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Network Working Group                                        D. EastlakeRequest for Comments: 2538                                           IBMCategory: Standards Track                                 O. Gudmundsson                                                                TIS Labs                                                              March 1999Storing 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...................................................11. Introduction............................................22. The CERT Resource Record................................22.1 Certificate Type Values................................32.2 Text Representation of CERT RRs........................42.3 X.509 OIDs.............................................43. Appropriate Owner Names for CERT RRs....................53.1 X.509 CERT RR Names....................................53.2 PGP CERT RR Names......................................64. Performance Considerations..............................65. IANA Considerations.....................................76. Security Considerations.................................7   References.................................................8   Authors' Addresses.........................................9   Full Copyright Notice.....................................10Eastlake & Gudmundsson      Standards Track                     [Page 1]

RFC 2538            Storing Certificates in the DNS           March 19991. 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.   Sections4,5, and6 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 insection 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   inRFC 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 matchingEastlake & 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 insection 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]

RFC 2538            Storing Certificates in the DNS           March 1999    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 04 273. 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 inRFC 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   theRFC 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 noRFC 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 isEastlake & Gudmundsson      Standards Track                     [Page 6]

RFC 2538            Storing Certificates in the DNS           March 1999   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.Eastlake & Gudmundsson      Standards Track                     [Page 7]

RFC 2538            Storing Certificates in the DNS           March 1999ReferencesRFC 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.Eastlake & Gudmundsson      Standards Track                     [Page 8]

RFC 2538            Storing Certificates in the DNS           March 1999Authors' 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.comEastlake & Gudmundsson      Standards Track                     [Page 9]

RFC 2538            Storing Certificates in the DNS           March 1999Full 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.Eastlake & Gudmundsson      Standards Track                    [Page 10]