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+Network Working Group D. Eastlake
+Request for Comments: 2538 IBM
+Category: Standards Track O. Gudmundsson
+ TIS Labs
+ March 1999
+
+
+ Storing Certificates in the Domain Name System (DNS)
+
+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 Notice
+
+ Copyright (C) The Internet Society (1999). All Rights Reserved.
+
+Abstract
+
+ Cryptographic public key are frequently published and their
+ authenticity demonstrated by certificates. A CERT resource record
+ (RR) is defined so that such certificates and related certificate
+ revocation lists can be stored in the Domain Name System (DNS).
+
+Table of Contents
+
+ Abstract...................................................1
+ 1. Introduction............................................2
+ 2. The CERT Resource Record................................2
+ 2.1 Certificate Type Values................................3
+ 2.2 Text Representation of CERT RRs........................4
+ 2.3 X.509 OIDs.............................................4
+ 3. Appropriate Owner Names for CERT RRs....................5
+ 3.1 X.509 CERT RR Names....................................5
+ 3.2 PGP CERT RR Names......................................6
+ 4. Performance Considerations..............................6
+ 5. IANA Considerations.....................................7
+ 6. Security Considerations.................................7
+ References.................................................8
+ Authors' Addresses.........................................9
+ Full Copyright Notice.....................................10
+
+
+
+
+
+
+Eastlake & Gudmundsson Standards Track [Page 1]
+
+RFC 2538 Storing Certificates in the DNS March 1999
+
+
+1. Introduction
+
+ Public keys are frequently published in the form of a certificate and
+ their authenticity is commonly demonstrated by certificates and
+ related certificate revocation lists (CRLs). A certificate is a
+ binding, through a cryptographic digital signature, of a public key,
+ a validity interval and/or conditions, and identity, authorization,
+ or other information. A certificate revocation list is a list of
+ certificates that are revoked, and incidental information, all signed
+ by the signer (issuer) of the revoked certificates. Examples are
+ X.509 certificates/CRLs in the X.500 directory system or PGP
+ certificates/revocations used by PGP software.
+
+ Section 2 below specifies a CERT resource record (RR) for the storage
+ of certificates in the Domain Name System.
+
+ Section 3 discusses appropriate owner names for CERT RRs.
+
+ Sections 4, 5, and 6 below cover performance, IANA, and security
+ considerations, respectively.
+
+ 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. The CERT Resource Record
+
+ The CERT resource record (RR) has the structure given below. Its RR
+ type code is 37.
+
+ 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | type | key tag |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+ | algorithm | /
+ +---------------+ certificate or CRL /
+ / /
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
+
+ The type field is the certificate type as define in section 2.1
+ below.
+
+ The algorithm field has the same meaning as the algorithm field in
+ KEY and SIG RRs [RFC 2535] except that a zero algorithm field
+ indicates the algorithm is unknown to a secure DNS, which may simply
+ be the result of the algorithm not having been standardized for
+ secure DNS.
+
+
+
+Eastlake & Gudmundsson Standards Track [Page 2]
+
+RFC 2538 Storing Certificates in the DNS March 1999
+
+
+ The key tag field is the 16 bit value computed for the key embedded
+ in the certificate as specified in the DNSSEC Standard [RFC 2535].
+ This field is used as an efficiency measure to pick which CERT RRs
+ may be applicable to a particular key. The key tag can be calculated
+ for the key in question and then only CERT RRs with the same key tag
+ need be examined. However, the key must always be transformed to the
+ format it would have as the public key portion of a KEY RR before the
+ key tag is computed. This is only possible if the key is applicable
+ to an algorithm (and limits such as key size limits) defined for DNS
+ security. If it is not, the algorithm field MUST BE zero and the tag
+ field is meaningless and SHOULD BE zero.
+
+2.1 Certificate Type Values
+
+ The following values are defined or reserved:
+
+ Value Mnemonic Certificate Type
+ ----- -------- ----------- ----
+ 0 reserved
+ 1 PKIX X.509 as per PKIX
+ 2 SPKI SPKI cert
+ 3 PGP PGP cert
+ 4-252 available for IANA assignment
+ 253 URI URI private
+ 254 OID OID private
+ 255-65534 available for IANA assignment
+ 65535 reserved
+
+ The PKIX type is reserved to indicate an X.509 certificate conforming
+ to the profile being defined by the IETF PKIX working group. The
+ certificate section will start with a one byte unsigned OID length
+ and then an X.500 OID indicating the nature of the remainder of the
+ certificate section (see 2.3 below). (NOTE: X.509 certificates do
+ not include their X.500 directory type designating OID as a prefix.)
+
+ The SPKI type is reserved to indicate a certificate formated as to be
+ specified by the IETF SPKI working group.
+
+ The PGP type indicates a Pretty Good Privacy certificate as described
+ in RFC 2440 and its extensions and successors.
+
+ The URI private type indicates a certificate format defined by an
+ absolute URI. The certificate portion of the CERT RR MUST begin with
+ a null terminated URI [RFC 2396] and the data after the null is the
+ private format certificate itself. The URI SHOULD be such that a
+ retrieval from it will lead to documentation on the format of the
+ certificate. Recognition of private certificate types need not be
+ based on URI equality but can use various forms of pattern matching
+
+
+
+Eastlake & Gudmundsson Standards Track [Page 3]
+
+RFC 2538 Storing Certificates in the DNS March 1999
+
+
+ so that, for example, subtype or version information can also be
+ encoded into the URI.
+
+ The OID private type indicates a private format certificate specified
+ by a an ISO OID prefix. The certificate section will start with a
+ one byte unsigned OID length and then a BER encoded OID indicating
+ the nature of the remainder of the certificate section. This can be
+ an X.509 certificate format or some other format. X.509 certificates
+ that conform to the IETF PKIX profile SHOULD be indicated by the PKIX
+ type, not the OID private type. Recognition of private certificate
+ types need not be based on OID equality but can use various forms of
+ pattern matching such as OID prefix.
+
+2.2 Text Representation of CERT RRs
+
+ The RDATA portion of a CERT RR has the type field as an unsigned
+ integer or as a mnemonic symbol as listed in section 2.1 above.
+
+ The key tag field is represented as an unsigned integer.
+
+ The algorithm field is represented as an unsigned integer or a
+ mnemonic symbol as listed in [RFC 2535].
+
+ The certificate / CRL portion is represented in base 64 and may be
+ divided up into any number of white space separated substrings, down
+ to single base 64 digits, which are concatenated to obtain the full
+ signature. These substrings can span lines using the standard
+ parenthesis.
+
+ Note that the certificate / CRL portion may have internal sub-fields
+ but these do not appear in the master file representation. For
+ example, with type 254, there will be an OID size, an OID, and then
+ the certificate / CRL proper. But only a single logical base 64
+ string will appear in the text representation.
+
+2.3 X.509 OIDs
+
+ OIDs have been defined in connection with the X.500 directory for
+ user certificates, certification authority certificates, revocations
+ of certification authority, and revocations of user certificates.
+ The following table lists the OIDs, their BER encoding, and their
+ length prefixed hex format for use in CERT RRs:
+
+
+
+
+
+
+
+
+
+Eastlake & Gudmundsson Standards Track [Page 4]
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+RFC 2538 Storing Certificates in the DNS March 1999
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+ id-at-userCertificate
+ = { joint-iso-ccitt(2) ds(5) at(4) 36 }
+ == 0x 03 55 04 24
+ id-at-cACertificate
+ = { joint-iso-ccitt(2) ds(5) at(4) 37 }
+ == 0x 03 55 04 25
+ id-at-authorityRevocationList
+ = { joint-iso-ccitt(2) ds(5) at(4) 38 }
+ == 0x 03 55 04 26
+ id-at-certificateRevocationList
+ = { joint-iso-ccitt(2) ds(5) at(4) 39 }
+ == 0x 03 55 04 27
+
+3. Appropriate Owner Names for CERT RRs
+
+ It is recommended that certificate CERT RRs be stored under a domain
+ name related to their subject, i.e., the name of the entity intended
+ to control the private key corresponding to the public key being
+ certified. It is recommended that certificate revocation list CERT
+ RRs be stored under a domain name related to their issuer.
+
+ Following some of the guidelines below may result in the use in DNS
+ names of characters that require DNS quoting which is to use a
+ backslash followed by the octal representation of the ASCII code for
+ the character such as \000 for NULL.
+
+3.1 X.509 CERT RR Names
+
+ Some X.509 versions permit multiple names to be associated with
+ subjects and issuers under "Subject Alternate Name" and "Issuer
+ Alternate Name". For example, x.509v3 has such Alternate Names with
+ an ASN.1 specification as follows:
+
+ GeneralName ::= CHOICE {
+ otherName [0] INSTANCE OF OTHER-NAME,
+ rfc822Name [1] IA5String,
+ dNSName [2] IA5String,
+ x400Address [3] EXPLICIT OR-ADDRESS.&Type,
+ directoryName [4] EXPLICIT Name,
+ ediPartyName [5] EDIPartyName,
+ uniformResourceIdentifier [6] IA5String,
+ iPAddress [7] OCTET STRING,
+ registeredID [8] OBJECT IDENTIFIER
+ }
+
+ The recommended locations of CERT storage are as follows, in priority
+ order:
+
+
+
+
+Eastlake & Gudmundsson Standards Track [Page 5]
+
+RFC 2538 Storing Certificates in the DNS March 1999
+
+
+ (1) If a domain name is included in the identification in the
+ certificate or CRL, that should be used.
+ (2) If a domain name is not included but an IP address is included,
+ then the translation of that IP address into the appropriate
+ inverse domain name should be used.
+ (3) If neither of the above it used but a URI containing a domain
+ name is present, that domain name should be used.
+ (4) If none of the above is included but a character string name is
+ included, then it should be treated as described for PGP names in
+ 3.2 below.
+ (5) If none of the above apply, then the distinguished name (DN)
+ should be mapped into a domain name as specified in RFC 2247.
+
+ Example 1: Assume that an X.509v3 certificate is issued to /CN=John
+ Doe/DC=Doe/DC=com/DC=xy/O=Doe Inc/C=XY/ with Subject Alternative
+ names of (a) string "John (the Man) Doe", (b) domain name john-
+ doe.com, and (c) uri <https://www.secure.john-doe.com:8080/>. Then
+ the storage locations recommended, in priority order, would be
+ (1) john-doe.com,
+ (2) www.secure.john-doe.com, and
+ (3) Doe.com.xy.
+
+ Example 2: Assume that an X.509v3 certificate is issued to /CN=James
+ Hacker/L=Basingstoke/O=Widget Inc/C=GB/ with Subject Alternate names
+ of (a) domain name widget.foo.example, (b) IPv4 address
+ 10.251.13.201, and (c) string "James Hacker
+ <hacker@mail.widget.foo.example>". Then the storage locations
+ recommended, in priority order, would be
+ (1) widget.foo.example,
+ (2) 201.13.251.10.in-addr.arpa, and
+ (3) hacker.mail.widget.foo.example.
+
+3.2 PGP CERT RR Names
+
+ PGP signed keys (certificates) use a general character string User ID
+ [RFC 2440]. However, it is recommended by PGP that such names include
+ the RFC 822 email address of the party, as in "Leslie Example
+ <Leslie@host.example>". If such a format is used, the CERT should be
+ under the standard translation of the email address into a domain
+ name, which would be leslie.host.example in this case. If no RFC 822
+ name can be extracted from the string name no specific domain name is
+ recommended.
+
+4. Performance Considerations
+
+ Current Domain Name System (DNS) implementations are optimized for
+ small transfers, typically not more than 512 bytes including
+ overhead. While larger transfers will perform correctly and work is
+
+
+
+Eastlake & Gudmundsson Standards Track [Page 6]
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+RFC 2538 Storing Certificates in the DNS March 1999
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+ underway to make larger transfers more efficient, it is still
+ advisable at this time to make every reasonable effort to minimize
+ the size of certificates stored within the DNS. Steps that can be
+ taken may include using the fewest possible optional or extensions
+ fields and using short field values for variable length fields that
+ must be included.
+
+5. IANA Considerations
+
+ Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can
+ only be assigned by an IETF standards action [RFC 2434] (and this
+ document assigns 0x0001 through 0x0003 and 0x00FD and 0x00FE).
+ Certificate types 0x0100 through 0xFEFF are assigned through IETF
+ Consensus [RFC 2434] based on RFC documentation of the certificate
+ type. The availability of private types under 0x00FD and 0x00FE
+ should satisfy most requirements for proprietary or private types.
+
+6. Security Considerations
+
+ By definition, certificates contain their own authenticating
+ signature. Thus it is reasonable to store certificates in non-secure
+ DNS zones or to retrieve certificates from DNS with DNS security
+ checking not implemented or deferred for efficiency. The results MAY
+ be trusted if the certificate chain is verified back to a known
+ trusted key and this conforms with the user's security policy.
+
+ Alternatively, if certificates are retrieved from a secure DNS zone
+ with DNS security checking enabled and are verified by DNS security,
+ the key within the retrieved certificate MAY be trusted without
+ verifying the certificate chain if this conforms with the user's
+ security policy.
+
+ CERT RRs are not used in connection with securing the DNS security
+ additions so there are no security considerations related to CERT RRs
+ and securing the DNS itself.
+
+
+
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+Eastlake & Gudmundsson Standards Track [Page 7]
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+RFC 2538 Storing Certificates in the DNS March 1999
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+References
+
+ RFC 1034 Mockapetris, P., "Domain Names - Concepts and Facilities",
+ STD 13, RFC 1034, November 1987.
+
+ RFC 1035 Mockapetris, P., "Domain Names - Implementation and
+ Specifications", STD 13, RFC 1035, November 1987.
+
+ RFC 2119 Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ RFC 2247 Kille, S., Wahl, M., Grimstad, A., Huber, R. and S.
+ Sataluri, "Using Domains in LDAP/X.500 Distinguished
+ Names", RFC 2247, January 1998.
+
+ RFC 2396 Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
+ Resource Identifiers (URI): Generic Syntax", RFC 2396,
+ August 1998.
+
+ RFC 2440 Callas, J., Donnerhacke, L., Finney, H. and R. Thayer,
+ "OpenPGP Message Format", RFC 2240, November 1998.
+
+ RFC 2434 Narten, T. and H. Alvestrand, "Guidelines for Writing an
+ IANA Considerations Section in RFCs", BCP 26, RFC 2434,
+ October 1998.
+
+ RFC 2535 Eastlake, D., "Domain Name System (DNS) Security
+ Extensions", RFC 2535, March 1999.
+
+ RFC 2459 Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
+ X.509 Public Key Infrastructure Certificate and CRL
+ Profile", RFC 2459, January 1999.
+
+
+
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+Eastlake & Gudmundsson Standards Track [Page 8]
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+RFC 2538 Storing Certificates in the DNS March 1999
+
+
+Authors' Addresses
+
+ Donald E. Eastlake 3rd
+ IBM
+ 65 Shindegan Hill Road
+ RR#1
+ Carmel, NY 10512 USA
+
+ Phone: +1-914-784-7913 (w)
+ +1-914-276-2668 (h)
+ Fax: +1-914-784-3833 (w-fax)
+ EMail: dee3@us.ibm.com
+
+
+ Olafur Gudmundsson
+ TIS Labs at Network Associates
+ 3060 Washington Rd, Route 97
+ Glenwood MD 21738
+
+ Phone: +1 443-259-2389
+ EMail: ogud@tislabs.com
+
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+Eastlake & Gudmundsson Standards Track [Page 9]
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+RFC 2538 Storing Certificates in the DNS March 1999
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+Full Copyright Statement
+
+ Copyright (C) The Internet Society (1999). All Rights Reserved.
+
+ This document and translations of it may be copied and furnished to
+ others, and derivative works that comment on or otherwise explain it
+ or assist in its implementation may be prepared, copied, published
+ and distributed, in whole or in part, without restriction of any
+ kind, provided that the above copyright notice and this paragraph are
+ included on all such copies and derivative works. However, this
+ document itself may not be modified in any way, such as by removing
+ the copyright notice or references to the Internet Society or other
+ Internet organizations, except as needed for the purpose of
+ developing Internet standards in which case the procedures for
+ copyrights defined in the Internet Standards process must be
+ followed, or as required to translate it into languages other than
+ English.
+
+ The limited permissions granted above are perpetual and will not be
+ revoked by the Internet Society or its successors or assigns.
+
+ This document and the information contained herein is provided on an
+ "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+ TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+ BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+ HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+ MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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