Redefinition of DNS Authenticated Data (AD) bit
Status of this Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Copyright © The Internet Society (2003). All Rights Reserved.
This document alters the specification defined in RFC 2535. Based on implementation experience, the Authenticated Data (AD) bit in the DNS header is not useful. This document redefines the AD bit such that it is only set if all answers or records proving that no answers exist in the response has been cryptographically verified or otherwise meets the server's local security policy.
As specified in RFC 2535 (section 6.1), the AD (Authenticated Data) bit indicates in a response that all data included in the answer and authority sections of the response have been authenticated by the server according to the policies of that server. This is not especially useful in practice, since a conformant server SHOULD never reply with data that failed its security policy.
This document redefines the AD bit such that it is only set if all data in the response has been cryptographically verified or otherwise meets the server's local security policy. Thus, neither a response containing properly delegated insecure data, nor a server configured without DNSSEC keys, will have the AD set. As before, data that failed to verify will not be returned. An application running on a host that has a trust relationship with the server performing the recursive query can now use the value of the AD bit to determine whether the data is secure.
A full DNSSEC capable resolver called directly from an application can return to the application the security status of the RRsets in the answer. However, most applications use a limited stub resolver that relies on an external recursive name server which incorporates a full resolver. The recursive nameserver can use the AD bit in a response to indicate the security status of the data in the answer, and the local resolver can pass this information to the application. The application in this context can be either a human using a DNS tool or a software application.
The AD bit SHOULD be used by the local resolver if and only if it has been explicitly configured to trust the remote resolver. The AD bit SHOULD be ignored when the recursive name server is not trusted.
An alternate solution would be to embed a full DNSSEC resolver into every application, but this has several disadvantages.
- DNSSEC validation is both CPU and network intensive, and caching SHOULD be used whenever possible.
- DNSSEC requires non-trivial configuration - the root key must be configured, as well as keys for any "islands of security" that will exist until DNSSEC is fully deployed. The number of configuration points should be minimized.
1.3. Updated documents and sections
The definition of the AD bit in RFC 2535, Section 6.1, is changed.
2. Setting of AD bit
The presence of the CD (Checking Disabled) bit in a query does not affect the setting of the AD bit in the response. If the CD bit is set, the server will not perform checking, but SHOULD still set the AD bit if the data has already been cryptographically verified or complies with local policy. The AD bit MUST only be set if DNSSEC records have been requested via the DO bit [RFC3225] and relevant SIG records are returned.
2.1. Setting of AD bit by recursive servers
Section 6.1 of RFC 2535 says:
"The AD bit MUST NOT be set on a response unless all of the RRs in the answer and authority sections of the response are either Authenticated or Insecure."
The replacement text reads:
"The AD bit MUST NOT be set on a response unless all of the RRsets in the answer and authority sections of the response are Authenticated."
"The AD bit SHOULD be set if and only if all RRs in the answer section and any relevant negative response RRs in the authority section are Authenticated."
A recursive DNS server following this modified specification will only set the AD bit when it has cryptographically verified the data in the answer.
2.2. Setting of AD bit by authoritative servers
A primary server for a secure zone MAY have the policy of treating authoritative secure zones as Authenticated. Secondary servers MAY have the same policy, but SHOULD NOT consider zone data Authenticated unless the zone was transferred securely and/or the data was verified. An authoritative server MUST only set the AD bit for authoritative answers from a secure zone if it has been explicitly configured to do so. The default for this behavior SHOULD be off.
Note that having the AD bit clear on an authoritative answer is normal and expected behavior.
2.2.1. Justification for setting AD bit w/o verifying data
The setting of the AD bit by authoritative servers affects only the small set of resolvers that are configured to directly query and trust authoritative servers. This only affects servers that function as both recursive and authoritative. Iterative resolvers SHOULD ignore the AD bit.
The cost of verifying all signatures on load by an authoritative server can be high and increases the delay before it can begin answering queries. Verifying signatures at query time is also expensive and could lead to resolvers timing out on many queries after the server reloads zones.
Organizations requiring that all DNS responses contain cryptographically verified data will need to separate the authoritative name server and signature verification functions, since name servers are not required to validate signatures of data for which they are authoritative.
3. Interpretation of the AD bit
A response containing data marked Insecure in the answer or authority section MUST never have the AD bit set. In this case, the resolver SHOULD treat the data as Insecure whether or not SIG records are present.
A resolver MUST NOT blindly trust the AD bit unless it communicates with a recursive nameserver over a secure transport mechanism or using a message authentication such as TSIG [RFC2845] or SIG(0) [RFC2931] and is explicitly configured to trust this recursive name server.
4. Applicability statement
The AD bit is intended to allow the transmission of the indication that a resolver has verified the DNSSEC signatures accompanying the records in the Answer and Authority section. The AD bit MUST only be trusted when the end consumer of the DNS data has confidence that the intermediary resolver setting the AD bit is trustworthy. This can only be accomplished via an out of band mechanism such as:
- Fiat: An organization that can dictate whether it is OK to trust certain DNS servers.
- Personal: Because of a personal relationship or the reputation of a recursive nameserver operator, a DNS consumer can decide to trust that recursive nameserver.
- Knowledge: If a recursive nameserver operator posts the configured policy of a recursive nameserver, a consumer can decide that recursive nameserver is trustworthy.
In the absence of one or more of these factors AD bit from a recursive name server SHOULD NOT be trusted. For example, home users frequently depend on their ISP to provide recursive DNS service; it is not advisable to trust these recursive nameservers. A roaming/traveling host SHOULD not use recursive DNS servers offered by DHCP when looking up information where security status matters.
In the latter two cases, the end consumer must also completely trust the path to the trusted recursive name servers, or a secure transport must be employed to protect the traffic.
When faced with a situation where there are no satisfactory recursive nameservers available, running one locally is RECOMMENDED. This has the advantage that it can be trusted, and the AD bit can still be used to allow applications to use stub resolvers.
5. Security Considerations
This document redefines a bit in the DNS header. If a resolver trusts the value of the AD bit, it must be sure that the responder is using the updated definition, which is any DNS server/resolver supporting the DO bit [RFC3225].
Authoritative servers can be explicitly configured to set the AD bit on answers without doing cryptographic checks. This behavior MUST be off by default. The only affected resolvers are those that directly query and trust the authoritative server, and this functionality SHOULD only be used on servers that act both as authoritative and recursive name servers.
Resolvers (full or stub) that blindly trust the AD bit without knowing the security policy of the server generating the answer can not be considered security aware.
A resolver MUST NOT blindly trust the AD bit unless it communicates such as IPsec, or using message authentication such as TSIG [RFC2845] or SIG(0) [RFC2931]. In addition, the resolver must have been explicitly configured to trust this recursive name server.
6. IANA Considerations
7. Internationalization Considerations
None. This document does not change any textual data in any protocol.
8. Intellectual Property Rights Notice
The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director.
The following people have provided input on this document: Robert Elz, Andreas Gustafsson, Bob Halley, Steven Jacob, Erik Nordmark, Edward Lewis, Jakob Schlyter, Roy Arends, Ted Lindgreen.
10. Normative References
[RFC1035] Mockapetris, P., "Domain Names - Implementation and Specification", STD 13, RFC 1035, November 1987. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2535] Eastlake, D., "Domain Name System Security Extensions", RFC 2535, March 1999.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D. and B.
Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", RFC 2845, May 2000.
11. Authors' Addresses
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Chevy Chase, MD, 20815
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