DNS 4 NOTIMP vs 5 REFUSED
Both DNS 4 (NOTIMP) and 5 (REFUSED) belong to the DNS Response Codes (RCODEs) category. 4 indicates that not Implemented. The name server does not support the requested query type or operation. Meanwhile, 5 means that query refused. The name server refuses to perform the requested operation for policy reasons.
Description
Not Implemented. The name server does not support the requested query type or operation.
When You See It
You sent a query type (e.g., IXFR, dynamic update) that the server does not support or has disabled.
How to Fix
Verify the server supports the query type you are using. If you need zone transfers or dynamic updates, enable them in the server configuration.
Description
Query refused. The name server refuses to perform the requested operation for policy reasons.
When You See It
The server rejected your query due to access control — for example, a recursive resolver that only serves its own network, or a zone transfer blocked by ACL.
How to Fix
Check the server's allow-query, allow-recursion, or allow-transfer ACLs. If you are not authorized to use this resolver, switch to a public DNS service.
Key Differences
DNS 4: Not Implemented. The name server does not support the requested query type or operation.
DNS 5: Query refused. The name server refuses to perform the requested operation for policy reasons.
You encounter 4 when you sent a query type (e.g., IXFR, dynamic update) that the server does not support or has disabled.
You encounter 5 when the server rejected your query due to access control — for example, a recursive resolver that only serves its own network, or a zone transfer blocked by ACL.
When to Use Which
For 4 (NOTIMP): Verify the server supports the query type you are using. If you need zone transfers or dynamic updates, enable them in the server configuration. For 5 (REFUSED): Check the server's allow-query, allow-recursion, or allow-transfer ACLs. If you are not authorized to use this resolver, switch to a public DNS service.