Windows Server 2016 fails to find domain controller

The May 2026 security update for Windows Server 2016 causes domain controller lookups to fail on systems whose hostname is exactly 15 characters. During DC lookup, a computer searches for an available domain controller to connect to. The failure occurs only at the 15-character length and does not affect hostnames with 14 or 16 characters. DCLocator calls return ERROR_INVALID_PARAMETER, which prevents applications and administrative tools from locating a domain controller. Administrative tasks that depend on DC access can also be impacted, including DFS Namespace management. Microsoft is investigating the cause and has not provided a fix or timeline.

"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."

"On systems with a hostname of exactly 15 characters, domain controller (DC) lookups fail after installing the update. A fix or timeline has not yet been provided. During DC lookups, a computer searches for an available DC to connect to. The error occurs only on servers whose hostname is exactly 15 characters long."

"Administrators with an affected system may therefore also encounter issues with DFS Namespace management and other administrative tasks that depend on DC access. Microsoft states that it is investigating the cause but has not yet announced a timeline for a fix."

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