Unable to access computer B on a windows network from computer A.

\\myNAS is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions.Access is denied

I ran into this nightmare of a problem last Friday. Honestly I don’t know what happened, but all of the sudden, from the workstation I cannot access anymore my Synology NAS. I mean by name or fully qualified domain name. By IP address it was working fine but you don’t want that approach to resolve the problem unless you are desperate.

It took Saturday, Sunday and Monday on and off grinding at the issue to finally realize what the problem was. I have to mention that from other machines I could access the NAS just fine. More, I could access other hosts on my network from my workstation with no problems. This was strictly an issue between my station and my NAS.

After numerous Wireshark traces and struggling to discern which ones are the records in those traces that represent the failed attempts to access my NAS, I started to believe more and more that this record was the one pointing out the problem: tree connect andx response error status_access_denied

A simple search on google takes me to this blog entry. Now even though it is only similar to my problem it made me remember a Kerberos error that I have seen in the Wireshark traces, in a reply from my DC: KRB5 KRB Error: KRB5KDC_ERR_C_PRINCIPAL_UNKNOWN . I should mention that my NAS is part of a domain and it uses AD authentication.

Naturally as the next step, I went automatically searching for the request related to the above reply: KRB5 AS-REQ. So, I’m expanding the NetBIOS section of the request in Wireshark, to see what is the name of the principal that my DC does not recognize. I was happy to see this:  Client Name (Principal): admin Realm: MYDOMAIN. I know for sure I don’t have such a user in MYDOMAIN so where is this coming from and why is it trying to authenticate using this specific account?

Thanks to my memory who’s still in decent shape I remember while analysing the Event Logs, seeing a very isolated warning in the System event log that was happening during the Windows booting procedure. This is the warning’s message:

The password stored in Credential Manager is invalid. This might be caused by the user changing the password from this computer or a different computer. To resolve this error, open Credential Manager in Control Panel, and reenter the password for the credential MYDOMAIN\admin.

This particular error was a confirmation that somewhere, Windows is storing the wrong credentials that are to be used automatically when accesing my NAS.

What do you want more clearer than that? Kudos to whomever was thoghtfull enough to put that error message. I immediatelly opened Control Panel, typed Credential Manager in the Search box. Clicked on Credential Manager link that showed up. It took me two seconds to spot the entry for my NAS’ name in the Windows Credentials section. I deleted the entry, went back to Windows explorer and tried both  \\myNAS and \\myNAS.mydomain.local and they worked!