Category Archives: Computer Networks

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!

Cheers.

Windows computer fails to resolve their names and browse other stations on the LAN

If one morning you find yourself surprised by the fact that you can browse the Internet just fine but your Windows Explorer fails to show you all the other computers on the local network (never mind their shares), read further.
So, you have been assigned (your machine) an IP address, broadcast address, gateway and DNS addresses and everything looks apparently fine. However, if you click on a mapped drive or attempt to type \\computer_name in the Address Bar in Windows Explorer an almost instantaneously pop-up message is displayed saying: “… the network mapped drive has not been restored …” or path/computer not found (those are not the exact error messages, sorry … but you get the picture). Basically you’re not allowed to access those computers by their NetBIOS name (access by IP address works).
Being able to browse the Internet means the DNS resolution was working fine but not for LAN computers. As we don’t have a domain on our network we don’t even need it so it doesn’t actually have to work. The computer names should be resolved by NBNS requests (NetBIOS Name Service). What I could tell clearly by capturing some network traffic with Packetizer is that my station was broadcasting DNS requests but not one NBSN request. No NBNS resolution means no IP address, which means no MAC address, which means you can’t address that computer.
So, why would no NBNS requests be issued by a host. I searched on google: “win xp fails to query NBNS” and found this:
NBNS Name query failure – Why?
Here, another fellow found out that the type of query which is attempted to resolve a name on the network is driven by the Node Type of the host. The Node Type is stored in the registry in either of the following values: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Netbt\Parameters\DhcpNodetype or
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Netbt\Parameters\Nodetype.
Now, even though the poster recommends deleting those values and then reboot, that is really not necessary. What you need to do is to set the node type to the value that suits your situation best and the attempt a Repair on the network adapter.
Our hosts can’t register themselves to the DNS running on the wireless router we use as a gateway so we don’t need DNS resolution for the internal machines. As a result I set the node type to 4 (see the Node Type link for an explanation) which attempts a NBNS broadcast query which if successful avoids another useless DNS request.
Thanks goes to the people who found the time to post about this issue and helped me found a solution for my problem.
Happy New Year!
References:
Can’t access remote computer because the Node type is setup Peer-Peer
http://www.chicagotech.net/Networking/nodetypeissue1.htm

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