Some Windows 10 users have issues with DNS resolution after installing the latest cumulative Windows 10 updates released this week.
As part of this week’s patch, April 2021, Microsoft released the cumulative Windows 10 KB5001330 and KB5001337 updates to fix various security vulnerabilities and errors found in the operating system.
Since installing updates, some corporate users have reported having DNS resolution issues that prevent access to shared folders on servers when trying to access them by server name. If they try to access the shared folder via the server’s IP address, users can successfully connect to the shared folders.
To access shared folders again, administrators had to uninstall the cumulative update, which is not a good solution because it removes security fixes.
After users started reporting the issue on the BleepingComputer forum, users found that many affected users use Dentrix Dental Practice Management software in their offices.
A long-standing recommendation for Dentrix customers to reduce network delays was to disable multi-case name resolution (LLMNR) on Windows workstations. This is done by activating the group policy “Disable multicast name resolution” Computer configuration > Administrative templates > Network > DNS client.
However, since installing the latest cumulative updates, this policy prevents users from accessing shared server folders.
Dentrix acknowledged the problem and said that after coordinating with Microsoft, disabling this group policy is the recommended way to resolve this issue.
“After working with Microsoft, we believe we’ve found a solution that allows you to keep your Microsoft security critical update installed. It seems that this problem can be solved by activating “Link Layer Multi-cast Name Resolution” (LLMNR). is a Windows Group Policy that many users have disabled over the years. Reactivation seems to restore the correct resolution of the IP address, which in turn restores connectivity to the Dentrix database. To activate LLMNR, follow the steps below. “
To disable the policy (enable LLMNR) and fix DNS resolution issues, the recommended steps are:
- Go to Start> Run and type GPEdit.msc
- This should open the Local Group Policy Editor.
- Use the Group Policy Editor to navigate to Local computer policy> Computer setup> Administrative template> Network> DNS client.
- Double click “Disable Multicast name resolution. ”
- Check the radio button labeled “Off”.
- Click apply and K.
- Open a Windows command window (Start> Cmd.exe) and enter the command ipconfig / flushdns .
Dentrix customers do not see this as a suitable solution, as LLMNR may cause delays in using the software. Instead, it recommends that users add the name and IP address of the affected server to the HOSTS file, which appears to resolve the issue.
“Enabling Multicast is definitely not an option for Dentrix due to the slowdown.
Microsoft should launch a fix as soon as possible.
The best resolution I have found so far is to fix the host file, the DNS service fix has not fixed any of them yet, but was reported by someone else as a solution. “- a Dentrix customer posted on our forums.
This error is not only related to Dentrix users, but to anyone with LLMNR disabled, who, for the most part, were Dentrix customers who were told to disable it.
Although it is not known what caused this error, Microsoft has fixed two DNS vulnerabilities [1, 2] this month, which could have contributed to this abnormal behavior.
BleepingComputer contacted Microsoft about this error, but did not hear.