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Independent Submission
Request for Comments: 7958
Category: Informational
ISSN: 2070-1721
J. Abley
Dyn, Inc.
J. Schlyter
Kirei AB
G. Bailey
Independent
P. Hoffman
ICANN
August 2016

DNSSEC Trust Anchor Publication for the Root Zone

Abstract

The root zone of the Domain Name System (DNS) has been cryptographically signed using DNS Security Extensions (DNSSEC).

In order to obtain secure answers from the root zone of the DNS using DNSSEC, a client must configure a suitable trust anchor. This document describes the format and publication mechanisms IANA has used to distribute the DNSSEC trust anchors.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

This is a contribution to the RFC Series, independently of any other RFC stream. The RFC Editor has chosen to publish this document at its discretion and makes no statement about its value for implementation or deployment. Documents approved for publication by the RFC Editor are not a candidate for any level of Internet Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7958.

Copyright Notice

Copyright © 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  IANA DNSSEC Root Zone Trust Anchor Formats and Semantics  . .   4
     2.1.  Hashes in XML . . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  XML Syntax  . . . . . . . . . . . . . . . . . . . . .   5
       2.1.2.  XML Semantics . . . . . . . . . . . . . . . . . . . .   5
       2.1.3.  Converting from XML to DS Records . . . . . . . . . .   7
       2.1.4.  XML Example . . . . . . . . . . . . . . . . . . . . .   8
     2.2.  Certificates  . . . . . . . . . . . . . . . . . . . . . .   8
     2.3.  Certificate Signing Requests  . . . . . . . . . . . . . .   9
   3.  Root Zone Trust Anchor Retrieval  . . . . . . . . . . . . . .   9
     3.1.  Retrieving Trust Anchors with HTTPS and HTTP  . . . . . .   9
   4.  Accepting DNSSEC Trust Anchors  . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Historical Note  . . . . . . . . . . . . . . . . . .  14
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1. Introduction

   The Domain Name System (DNS) is described in [RFC1034] and [RFC1035].
   DNS Security Extensions (DNSSEC) are described in [RFC4033],
   [RFC4034], [RFC4035], [RFC4509], [RFC5155], and [RFC5702].

A discussion of operational practices relating to DNSSEC can be found in [RFC6781].

In the DNSSEC protocol, Resource Record Sets (RRSets) are signed cryptographically. This means that a response to a query contains signatures that allow the integrity and authenticity of the RRSet to be verified. DNSSEC signatures are validated by following a chain of signatures to a "trust anchor". The reason for trusting a trust anchor is outside the DNSSEC protocol, but having one or more trust anchors is required for the DNSSEC protocol to work.

The publication of trust anchors for the root zone of the DNS is an IANA function performed by ICANN. A detailed description of corresponding key management practices can be found in [DPS], which can be retrieved from the IANA Repository at <https://www.iana.org/dnssec/>.

This document describes the formats and distribution methods of DNSSEC trust anchors that have been used by IANA for the root zone of the DNS since 2010. Other organizations might have different formats and mechanisms for distributing DNSSEC trust anchors for the root zone; however, most operators and software vendors have chosen to rely on the IANA trust anchors.

It is important to note that at the time of this writing, IANA intends to change the formats and distribution methods in the future. If such a change happens, IANA will publish the changes on its web site at <https://www.iana.org/dnssec/files>.

The formats and distribution methods described in this document are a complement to, not a substitute for, the automated DNSSEC trust anchor update protocol described in [RFC5011]. That protocol allows for secure in-band succession of trust anchors when trust has already been established. This document describes one way to establish an initial trust anchor that can be used by [RFC5011].

1.1. Definitions

The term "trust anchor" is used in many different contexts in the security community. Many of the common definitions conflict because they are specific to a specific system, such as just for DNSSEC or just for S/MIME messages.

In cryptographic systems with hierarchical structure, a trust anchor is an authoritative entity for which trust is assumed and not derived. The format of the entity differs in different systems, but the basic idea, that trust is assumed and not derived, is common to all the common uses of the term "trust anchor".

The root zone trust anchor formats published by IANA are defined in Section 2. [RFC4033] defines a trust anchor as "A configured DNSKEY RR or DS RR hash of a DNSKEY RR". Note that the formats defined here do not match the definition of "trust anchor" from [RFC4033]; however, a system that wants to convert the trusted material from IANA into a Delegation Signer (DS) RR can do so.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

2. IANA DNSSEC Root Zone Trust Anchor Formats and Semantics

IANA publishes trust anchors for the root zone in three formats:

  • an XML document that contains the hashes of the DNSKEY records
  • certificates in PKIX format [RFC5280] that contain DS records and the full public key of DNSKEY records
  • Certificate Signing Requests (CSRs) in PKCS #10 format [RFC2986] that contain DS records and the full public key of DNSKEY records

These formats and the semantics associated with each are described in the rest of this section.

2.1. Hashes in XML

The XML document contains a set of hashes for the DNSKEY records that can be used to validate the root zone. The hashes are consistent with the defined presentation format of DS resource records from [RFC4034].

2.1.1. XML Syntax

A RELAX NG Compact Schema [RELAX-NG] for the documents used to publish trust anchors is given in Figure 1.

datatypes xsd = "http://www.w3.org/2001/XMLSchema-datatypes"

start = element TrustAnchor { 

       attribute id { xsd:string },
       attribute source { xsd:string },
       element Zone { xsd:string },
   
       keydigest+
   }

keydigest = element KeyDigest { 

       attribute id { xsd:string },
       attribute validFrom { xsd:dateTime },
       attribute validUntil { xsd:dateTime }?,
   
       element KeyTag {
               xsd:nonNegativeInteger { maxInclusive = "65535" } },
       element Algorithm {
               xsd:nonNegativeInteger { maxInclusive = "255" } },
       element DigestType {
               xsd:nonNegativeInteger { maxInclusive = "255" } },
       element Digest { xsd:hexBinary }
   }

Figure 1

2.1.2. XML Semantics

The TrustAnchor element is the container for all of the trust anchors in the file.

The id attribute in the TrustAnchor element is an opaque string that identifies the set of trust anchors. Its value has no particular semantics. Note that the id element in the TrustAnchor element is different than the id element in the KeyDigest element, described below.

The source attribute in the TrustAnchor element gives information about where to obtain the TrustAnchor container. It is likely to be a URL and is advisory only.

The Zone element in the TrustAnchor element states to which DNS zone this container applies. The root zone is indicated by a single period (.) character without any quotation marks.

The TrustAnchor element contains one or more KeyDigest elements. Each KeyDigest element represents the digest of a DNSKEY record in the zone defined in the Zone element.

The id attribute in the KeyDigest element is an opaque string that identifies the hash. Its value is used in the file names and URI of the other trust anchor formats. This is described in Section 3.1. For example, if the value of the id attribute in the KeyDigest element is "Kjqmt7v", the URI for the CSR that is associated with this hash will be <https://data.iana.org/root-anchors/Kjqmt7v.csr>. Note that the id element in the KeyDigest element is different than the id element in the TrustAnchor element described above.

The validFrom and validUntil attributes in the KeyDigest element specify the range of times that the KeyDigest element can be used as a trust anchor. Note that the KeyDigest element is optional; if it is not given, the trust anchor can be used until a KeyDigest element covering the same DNSKEY record, but having a validUntil attribute, is trusted by the relying party. Relying parties SHOULD NOT use a KeyDigest outside of the time range given in the validFrom and validUntil attributes.

The KeyTag element in the KeyDigest element contains the key tag for the DNSKEY record represented in this KeyDigest.

The Algorithm element in the KeyDigest element contains the signing algorithm identifier for the DNSKEY record represented in this KeyDigest.

The DigestType element in the KeyDigest element contains the digest algorithm identifier for the DNSKEY record represented in this KeyDigest.

The Digest element in the KeyDigest element contains the hexadecimal representation of the hash for the DNSKEY record represented in this KeyDigest.

2.1.3. Converting from XML to DS Records

The display format for the DS record that is the equivalent of a KeyDigest element can be constructed by marshaling the KeyTag, Algorithm, DigestType, and Digest elements. For example, assume that the TrustAnchor element contains: 

   <?xml version="1.0" encoding="UTF-8"?>
   <TrustAnchor
      id="AD42165F-3B1A-4778-8F42-D34A1D41FD93"
      source="http://data.iana.org/root-anchors/root-anchors.xml">
   <Zone>.</Zone>
   <KeyDigest id="Kjqmt7v" validFrom="2010-07-15T00:00:00+00:00">
   <KeyTag>19036</KeyTag>
   <Algorithm>8</Algorithm>
   <DigestType>2</DigestType>
   <Digest>
   49AAC11D7B6F6446702E54A1607371607A1A41855200FD2CE1CDDE32F24E8FB5
   </Digest>
   </KeyDigest>
   </TrustAnchor>

The DS record would be: 

   . IN DS 19036 8 2
      49AAC11D7B6F6446702E54A1607371607A1A41855200FD2CE1CDDE32F24E8FB5

2.1.4. XML Example

Figure 2 describes two fictitious trust anchors for the root zone. 

   <?xml version="1.0" encoding="UTF-8"?>
   
   <TrustAnchor
       id="AD42165F-B099-4778-8F42-D34A1D41FD93"
       source="http://data.iana.org/root-anchors/root-anchors.xml">
       <Zone>.</Zone>
       <KeyDigest id="42"
                  validFrom="2010-07-01T00:00:00-00:00"
                  validUntil="2010-08-01T00:00:00-00:00">
           <KeyTag>34291</KeyTag>
           <Algorithm>5</Algorithm>
           <DigestType>1</DigestType>
           <Digest>c8cb3d7fe518835490af8029c23efbce6b6ef3e2</Digest>
       </KeyDigest>
       <KeyDigest id="53"
                  validFrom="2010-08-01T00:00:00-00:00">
           <KeyTag>12345</KeyTag>
           <Algorithm>5</Algorithm>
           <DigestType>1</DigestType>
           <Digest>a3cf809dbdbc835716ba22bdc370d2efa50f21c7</Digest>
       </KeyDigest>
   </TrustAnchor>

Figure 2

2.2. Certificates

Each public key that can be used as a trust anchor is represented as a certificate in PKIX format. Each certificate is signed by the ICANN certificate authority. The SubjectPublicKeyInfo in the certificate represents the public key of the Key Signing Key (KSK). The Subject field has the following attributes: 

   O:   the string "ICANN".
   
   OU:  the string "IANA".
   
   CN:  the string "Root Zone KSK" followed by the time and date of key
      generation in the format specified in [RFC3339].  For example, a
      CN might be "Root Zone KSK 2010-06-16T21:19:24+00:00".
   
   resourceRecord:  a string in the presentation format of the DS
      [RFC4034] resource record for the DNSSEC public key.

The "resourceRecord" attribute in the Subject is defined as follows:

ResourceRecord 

     { iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-dns-resource-record(70) }

DEFINITIONS IMPLICIT TAGS ::= 

   BEGIN
   
   -- EXPORTS ALL --
   
   IMPORTS

caseIgnoreMatch FROM SelectedAttributeTypes 

       { joint-iso-itu-t ds(5) module(1) selectedAttributeTypes(5) 4 }
   
   ;
   
   iana OBJECT IDENTIFIER ::= { iso(1) identified-organization(3)
       dod(6) internet(1) private(4) enterprise(1) 1000 }
   
   iana-dns OBJECT IDENTIFIER ::= { iana 53 }
   
   resourceRecord ATTRIBUTE ::= {
       WITH SYNTAX IA5String
       EQUALITY MATCHING RULE caseIgnoreMatch
       ID iana-dns
   }
   
   END

2.3. Certificate Signing Requests

Each public key that can be used as a trust anchor is represented as a CSR in PKCS #10 format. The SubjectPublicKeyInfo and Subject field are the same as for certificates (see Section 2.2 above).

3. Root Zone Trust Anchor Retrieval

3.1. Retrieving Trust Anchors with HTTPS and HTTP

Trust anchors are available for retrieval using HTTPS and HTTP.

In this section, all URLs are given using the "https:" scheme. If HTTPS cannot be used, replace the "https:" scheme with "http:". 

   The URL for retrieving the set of hashes described in Section 2.1 is
   <https://data.iana.org/root-anchors/root-anchors.xml>.

The URL for retrieving the PKIX certificate described in Section 2.2 is <https://data.iana.org/root-anchors/KEYDIGEST-ID.crt>, with the string "KEYDIGEST-ID" replacing the "id" attribute from the KeyDigest element from the XML file, as described in Section 2.1.2.

The URL for retrieving the CSR described in Section 2.3 is <https://data.iana.org/root-anchors/KEYDIGEST-ID.csr>, with the string "KEYDIGEST-ID" replacing the "id" attribute from the KeyDigest element from the XML file, as described in Section 2.1.2.

4. Accepting DNSSEC Trust Anchors

A validator operator can choose whether or not to accept the trust anchors described in this document using whatever policy they want. In order to help validator operators verify the content and origin of trust anchors they receive, IANA uses digital signatures that chain to an ICANN-controlled Certificate Authority (CA) over the trust anchor data.

It is important to note that the ICANN CA is not a DNSSEC trust anchor. Instead, it is an optional mechanism for verifying the content and origin of the XML and certificate trust anchors. It is also important to note that the ICANN CA cannot be used to verify the origin of the trust anchor in the CSR format.

The content and origin of the XML file can be verified using a digital signature on the file. IANA provides a detached Cryptographic Message Syntax (CMS) [RFC5652] signature that chains to the ICANN CA with the XML file. The URL for a detached CMS signature for the XML file is <https://data.iana.org/root-anchors/root-anchors.p7s>.

(IANA also provided a detached OpenPGP [RFC4880] signature as a second parallel verification mechanism for the first trust anchor publication but has indicated that it will not use this parallel mechanism in the future.)

Another method IANA uses to help validator operators verify the content and origin of trust anchors they receive is to use the Transport Layer Security (TLS) protocol for distributing the trust anchors. Currently, the CA used for data.iana.org is well known, that is, one that is a WebTrust-accredited CA. If a system retrieving the trust anchors trusts the CA that IANA uses for the "data.iana.org" web server, HTTPS SHOULD be used instead of HTTP in order to have assurance of data origin.

5. IANA Considerations

   This document defines id-mod-dns-resource-record, value 70 (see
   Section 2.2), in the "SMI Security for PKIX Module Identifier"
   registry.

6. Security Considerations

This document describes how DNSSEC trust anchors for the root zone of the DNS are published. Many DNSSEC clients will only configure IANA- issued trust anchors for the DNS root to perform validation. As a consequence, reliable publication of trust anchors is important.

This document aims to specify carefully the means by which such trust anchors are published, with the goal of making it easier for those trust anchors to be integrated into user environments.

7. References

7.1. Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <http://www.rfc-editor.org/info/rfc1034>.
   
   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.
   
   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <http://www.rfc-editor.org/info/rfc2986>.
   
   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <http://www.rfc-editor.org/info/rfc3339>.
   
   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <http://www.rfc-editor.org/info/rfc4033>.
   
   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <http://www.rfc-editor.org/info/rfc4034>.
   
   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <http://www.rfc-editor.org/info/rfc4035>.
   
   [RFC4509]  Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer
              (DS) Resource Records (RRs)", RFC 4509,
              DOI 10.17487/RFC4509, May 2006,
              <http://www.rfc-editor.org/info/rfc4509>.
   
   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
              September 2007, <http://www.rfc-editor.org/info/rfc5011>.
   
   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
              <http://www.rfc-editor.org/info/rfc5155>.
   
   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.
   
   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <http://www.rfc-editor.org/info/rfc5652>.
   
   [RFC5702]  Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
              and RRSIG Resource Records for DNSSEC", RFC 5702,
              DOI 10.17487/RFC5702, October 2009,
              <http://www.rfc-editor.org/info/rfc5702>.
   
   [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
              Operational Practices, Version 2", RFC 6781,
              DOI 10.17487/RFC6781, December 2012,
              <http://www.rfc-editor.org/info/rfc6781>.

7.2. Informative References

   [DPS]      Ljunggren, F., Okubo, T., Lamb, R., and J. Schlyter,
              "DNSSEC Practice Statement for the Root Zone KSK
              Operator", October 2015,
              <https://www.iana.org/dnssec/icann-dps.txt>.
   
   [RELAX-NG] Clark, J., "RELAX NG Compact Syntax",
              Committee Specification, November 2002,
              <https://www.oasis-open.org/committees/relax-ng/
              compact-20021121.html>.
   
   [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
              Thayer, "OpenPGP Message Format", RFC 4880,
              DOI 10.17487/RFC4880, November 2007,
              <http://www.rfc-editor.org/info/rfc4880>.

Appendix A. Historical Note

The first KSK for use in the root zone of the DNS was generated at a key ceremony at an ICANN Key Management Facility (KMF) in Culpeper, Virginia, USA on 2010-06-16. This key entered production during a second key ceremony held at an ICANN KMF in El Segundo, California, USA on 2010-07-12. The resulting trust anchor was first published on 2010-07-15.

Acknowledgements

Many pioneers paved the way for the deployment of DNSSEC in the root zone of the DNS, and the authors hereby acknowledge their substantial collective contribution.

This document incorporates suggestions made by Alfred Hoenes and Russ Housley, whose contributions are appreciated.

Authors' Addresses

   Joe Abley
   Dyn, Inc.
   300-184 York Street
   London, ON  N6A 1B5
   Canada
   
   Phone: +1 519 670 9327
   Email: jabley@dyn.com

Jakob Schlyter
Kirei AB

Email: 

          jakob@kirei.se

Guillaume Bailey
Independent

Email: 

          GuillaumeBailey@outlook.com

Paul Hoffman
ICANN 

   Email: paul.hoffman@icann.org