What does Windows error code 9551 (DNS_ERROR_INVALID_TYPE) mean?

DNS_ERROR_INVALID_TYPE

Validate the RR type before querying

DNS_ERROR_INVALID_TYPE means the DNS type supplied to the Windows API is not acceptable for that operation. DNS record types are numeric protocol identifiers, not arbitrary application enums. A valid type can legitimately return no records; this status instead points to the request construction, conversion, or API contract.

Common mistakes include passing a string token where a numeric DNS_TYPE_* value is required, truncating or sign-extending an application value, confusing address family with RR type, or forwarding a private sentinel such as “ANY/default” into an API that does not support it in that position.

What to inspect

• Log the exact numeric type in decimal and hexadecimal before the call.
• Map user-facing names such as A, AAAA, SRV, TXT, PTR, and CAA through a fixed validated table.
• Check the IANA registry and the Windows header for the intended type; do not assume every newer type is supported by every API path.
• Keep invalid type separate from DNS_INFO_NO_RECORDS, which is a valid query with an empty matching RRset.

References

• IANA: Resource Record TYPE values
• Microsoft: DNS constants
• Microsoft: DnsQuery_W