Microsoft tests the 15-character limit of Windows Server admins' patience

OSes

May security update trips over hostnames of a very specific length

Windows Server 2016 might be long in the tooth but that isn't about to stop Microsoft breaking stuff.

The May 12 security update introduced another bug for administrators to worry about. According to Microsoft, if the server hostname is exactly 15 characters long (like, for example, THEY-NEVER-TEST), domain controller discovery might fail.

In the notes for the glitch, Microsoft wrote: "When the hostname is 15 characters long, DCLocator calls (for example, using nltest /dsgetdc:<domain> /pdc) will return ERROR_INVALID_PARAMETER, preventing applications and administrative tools from locating a domain controller."

In other words, anything that depends on a domain controller lookup might stop working. As an example, Microsoft gave Distributed File System (DFS) Namespace management, which would certainly be inconvenient. DFS Namespaces is a Windows Server role that allows admins to group shared folders across different servers into a single namespace. A single path can lead to files located on multiple servers. Unless, of course, the domain controller lookup is broken.

Microsoft lists no workaround for affected users, though changing the server hostname to something other than 15 characters would presumably avoid the trigger. "The issue is under investigation, and additional information will be shared as soon as it becomes available," it said.

Microsoft still officially supports Windows Server 2016. Mainstream support ended in 2022, but extended support will continue until January 12, 2027. Microsoft is offering up to three more years of support via the Extended Security Updates (ESU) program after that.

Earlier this year, Esben Dochy of Lansweeper told The Register that the operating system accounted for just 2.2 percent of all Windows devices it tracks, but 20.3 percent of all servers. That figure is unlikely to have dropped dramatically in the months since, so there is a fair chance that an administrator with a 15-character hostname could be affected.

In addition to the Windows Server 2016 problems, the May 2026 security update has failed during installation on some Windows 11 devices when the EFI System Partition is insufficient in size.

It is reassuring to know Microsoft's talent for breakage shows no bias toward any particular vintage.   ®