| Network Working Group | J. Abley |
| Internet-Draft | Afilias |
| Intended status: Best Current Practice | D. Knight |
| Expires: September 20, 2018 | Neustar |
| March 19, 2018 |
Establishing an Appropriate Root Zone DNSSEC Trust Anchor at Startup
draft-jabley-dnsop-bootstrap-validator-00
Domain Name System Security Extensions (DNSSEC) allow cryptographic signatures to be used to validate responses received from the Domain Name System (DNS). A DNS client which validates such signatures is known as a validator.
The choice of appropriate root zone trust anchor for a validator is expected to vary over time as the corresponding cryptographic keys used in DNSSEC are changed.
This document provides guidance on how validators might determine an appropriate trust anchor for the root zone to use at start-up, or when other mechanisms intended to allow key rollover to be tolerated gracefully are not available.
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The terms Key Signing Key (KSK) and Trust Anchor are used as defined in [RFC4033].
The term Validator is used in this document to mean a Validating Security-Aware Stub Resolver, as defined in [RFC4033].
The Domain Name System (DNS) is described in [RFC1034] and [RFC1035]. DNS Security Extensions (DNSSEC) are described in [RFC4033], [RFC4034] and [RFC4035].
The root zone of the DNS was signed using DNSSEC in July 2011, and many top-level domain registries have since signed their zones, installing secure delegations for them in the root zone. A single trust anchor for the root zone is hence increasingly sufficient for validators.
Validators are deployed in a variety of environments, and there is variation in the amount of system administration that might reasonably be expected to be available. For example, embedded devices might never be administered by a human operator, whereas validators deployed on general-purpose operating systems in enterprise networks might have technical staff available to assist with their configuration.
This document includes descriptions of mechanisms for validator bootstrapping, intended to be sufficient for embedded devices. The implementation of those mechanisms might be automatic in the case of unattended devices, or manual, carried out by a systems administrator, depending on local circumstances.
The choice of appropriate trust anchor for a DNSSEC Validator is expected to vary over time as the corresponding KSK used in the root zone is changed. The DNSSEC Policy and Practice Statement (DPS) for the root zone KSK maintainer [KSK-DPS] specifies that scheduled KSK rollover will be undertaken according to the semantics specified in [RFC5011]. Validators which are able to recognise and accommodate those semantics should need no additional support to be able to maintain an appropriate trust anchor over a root zone KSK rollover event.
The possibility remains, however, that [RFC5011] signalling will not be available to a validator: e.g. certain classes of emergency KSK rollover may require a compromised KSK to be discarded more quickly than [RFC5011] specifies, or a validator might be off-line over the whole key-roll event.
This document provides guidance on how DNSSEC Validators might determine an appropriate set of trust anchors to use at start-up, or when other mechanisms intended to allow key rollover to be tolerated gracefully are not available.
The bootstrapping procedures described in this document are also expected to be useful for a deployed, running validator which is not able to accommodate a KSK roll using [RFC5011] signalling.
A validator that has no valid trust anchor initialises itself as follows.
A validator in its initial state is capable of sending and receiving DNS queries and responses, but is not capable of validating signatures received in responses.
A validator must confirm that its local clock is sufficiently accurate before trust anchors can be established, and before processing of DNSSEC signatures can proceed. Discussion of timing considerations can be found in Section 4.
Once the local clock has been synchronised, a validator may proceed to gather candidate trust anchors for consideration. Discussion of trust anchor retrieval can be found in Section 5.
Once a set of candidate trust anchors has been obtained, a validator attempts to find one trust anchor in the set which is appropriate for use. This process involves verification of cryptographic signatures, and is discussed in Section 6.
The validator now has an accurate trust anchor for the root zone, and is capable of validating signatures on responses from the DNS.
DNSSEC signatures are valid for particular periods of time, as specified by the administrator of the zone containing the signatures. It follows that any validator must maintain an accurate local clock in order to verify that signatures are accurate.
Trust anchors correspond to KSKs in particular zones. Zone administrators may choose to replace KSKs from time to time, e.g. due to a key compromise or local key management policy, and the corresponding appropriate choice in trust anchor will change as KSKs are replaced.
Trust anchors for the root zone in particular are published with intended validity periods, as discussed in Section 5. A validator making use of such trust anchors also requires an accurate local clock in order to avoid configuring a local trust anchor which corresponds to an old key.
Validators should take appropriate steps to ensure that their local clocks are set with sufficient accuracy, and in the case where local clocks are set with reference to external time sources over a network [RFC5905] that the time information received from those sources is authentic.
Candidate trust anchors may be retrieved using several mechanisms. The process of gaining trust in particular candidate trust anchors before using them is discussed in Section 6.
A trust anchor which is packaged with validator software can never be trusted, since the corresponding root zone KSK may have rolled since the software was packaged, and the trust anchor may be derived from a root zone KSK that was retired due to compromise.
Validators should never use local trust anchors for bootstrapping.
The current root zone trust anchor is a hash (in DS RDATA format) of a member of the root zone apex DNSKEY RRSet that has the SEP bit set. Such a trust anchor could be derived from a response to the query ". IN DNSKEY?", but there is no mechanism available to trust the result: without an existing, accurate trust anchor the validator has no means to gauge the authenticity of the response.
Validators should never derive trust anchors from DNSKEY RRSets obtained from the DNS.
The Root Zone KSK Manager publishes trust anchors corresponding to the root zone KSK as described in [RFC7958].
A full history of previously-published trust anchors, including the trust anchor recommended for immediate use, is made available in an XML document at the following stable URLs: Section 4).
Validity periods for each trust anchor packaged in the root-anchors.xml document are provided as XML attributes, allowing an appropriate trust anchor for immediate use to be identified (but see
Individual trust anchors are also packaged as X.509 identity certificates, signed by various Certificate Authorities (CAs). URLs to allow those certificates to be retrieved are included as optional elements in the XML document.
For automatic bootstrapping, the recommended approach is as follows.
Once a candidate trust anchor has been retrieved, the validator must establish that it is authentic before it can be used. This document recommends that this be carried out by checking the signatures on each of the X.509 identity certificates retrieved in the previous step until a certificate is found which matches a CA trust anchor.
This verification phase requires that validators ship with a useful set of CA trust anchors, and that corresponding identity certificates are published by the root zone KSK manager. In some cases validator implementors may decide to use commercial CA services, perhaps a subset of the "browser list" that is commonly distributed with web browsers; alternatively a vendor may instantiate its own CA and make arrangements with the root zone KSK manager to have the corresponding identity certificate locations published in root-anchors.xml.
The CA trust anchors packaged with validators should have an expected lifetime in excess of the anticipated life of the validator. As a protection against CA failure, validators are recommended to ship with more than one CA trust anchor.
A validator that has failed to locate a valid trust anchor may re-try the retrieval and trust establishment phases indefinitely, but must not perform validation on DNS responses until a valid trust anchor has been identified.
This document has no IANA actions.
This document discusses an approach for automatic configuration of trust anchors in a DNSSEC validator.
| [RFC5905] | Mills, D., Martin, J., Burbank, J. and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010. |
This document contains material first discussed at VeriSign and ICANN during the deployment of DNSSEC in the root zone, and also draws upon subsequent technical discussion from public mailing lists. The contributions of all those who voiced opinions are acknowledged.
This section (and sub-sections) to be removed prior to publication.
This is not a working group document. However, the topics discussed in this document are consistent with the general subject area of the DNSOP working group, and discussion of this document could reasonably take place on the dnsop mailing list.