var/root/Library/Group Containers/group.com.apple.NetworkAuthorization.ServerMarkers/serverMarkers.plist'

I have the same issue with backups on a Synology NAS. It was working fine in 26.3 and stopped working altogether, in 26.4.

• I checked and the connection is properly establishing in SMB3.
• I made sure there wasn't any non ASCI character anywhere (drive name, username, etc).
• Running this command while trying to backup:

log stream --level debug --predicate 'subsystem == "com.apple.TimeMachine"' --style compact &> ~/Desktop/tm_debug.log

confirms an authentication error but only as the Mac is attempting to back data up (everything is fine to add the destination, or if I attempt to connect through the Finder).

• I changed the username and password to make sure I didn't have any special character (no space, no $% or &).
• I wiped out the system and login keychains (which wasn't fun) to rule out any keychain corruption, password or certificate issue.
• I wiped out all Network-related preferences from /Library/Preferences
• I reset all SMB related configuration files.
• I reset my list of known hosts (which shouldn't affect SMB all that much, but just in case…)
• I attempted to connect to the drive either through Bonjour or directly by IP.
• I connected straight to the NAS by direct ethernet connection, bypassing my switch and router.

Nothing did it! It just keeps on failing to authenticate through Time Machine backups.

@Broeseli it did work (I ran the command and rebooted).

If I summarize and simplify what I read on other sites, the 26.4 update triggered a pre-existing bug in older versions of samba.

backupd, the Time Machine process runs as root, and could not see the credentials stored in the keychain. As a workaround, this command adds the server to the ServerMarker list to restores proper authentication.

I reported the issue to Synology though. They only run Samba 4.15.13 as of today and my understanding is that it's only addressed in a much older version

Nicholas Sayer wrote:

Now, I am really, really, REALLY, REALLY, sure that both the password to connect to the NAS and the password to unlock the (encrypted) volume are correct. I have deleted and recreated the TM connection in the TM control panel approximately 39,241 times and it doesn't help. And this same thing happens on 5 different macs connecting to two different Synology NASs all running the latest DSM.

Have you verified it by successfully mounting the volume on your Mac? Can you access the TM backups using the TM browser, or does that fail, too?

DSM configured as recommended by Synology? They modified that kbase article with some Tahoe-specific changes.

Had the issue when tried to back-up to a disk connected to my Raspberry Pi running samba 2:4.17.12, the fix via serverMarkers.plist worked for me. Thanks @Broeseli!

Updated to 26.4 this afternoon. Last backup to my Synology DS223j completed less than 10 minutes ago. No problems, business as usual.

LLMs are usually GIGO (garbage in garbage out) for this sort of thing, they scrape forums like this one and find wrong solutions occasionally mixed with correct ones.

Perhaps you might have more success posting the error(s) you are seeing and the troubleshooting you've done. Have you rebooted your Mac? Can you mount the volume used for the TM backup?

group.com.apple.NetworkAuthorization.ServerMarkers (and therefore) serverMarkers.plist do not exist on my Mac…

I'm not aware of what they are supposed to do and how they function (and there isn't much out there if you Google them up).

Did you have to create them yourself or were they already there on your system?

As a side note, nas.localdomain needs to be replaced by whatever is used on your LAN (like MyNAS.local or whatever).