What does Windows error code 9915 (DNS_ERROR_RRL_INVALID_TC_RATE) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: The rate for sending truncated replies under RRL is outside the accepted range.

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DNS_ERROR_RRL_INVALID_IPV6_PREFIX	DNS_ERROR_RRL_INVALID_LEAK_RATE

DNS_ERROR_RRL_INVALID_TC_RATE

TC rate is Windows' truncated-reply cadence

DNS_ERROR_RRL_INVALID_TC_RATE is returned when Windows validates the RRL truncation-rate field, named dwTCRate in the DNS Server protocol and exposed as TruncateRate by PowerShell. For queries that RRL would otherwise drop, Windows can send a truncated DNS response periodically. The PowerShell documentation describes this as one truncated reply per configured number of affected queries; the truncated reply gives a valid client an opportunity to retry over TCP.

The DNS TC flag is a protocol signal, not an application error text. DNS standards require support for TCP so that clients can retry truncated UDP responses when necessary. RRL uses the same established transport signal as a mitigation option; it does not change the zone's answer data or make a reply permanently unavailable.

Validate the intended setting

Check the current TruncateRate and the value passed by automation. The current PowerShell documentation allows values from 2 through 10, while zero disables truncated-response behaviour.
The short Win32 message says the TC rate must be below 10. When documentation and a short error string appear to differ at an endpoint, rely on the server's validation and the current cmdlet documentation rather than trying to infer an off-by-one rule.
Do not confuse TruncateRate with LeakRate. A truncation tells the client to retry with TCP; a leak is an occasional response that would otherwise have been dropped.
Verify that the DNS service and network path accept TCP port 53 before relying on truncated replies as a client recovery path.

References

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