Handling Verification Failures of TSIG-Signed DNS Messages

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Table of Contents

1. Introduction

Transaction Signatures are specified in [RFC8945] and provides a mechanism for transaction-level authentication of DNS messages using shared secrets and one-way hash functions. It can be used to authenticate messages exchanged between DNS clients and servers, e.g. in cases where message integrity and authorisation are important, such as DNS Update [RFC2136] and the DNS Zone Transfer Protocol [RFC5936].¶

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

This document assumes familiarity with terminology specific to the Domain Name System (DNS) as described in [RFC9499].¶

3. Processing of Signed Responses

The processing of a signed response by a DNS client is described in section 5.4 of [RFC8945], which includes the following in the case of signed messages that fail verification:¶

• Regardless of the RCODE, a message containing a TSIG RR that is unsigned as specified in Section 5.3.2 or that fails verification SHOULD NOT be considered an acceptable response, as it may have been spoofed or manipulated. Instead, the client SHOULD log an error and continue to wait for a signed response until the request times out.¶

There are basically two scenarios how such a message can be received by client. Either the server is misconfigured or the client is under attack.¶

If we assume that such a response has been sent by misconfigured server, e.g. server that doesn't have the correct TSIG keys or isn't configured to use TSIG keys to sign messages to this particular client then no amount of waiting will allow the DNS communication to recover as the server will never send a correct message back.¶

If we assume that such a response is malicious, then the possible attacks can be categorised as on-path or off-path.¶

In the case of an off-path attack, the specified behaviour is not ideal since it provides an increased window of opportunity for an attacker that has correctly guessed parameters such as source port and query id to continue sending responses with different signatures. In the case of an on-path attack, there is even less value in waiting for a valid response, since the attacker can be assumed to have full control over what messages are and are not allowed to reach the client, and once again waiting only provides the attacker with more opportunity to conduct their attack.¶

This document updates the specification in this case to clarify that clients that receive responses that fit the description quoted above SHOULD log an error and MUST treat the message as corrupt, MUST discard it immediately and MUST NOT continue to wait.¶

4. Security Considerations

This document updates [RFC8945] and provides new guidance in order to mitigate on-path and off-path attacks on signed DNS responses.¶

5. IANA Considerations

This document has no IANA actions.¶

6. References

Acknowledgments

TODO acknowledge.¶

Authors' Addresses