Windows error 9556 (DNS_ERROR_NON_RFC_NAME): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: The DNS name uses syntax Windows may accept but other DNS implementations might reject.

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DNS_ERROR_NOT_UNIQUE	DNS_STATUS_FQDN

DNS_ERROR_NON_RFC_NAME

A portability warning rather than every time a fatal error

DnsValidateName can return DNS_ERROR_NON_RFC_NAME for extended or Unicode characters and for underscores outside the permitted SRV-name form. Microsoft documents this result as a warning: a suitably configured Windows DNS server can accept the name, while other DNS server software may not. It is therefore different from DNS_ERROR_INVALID_NAME_CHAR, which identifies characters Windows treats as invalid for the selected format.

The supplied DNS_NAME_FORMAT is part of the validation contract. A service record, wildcard, hostname label, domain label, and complete domain name have different permitted syntax. Internationalized names also require a deliberate Unicode/IDNA strategy rather than byte-by-byte ANSI validation.

What to inspect

Record the validation format and the Unicode code points in the original name.
Use the SRV format only for the underscore-prefixed labels defined by the service-discovery convention.
Convert user-facing internationalized names with a standards-compliant IDNA implementation before interoperating with systems that expect A-labels.
Test the authoritative and recursive servers actually used in production before accepting a Windows-only spelling.

References

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