Three years ago I ran into a problem with a Synology DSM update preventing vintage SMB clients from connecting to my NAS. Now I ran into a similar but different problem.
The symptom was DOS and (at least old) OS/2 clients refusing to connect to my NAS, like this:
C:\>net view \\mynas
Error 58: The network has responded incorrectly
C:\>dir n:\
Incorrect response from network
Abort, Retry, Fail?
I had updated DSM not long ago, but I suspected that that hadn’t been the trigger. More likely this happened because I got tired of fresh Windows 10 installs complaining that they don’t want to talk to SMB1, and changed the DSM settings to support everything from SMB1 to SMB3. But DOS uses SMB1, right? Well…
While narrowing the problem down I determined that Windows XP had no trouble, NT 4 had no trouble, and even NT 3.1 still could connect to my NAS. Windows 98 could also connect. But DOS with the latest Microsoft client could not, and OS/2 with LAN Manager 2.2 couldn’t either.
Going back to the old article made me look at /etc/samba/smb.conf and the problem was more or less obvious:
min protocol=NT1
Hmm, that looks like a problem. So what are the other possible protocol values? A quick trip to the documentation had the answer: LANMAN1. After editing /etc/samba/smb.conf and changing the line min protocol=NT1 to read instead min protocol=LANMAN1, and then running sudo smbcontrol smbd reload-config to apply the modified configuration, lo and behold—the DOS client is working again!
If I change the supported SMB protocols through the DSM user interface, the vintage client connectivity will likely break again, but at least I know what to do now.