draft-ietf-dnsext-rfc2538bis-09.txt   draft-ietf-dnsext-rfc2538bis.txt 
Network Working Group S. Josefsson Network Working Group S. Josefsson
Obsoletes: 2538 (if approved) Obsoletes: 2538 (if approved)
Expires: April 21, 2006 Expires: September 2, 2006
Storing Certificates in the Domain Name System (DNS) Storing Certificates in the Domain Name System (DNS)
draft-ietf-dnsext-rfc2538bis-09 draft-ietf-dnsext-rfc2538bis-10
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). Copyright (C) The Internet Society (2006).
Abstract Abstract
Cryptographic public keys are frequently published and their Cryptographic public keys are frequently published, and their
authenticity demonstrated by certificates. A CERT resource record authenticity is demonstrated by certificates. A CERT resource record
(RR) is defined so that such certificates and related certificate (RR) is defined so that such certificates and related certificate
revocation lists can be stored in the Domain Name System (DNS). revocation lists can be stored in the Domain Name System (DNS).
This document obsoletes RFC 2538. This document obsoletes RFC 2538.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The CERT Resource Record . . . . . . . . . . . . . . . . . . . 3 2. The CERT Resource Record . . . . . . . . . . . . . . . . . . . 3
2.1. Certificate Type Values . . . . . . . . . . . . . . . . . 4 2.1. Certificate Type Values . . . . . . . . . . . . . . . . . 4
2.2. Text Representation of CERT RRs . . . . . . . . . . . . . 5 2.2. Text Representation of CERT RRs . . . . . . . . . . . . . 6
2.3. X.509 OIDs . . . . . . . . . . . . . . . . . . . . . . . . 6 2.3. X.509 OIDs . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Appropriate Owner Names for CERT RRs . . . . . . . . . . . . . 6 3. Appropriate Owner Names for CERT RRs . . . . . . . . . . . . . 7
3.1. Content-based X.509 CERT RR Names . . . . . . . . . . . . 7 3.1. Content-Based X.509 CERT RR Names . . . . . . . . . . . . 8
3.2. Purpose-based X.509 CERT RR Names . . . . . . . . . . . . 8 3.2. Purpose-Based X.509 CERT RR Names . . . . . . . . . . . . 9
3.3. Content-based OpenPGP CERT RR Names . . . . . . . . . . . 9 3.3. Content-Based OpenPGP CERT RR Names . . . . . . . . . . . 9
3.4. Purpose-based OpenPGP CERT RR Names . . . . . . . . . . . 9 3.4. Purpose-Based OpenPGP CERT RR Names . . . . . . . . . . . 10
3.5. Owner names for IPKIX, ISPKI, IPGP, and IACPKIX . . . . . 10 3.5. Owner Names for IPKIX, ISPKI, IPGP, and IACPKIX . . . . . 10
4. Performance Considerations . . . . . . . . . . . . . . . . . . 10 4. Performance Considerations . . . . . . . . . . . . . . . . . . 10
5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11 7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Changes since RFC 2538 . . . . . . . . . . . . . . . . . . . . 12 9. Changes since RFC 2538 . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Copying conditions . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . . 13 10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . . 14 10.2. Informative References . . . . . . . . . . . . . . . . . . 14
Appendix A. Copying Conditions . . . . . . . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17 Intellectual Property and Copyright Statements . . . . . . . . . . 17
1. Introduction 1. Introduction
Public keys are frequently published in the form of a certificate and Public keys are frequently published in the form of a certificate,
their authenticity is commonly demonstrated by certificates and and their authenticity is commonly demonstrated by certificates and
related certificate revocation lists (CRLs). A certificate is a related certificate revocation lists (CRLs). A certificate is a
binding, through a cryptographic digital signature, of a public key, binding, through a cryptographic digital signature, of a public key,
a validity interval and/or conditions, and identity, authorization, a validity interval and/or conditions, and identity, authorization,
or other information. A certificate revocation list is a list of or other information. A certificate revocation list is a list of
certificates that are revoked, and incidental information, all signed certificates that are revoked, and of incidental information, all
by the signer (issuer) of the revoked certificates. Examples are signed by the signer (issuer) of the revoked certificates. Examples
X.509 certificates/CRLs in the X.500 directory system or OpenPGP are X.509 certificates/CRLs in the X.500 directory system or OpenPGP
certificates/revocations used by OpenPGP software. certificates/revocations used by OpenPGP software.
Section 2 below specifies a CERT resource record (RR) for the storage Section 2 specifies a CERT resource record (RR) for the storage of
of certificates in the Domain Name System [1] [2]. certificates in the Domain Name System [1] [2].
Section 3 discusses appropriate owner names for CERT RRs. Section 3 discusses appropriate owner names for CERT RRs.
Sections 4, 5, and 6 below cover performance, IANA, and security Sections 4, 7, and 8 cover performance, security, and IANA
considerations, respectively. considerations, respectively.
Section 9 explain the changes in this document compared to RFC 2538. Section 9 explains the changes in this document compared to RFC 2538.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [3]. document are to be interpreted as described in [3].
2. The CERT Resource Record 2. The CERT Resource Record
The CERT resource record (RR) has the structure given below. Its RR The CERT resource record (RR) has the structure given below. Its RR
type code is 37. 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 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 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 | | type | key tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| algorithm | / | algorithm | /
+---------------+ certificate or CRL / +---------------+ certificate or CRL /
/ / / /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
The type field is the certificate type as defined in section 2.1 The type field is the certificate type as defined in Section 2.1
below. below.
The key tag field is the 16 bit value computed for the key embedded The key tag field is the 16-bit value computed for the key embedded
in the certificate, using the RRSIG Key Tag algorithm described in in the certificate, using the RRSIG Key Tag algorithm described in
Appendix B of [12]. This field is used as an efficiency measure to Appendix B of [12]. This field is used as an efficiency measure to
pick which CERT RRs may be applicable to a particular key. The key 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 tag can be calculated for the key in question, and then only CERT RRs
with the same key tag need be examined. Note that two different keys with the same key tag need to be examined. Note that two different
can have the same key tag. However, the key MUST be transformed to keys can have the same key tag. However, the key MUST be transformed
the format it would have as the public key portion of a DNSKEY RR to the format it would have as the public key portion of a DNSKEY RR
before the key tag is computed. This is only possible if the key is before the key tag is computed. This is only possible if the key is
applicable to an algorithm and complies to limits (such as key size) applicable to an algorithm and complies to limits (such as key size)
defined for DNS security. If it is not, the algorithm field MUST be defined for DNS security. If it is not, the algorithm field MUST be
zero and the tag field is meaningless and SHOULD be zero. zero and the tag field is meaningless and SHOULD be zero.
The algorithm field has the same meaning as the algorithm field in The algorithm field has the same meaning as the algorithm field in
DNSKEY and RRSIG RRs [12], except that a zero algorithm field DNSKEY and RRSIG RRs [12], except that a zero algorithm field
indicates the algorithm is unknown to a secure DNS, which may simply indicates that the algorithm is unknown to a secure DNS, which may
be the result of the algorithm not having been standardized for simply be the result of the algorithm not having been standardized
DNSSEC [11]. for DNSSEC [11].
2.1. Certificate Type Values 2.1. Certificate Type Values
The following values are defined or reserved: The following values are defined or reserved:
Value Mnemonic Certificate Type Value Mnemonic Certificate Type
----- -------- ---------------- ----- -------- ----------------
0 reserved 0 Reserved
1 PKIX X.509 as per PKIX 1 PKIX X.509 as per PKIX
2 SPKI SPKI certificate 2 SPKI SPKI certificate
3 PGP OpenPGP packet 3 PGP OpenPGP packet
4 IPKIX The URL of an X.509 data object 4 IPKIX The URL of an X.509 data object
5 ISPKI The URL of an SPKI certificate 5 ISPKI The URL of an SPKI certificate
6 IPGP The URL of an OpenPGP packet 6 IPGP The fingerprint and URL of an OpenPGP packet
7 ACPKIX Attribute Certificate 7 ACPKIX Attribute Certificate
8 IACPKIX The URL of an Attribute Certificate 8 IACPKIX The URL of an Attribute Certificate
9-252 available for IANA assignment 9-252 Available for IANA assignment
253 URI URI private 253 URI URI private
254 OID OID private 254 OID OID private
255-65023 available for IANA assignment 255 Reserved
65024-65534 experimental 256-65279 Available for IANA assignment
65535 reserved 65280-65534 Experimental
65535 Reserved
These values represent the initial content of the IANA registry, see These values represent the initial content of the IANA registry; see
section 8. Section 8.
The PKIX type is reserved to indicate an X.509 certificate conforming The PKIX type is reserved to indicate an X.509 certificate conforming
to the profile defined by the IETF PKIX working group [9]. The to the profile defined by the IETF PKIX working group [8]. The
certificate section will start with a one-octet unsigned OID length certificate section will start with a one-octet unsigned OID length
and then an X.500 OID indicating the nature of the remainder of the 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 certificate section (see Section 2.3, below). (NOTE: X.509
not include their X.500 directory type designating OID as a prefix.) certificates do not include their X.500 directory-type-designating
The SPKI type is reserved to indicate the SPKI certificate format OID as a prefix.)
[15], for use when the SPKI documents are moved from experimental
status.
The PGP type indicates an OpenPGP packet as described in [6] and its The SPKI and ISPKI types are reserved to indicate the SPKI
extensions and successors. Two uses are to transfer public key certificate format [15], for use when the SPKI documents are moved
material and revocation signatures. The data is binary, and MUST NOT from experimental status. The format for these two CERT RR types
will need to be specified later.
The PGP type indicates an OpenPGP packet as described in [5] and its
extensions and successors. This is used to transfer public key
material and revocation signatures. The data is binary and MUST NOT
be encoded into an ASCII armor. An implementation SHOULD process be encoded into an ASCII armor. An implementation SHOULD process
transferable public keys as described in section 10.1 of [6], but it transferable public keys as described in Section 10.1 of [5], but it
MAY handle additional OpenPGP packets. MAY handle additional OpenPGP packets.
The ACPKIX type indicate an Attribute Certificate format [10]. The ACPKIX type indicates an Attribute Certificate format [9].
The IPKIX, ISPKI, IPGP, IACPKIX types indicate a URL which will serve The IPKIX and IACPKIX types indicate a URL that will serve the
the content that would have been in the "certificate, CRL or URL" content that would have been in the "certificate, CRL, or URL" field
field of the corresponding types; PKIX, SPKI, PGP, or ACPKIX of the corresponding type (PKIX or ACPKIX, respectively).
respectively. The IPKIX, ISPKI, IPGP and IACPKIX types are known as
"indirect". These types MUST be used when the content is too large The IPGP type contains both an OpenPGP fingerprint for the key in
to fit in the CERT RR, and MAY be used at the implementer's question, as well as a URL. The certificate portion of the IPGP CERT
discretion. They SHOULD NOT be used where the DNS message is 512 RR is defined as a one-octet fingerprint length, followed by the
octets or smaller, and could thus be expected to fit a UDP packet. OpenPGP fingerprint, followed by the URL. The OpenPGP fingerprint is
calculated as defined in RFC 2440 [5]. A zero-length fingerprint or
a zero-length URL are legal, and indicate URL-only IPGP data or
fingerprint-only IPGP data, respectively. A zero-length fingerprint
and a zero-length URL are meaningless and invalid.
The IPKIX, ISPKI, IPGP, and IACPKIX types are known as "indirect".
These types MUST be used when the content is too large to fit in the
CERT RR and MAY be used at the implementer's discretion. They SHOULD
NOT be used where the DNS message is 512 octets or smaller and could
thus be expected to fit a UDP packet.
The URI private type indicates a certificate format defined by an The URI private type indicates a certificate format defined by an
absolute URI. The certificate portion of the CERT RR MUST begin with absolute URI. The certificate portion of the CERT RR MUST begin with
a null terminated URI [5] and the data after the null is the private a NUL-terminated URI [10] and the data after the NUL is the private
format certificate itself. The URI SHOULD be such that a retrieval format certificate itself. The URI SHOULD be such that a retrieval
from it will lead to documentation on the format of the certificate. from it will lead to documentation on the format of the certificate.
Recognition of private certificate types need not be based on URI Recognition of private certificate types need not be based on URI
equality but can use various forms of pattern matching so that, for equality but can use various forms of pattern matching so that, for
example, subtype or version information can also be encoded into the example, subtype or version information can also be encoded into the
URI. URI.
The OID private type indicates a private format certificate specified The OID private type indicates a private format certificate specified
by an ISO OID prefix. The certificate section will start with a one- by an ISO OID prefix. The certificate section will start with a one-
octet unsigned OID length and then a BER encoded OID indicating the octet unsigned OID length and then a BER-encoded OID indicating the
nature of the remainder of the certificate section. This can be an nature of the remainder of the certificate section. This can be an
X.509 certificate format or some other format. X.509 certificates X.509 certificate format or some other format. X.509 certificates
that conform to the IETF PKIX profile SHOULD be indicated by the PKIX that conform to the IETF PKIX profile SHOULD be indicated by the PKIX
type, not the OID private type. Recognition of private certificate type, not the OID private type. Recognition of private certificate
types need not be based on OID equality but can use various forms of types need not be based on OID equality but can use various forms of
pattern matching such as OID prefix. pattern matching such as OID prefix.
2.2. Text Representation of CERT RRs 2.2. Text Representation of CERT RRs
The RDATA portion of a CERT RR has the type field as an unsigned The RDATA portion of a CERT RR has the type field as an unsigned
decimal integer or as a mnemonic symbol as listed in section 2.1 decimal integer or as a mnemonic symbol as listed in Section 2.1,
above. above.
The key tag field is represented as an unsigned decimal integer. The key tag field is represented as an unsigned decimal integer.
The algorithm field is represented as an unsigned decimal integer or The algorithm field is represented as an unsigned decimal integer or
a mnemonic symbol as listed in [12]. a mnemonic symbol as listed in [12].
The certificate / CRL portion is represented in base 64 [16] and may The certificate/CRL portion is represented in base 64 [16] and may be
be divided up into any number of white space separated substrings, divided into any number of white-space-separated substrings, down to
down to single base 64 digits, which are concatenated to obtain the single base-64 digits, which are concatenated to obtain the full
full signature. These substrings can span lines using the standard signature. These substrings can span lines using the standard
parenthesis. parenthesis.
Note that the certificate / CRL portion may have internal sub-fields, Note that the certificate / CRL portion may have internal sub-fields,
but these do not appear in the master file representation. For 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 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 the certificate/CRL proper. However, only a single logical base-64
string will appear in the text representation. string will appear in the text representation.
2.3. X.509 OIDs 2.3. X.509 OIDs
OIDs have been defined in connection with the X.500 directory for OIDs have been defined in connection with the X.500 directory for
user certificates, certification authority certificates, revocations user certificates, certification authority certificates, revocations
of certification authority, and revocations of user certificates. of certification authority, and revocations of user certificates.
The following table lists the OIDs, their BER encoding, and their The following table lists the OIDs, their BER encoding, and their
length-prefixed hex format for use in CERT RRs: length-prefixed hex format for use in CERT RRs:
skipping to change at page 7, line 4 skipping to change at page 7, line 27
3. Appropriate Owner Names for CERT RRs 3. Appropriate Owner Names for CERT RRs
It is recommended that certificate CERT RRs be stored under a domain 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 name related to their subject, i.e., the name of the entity intended
to control the private key corresponding to the public key being to control the private key corresponding to the public key being
certified. It is recommended that certificate revocation list CERT certified. It is recommended that certificate revocation list CERT
RRs be stored under a domain name related to their issuer. RRs be stored under a domain name related to their issuer.
Following some of the guidelines below may result in DNS names with Following some of the guidelines below may result in DNS names with
characters that require DNS quoting as per section 5.1 of RFC 1035 characters that require DNS quoting as per Section 5.1 of RFC 1035
[2]. [2].
The choice of name under which CERT RRs are stored is important to The choice of name under which CERT RRs are stored is important to
clients that perform CERT queries. In some situations, the clients clients that perform CERT queries. In some situations, the clients
may not know all information about the CERT RR object it wishes to may not know all information about the CERT RR object it wishes to
retrieve. For example, a client may not know the subject name of an retrieve. For example, a client may not know the subject name of an
X.509 certificate, or the e-mail address of the owner of an OpenPGP X.509 certificate, or the email address of the owner of an OpenPGP
key. Further, the client might only know the hostname of a service key. Further, the client might only know the hostname of a service
that uses X.509 certificates or the Key ID of an OpenPGP key. that uses X.509 certificates or the Key ID of an OpenPGP key.
Therefore, two owner name guidelines are defined: content-based owner Therefore, two owner name guidelines are defined: content-based owner
names and purpose-based owner names. A content-based owner name is names and purpose-based owner names. A content-based owner name is
derived from the content of the CERT RR data; for example, the derived from the content of the CERT RR data; for example, the
Subject field in an X.509 certificate or the User ID field in OpenPGP Subject field in an X.509 certificate or the User ID field in OpenPGP
keys. A purpose-based owner name is a name that a client retrieving keys. A purpose-based owner name is a name that a client retrieving
CERT RRs ought to already know; for example, the host name of an CERT RRs ought to know already; for example, the host name of an
X.509 protected service or the Key ID of an OpenPGP key. The X.509 protected service or the Key ID of an OpenPGP key. The
content-based and purpose-based owner name may be the same; for content-based and purpose-based owner name may be the same; for
example, when a client looks up a key based on the From: address of example, when a client looks up a key based on the From: address of
an incoming e-mail. an incoming email.
Implementations SHOULD use the purpose-based owner name guidelines Implementations SHOULD use the purpose-based owner name guidelines
described in this document, and MAY use CNAME RRs at content-based described in this document and MAY use CNAME RRs at content-based
owner names (or other names), pointing to the purpose-based owner owner names (or other names), pointing to the purpose-based owner
name. name.
Note that this section describes an application-based mapping from Note that this section describes an application-based mapping from
the name space used in a certificate to the name space used by DNS. the name space used in a certificate to the name space used by DNS.
The DNS does not infer any relationship amongst CERT resource records The DNS does not infer any relationship amongst CERT resource records
based on similarities or differences of the DNS owner name(s) of CERT based on similarities or differences of the DNS owner name(s) of CERT
resource records. For example, if multiple labels are used when resource records. For example, if multiple labels are used when
mapping from a CERT identifier to a domain name then care must be mapping from a CERT identifier to a domain name, then care must be
taken in understanding wildcard record synthesis. taken in understanding wildcard record synthesis.
3.1. Content-based X.509 CERT RR Names 3.1. Content-Based X.509 CERT RR Names
Some X.509 versions, such as the PKIX profile of X.509 [9], permit Some X.509 versions, such as the PKIX profile of X.509 [8], permit
multiple names to be associated with subjects and issuers under multiple names to be associated with subjects and issuers under
"Subject Alternative Name" and "Issuer Alternative Name". For "Subject Alternative Name" and "Issuer Alternative Name". For
example, the PKIX profile has such Alternate Names with an ASN.1 example, the PKIX profile has such Alternate Names with an ASN.1
specification as follows: specification as follows:
GeneralName ::= CHOICE { GeneralName ::= CHOICE {
otherName [0] OtherName, otherName [0] OtherName,
rfc822Name [1] IA5String, rfc822Name [1] IA5String,
dNSName [2] IA5String, dNSName [2] IA5String,
x400Address [3] ORAddress, x400Address [3] ORAddress,
directoryName [4] Name, directoryName [4] Name,
ediPartyName [5] EDIPartyName, ediPartyName [5] EDIPartyName,
uniformResourceIdentifier [6] IA5String, uniformResourceIdentifier [6] IA5String,
iPAddress [7] OCTET STRING, iPAddress [7] OCTET STRING,
registeredID [8] OBJECT IDENTIFIER } registeredID [8] OBJECT IDENTIFIER }
The recommended locations of CERT storage are as follows, in priority The recommended locations of CERT storage are as follows, in priority
order: order:
1. If a domain name is included in the identification in the 1. If a domain name is included in the identification in the
certificate or CRL, that ought be used. certificate or CRL, that ought to be used.
2. If a domain name is not included but an IP address is included, 2. If a domain name is not included but an IP address is included,
then the translation of that IP address into the appropriate then the translation of that IP address into the appropriate
inverse domain name ought to be used. inverse domain name ought to be used.
3. If neither of the above is used, but a URI containing a domain 3. If neither of the above is used, but a URI containing a domain
name is present, that domain name ought to be used. name is present, that domain name ought to be used.
4. If none of the above is included but a character string name is 4. If none of the above is included but a character string name is
included, then it ought to be treated as described for OpenPGP included, then it ought to be treated as described below for
names below. OpenPGP names.
5. If none of the above apply, then the distinguished name (DN) 5. If none of the above apply, then the distinguished name (DN)
ought to be mapped into a domain name as specified in [4]. ought to be mapped into a domain name as specified in [4].
Example 1: An X.509v3 certificate is issued to /CN=John Doe /DC=Doe/ Example 1: 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) 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) string "John (the Man) Doe", (b) domain name john-doe.com, and (c)
URI <https://www.secure.john-doe.com:8080/>. The storage locations URI <https://www.secure.john-doe.com:8080/>. The storage locations
recommended, in priority order, would be recommended, in priority order, would be
1. john-doe.com, 1. john-doe.com,
2. www.secure.john-doe.com, and 2. www.secure.john-doe.com, and
skipping to change at page 8, line 49 skipping to change at page 9, line 18
Example 2: An X.509v3 certificate is issued to /CN=James Hacker/ Example 2: An X.509v3 certificate is issued to /CN=James Hacker/
L=Basingstoke/O=Widget Inc/C=GB/ with Subject Alternate names of (a) 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 domain name widget.foo.example, (b) IPv4 address 10.251.13.201, and
(c) string "James Hacker <hacker@mail.widget.foo.example>". The (c) string "James Hacker <hacker@mail.widget.foo.example>". The
storage locations recommended, in priority order, would be storage locations recommended, in priority order, would be
1. widget.foo.example, 1. widget.foo.example,
2. 201.13.251.10.in-addr.arpa, and 2. 201.13.251.10.in-addr.arpa, and
3. hacker.mail.widget.foo.example. 3. hacker.mail.widget.foo.example.
3.2. Purpose-based X.509 CERT RR Names 3.2. Purpose-Based X.509 CERT RR Names
Due to the difficulty for clients that do not already possess a Due to the difficulty for clients that do not already possess a
certificate to reconstruct the content-based owner name, purpose- certificate to reconstruct the content-based owner name, purpose-
based owner names are recommended in this section. Recommendations based owner names are recommended in this section. Recommendations
for purpose-based owner names vary per scenario. The following table for purpose-based owner names vary per scenario. The following table
summarizes the purpose-based X.509 CERT RR owner name guidelines for summarizes the purpose-based X.509 CERT RR owner name guidelines for
use with S/MIME [17], SSL/TLS [13], and IPsec [14]: use with S/MIME [17], SSL/TLS [13], and IPsec [14]:
Scenario Owner name Scenario Owner name
------------------ ---------------------------------------------- ------------------ ----------------------------------------------
skipping to change at page 9, line 25 skipping to change at page 9, line 43
"postmaster.example.org". "postmaster.example.org".
TLS Certificate Hostname of the TLS server. TLS Certificate Hostname of the TLS server.
IPsec Certificate Hostname of the IPsec machine and/or, for IPv4 IPsec Certificate Hostname of the IPsec machine and/or, for IPv4
or IPv6 addresses, the fully qualified domain or IPv6 addresses, the fully qualified domain
name in the appropriate reverse domain. name in the appropriate reverse domain.
An alternate approach for IPsec is to store raw public keys [18]. An alternate approach for IPsec is to store raw public keys [18].
3.3. Content-based OpenPGP CERT RR Names 3.3. Content-Based OpenPGP CERT RR Names
OpenPGP signed keys (certificates) use a general character string OpenPGP signed keys (certificates) use a general character string
User ID [6]. However, it is recommended by OpenPGP that such names User ID [5]. However, it is recommended by OpenPGP that such names
include the RFC 2822 [8] email address of the party, as in "Leslie include the RFC 2822 [7] email address of the party, as in "Leslie
Example <Leslie@host.example>". If such a format is used, the CERT Example <Leslie@host.example>". If such a format is used, the CERT
ought to be under the standard translation of the email address into ought to be under the standard translation of the email address into
a domain name, which would be leslie.host.example in this case. If a domain name, which would be leslie.host.example in this case. If
no RFC 2822 name can be extracted from the string name, no specific no RFC 2822 name can be extracted from the string name, no specific
domain name is recommended. domain name is recommended.
If a user has more than one email address, the CNAME type can be used If a user has more than one email address, the CNAME type can be used
to reduce the amount of data stored in the DNS. Example: to reduce the amount of data stored in the DNS. For example:
$ORIGIN example.org. $ORIGIN example.org.
smith IN CERT PGP 0 0 <OpenPGP binary> smith IN CERT PGP 0 0 <OpenPGP binary>
john.smith IN CNAME smith john.smith IN CNAME smith
js IN CNAME smith js IN CNAME smith
3.4. Purpose-based OpenPGP CERT RR Names 3.4. Purpose-Based OpenPGP CERT RR Names
Applications that receive an OpenPGP packet containing encrypted or Applications that receive an OpenPGP packet containing encrypted or
signed data but do not know the email address of the sender will have signed data but do not know the email address of the sender will have
difficulties constructing the correct owner name and cannot use the difficulties constructing the correct owner name and cannot use the
content-based owner name guidelines. However, these clients commonly content-based owner name guidelines. However, these clients commonly
know the key fingerprint or the Key ID. The key ID is found in know the key fingerprint or the Key ID. The key ID is found in
OpenPGP packets, and the key fingerprint is commonly found in OpenPGP packets, and the key fingerprint is commonly found in
auxiliary data that may be available. In this case, use of an owner auxiliary data that may be available. In this case, use of an owner
name identical to the key fingerprint and the key ID expressed in name identical to the key fingerprint and the key ID expressed in
hexadecimal [16] is recommended. Example: hexadecimal [16] is recommended. For example:
$ORIGIN example.org. $ORIGIN example.org.
0424D4EE81A0E3D119C6F835EDA21E94B565716F IN CERT PGP ... 0424D4EE81A0E3D119C6F835EDA21E94B565716F IN CERT PGP ...
F835EDA21E94B565716F IN CERT PGP ... F835EDA21E94B565716F IN CERT PGP ...
B565716F IN CERT PGP ... B565716F IN CERT PGP ...
If the same key material is stored for several owner names, the use If the same key material is stored for several owner names, the use
of CNAME may help to avoid data duplication. Note that CNAME is not of CNAME may help avoid data duplication. Note that CNAME is not
always applicable, because it maps one owner name to the other for always applicable, because it maps one owner name to the other for
all purposes, which may be sub-optimal when two keys with the same all purposes, which may be sub-optimal when two keys with the same
Key ID are stored. Key ID are stored.
3.5. Owner names for IPKIX, ISPKI, IPGP, and IACPKIX 3.5. Owner Names for IPKIX, ISPKI, IPGP, and IACPKIX
These types are stored under the same owner names, both purpose- and These types are stored under the same owner names, both purpose- and
content-based, as the PKIX, SPKI, PGP and ACPKIX types. content-based, as the PKIX, SPKI, PGP, and ACPKIX types.
4. Performance Considerations 4. Performance Considerations
The Domain Name System (DNS) protocol was designed for small The Domain Name System (DNS) protocol was designed for small
transfers, typically below 512 octets. While larger transfers will transfers, typically below 512 octets. While larger transfers will
perform correctly and work is underway to make larger transfers more perform correctly and work is underway to make larger transfers more
efficient, it is still advisable at this time to make every efficient, it is still advisable at this time that every reasonable
reasonable effort to minimize the size of certificates stored within effort be made to minimize the size of certificates stored within the
the DNS. Steps that can be taken may include using the fewest DNS. Steps that can be taken may include using the fewest possible
possible optional or extension fields and using short field values optional or extension fields and using short field values for
for necessary variable length fields. necessary variable-length fields.
The RDATA field in the DNS protocol may only hold data of size 65535 The RDATA field in the DNS protocol may only hold data of size 65535
octets (64kb) or less. This means that each CERT RR MUST NOT contain octets (64kb) or less. This means that each CERT RR MUST NOT contain
more than 64kb of payload, even if the corresponding certificate or more than 64kb of payload, even if the corresponding certificate or
certificate revocation list is larger. This document addresses this certificate revocation list is larger. This document addresses this
by defining "indirect" data types for each normal type. by defining "indirect" data types for each normal type.
Deploying CERT RRs to support digitally signed e-mail change the Deploying CERT RRs to support digitally signed email changes the
access patterns of DNS lookups from per-domain to per-user. If access patterns of DNS lookups from per-domain to per-user. If
digitally signed e-mail, and a key/certificate lookup based on CERT digitally signed email and a key/certificate lookup based on CERT RRs
RRs, is deployed on a wide scale, this may lead to an increased DNS are deployed on a wide scale, this may lead to an increased DNS load,
load, with potential performance and cache effectiveness with potential performance and cache effectiveness consequences.
consequencess. Whether this load increase will be noticable or not Whether or not this load increase will be noticeable is not known.
is not known.
5. Contributors 5. Contributors
The majority of this document is copied verbatim from RFC 2538, by The majority of this document is copied verbatim from RFC 2538, by
Donald Eastlake 3rd and Olafur Gudmundsson. Donald Eastlake 3rd and Olafur Gudmundsson.
6. Acknowledgements 6. Acknowledgements
Thanks to David Shaw and Michael Graff for their contributions to Thanks to David Shaw and Michael Graff for their contributions to
earlier works that motivated, and served as inspiration for, this earlier works that motivated, and served as inspiration for, this
document. document.
This document was improved by suggestions and comments from Olivier This document was improved by suggestions and comments from Olivier
Dubuisson, Scott Hollenbeck, Russ Housley, Peter Koch, Olaf M. Dubuisson, Scott Hollenbeck, Russ Housley, Peter Koch, Olaf M.
Kolkman, Ben Laurie, Edward Lewis, John Loughney, Allison Mankin, Kolkman, Ben Laurie, Edward Lewis, John Loughney, Allison Mankin,
Douglas Otis, Marcos Sanz, Pekka Savola, Jason Sloderbeck, Samuel Douglas Otis, Marcos Sanz, Pekka Savola, Jason Sloderbeck, Samuel
Weiler, and Florian Weimer. No doubt the list is incomplete. We Weiler, Florian Weimer, and the IANA. No doubt the list is
apologize to anyone we left out. incomplete. We apologize to anyone we left out.
7. Security Considerations 7. Security Considerations
By definition, certificates contain their own authenticating By definition, certificates contain their own authenticating
signature. Thus, it is reasonable to store certificates in non- signatures. Thus, it is reasonable to store certificates in non-
secure DNS zones or to retrieve certificates from DNS with DNS secure DNS zones or to retrieve certificates from DNS with DNS
security checking not implemented or deferred for efficiency. The security checking not implemented or deferred for efficiency. The
results may be trusted if the certificate chain is verified back to a 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. known trusted key and this conforms with the user's security policy.
Alternatively, if certificates are retrieved from a secure DNS zone Alternatively, if certificates are retrieved from a secure DNS zone
with DNS security checking enabled and are verified by DNS security, with DNS security checking enabled and are verified by DNS security,
the key within the retrieved certificate may be trusted without the key within the retrieved certificate may be trusted without
verifying the certificate chain if this conforms with the user's verifying the certificate chain if this conforms with the user's
security policy. security policy.
If an organization chooses to issue certificates for its employees, If an organization chooses to issue certificates for its employees,
placing CERT RR's in the DNS by owner name, and if DNSSEC (with NSEC) placing CERT RR's in the DNS by owner name, and if DNSSEC (with NSEC)
is in use, it is possible for someone to enumerate all employees of is in use, it is possible for someone to enumerate all employees of
the organization. This is usually not considered desirable, for the the organization. This is usually not considered desirable, for the
same reason enterprise phone listings are not often publicly same reason that enterprise phone listings are not often publicly
published and are even mark confidential. published and are even marked confidential.
Using the URI type introduces another level of indirection that may Using the URI type introduces another level of indirection that may
open a new vulnerability. One method to secure that indirection is open a new vulnerability. One method of securing that indirection is
to include a hash of the certificate in the URI itself. to include a hash of the certificate in the URI itself.
If DNSSEC is used, then the non-existence of a CERT RR and, If DNSSEC is used, then the non-existence of a CERT RR and,
consequently, certificates or revocation lists can be securely consequently, certificates or revocation lists can be securely
asserted. Without DNSSEC, this is not possible. asserted. Without DNSSEC, this is not possible.
8. IANA Considerations 8. IANA Considerations
IANA needs to create a new registry for CERT RR, certificate types. The IANA has created a new registry for CERT RR: certificate types.
The initial contents of this registry is: The initial contents of this registry is: