rfc2538.txt		rfc4398.txt
	Network Working Group D. Eastlake		Network Working Group S. Josefsson
	Request for Comments: 2538 IBM		Request for Comments: 4398 March 2006
	Category: Standards Track O. Gudmundsson		Obsoletes: 2538
	TIS Labs		Category: Standards Track
	March 1999
	Storing Certificates in the Domain Name System (DNS)		Storing Certificates in the Domain Name System (DNS)
	Status of this Memo		Status of This Memo
	This document specifies an Internet standards track protocol for the		This document specifies an Internet standards track protocol for the
	Internet community, and requests discussion and suggestions for		Internet community, and requests discussion and suggestions for
	improvements. Please refer to the current edition of the "Internet		improvements. Please refer to the current edition of the "Internet
	Official Protocol Standards" (STD 1) for the standardization state		Official Protocol Standards" (STD 1) for the standardization state
	and status of this protocol. Distribution of this memo is unlimited.		and status of this protocol. Distribution of this memo is unlimited.
	Copyright Notice		Copyright Notice
	Copyright (C) The Internet Society (1999). All Rights Reserved.		Copyright (C) The Internet Society (2006).
	Abstract		Abstract
	Cryptographic public key 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.
	Table of Contents		Table of Contents
	Abstract...................................................1		1. Introduction ....................................................3
	1. Introduction............................................2		2. The CERT Resource Record ........................................3
	2. The CERT Resource Record................................2		2.1. Certificate Type Values ....................................4
	2.1 Certificate Type Values................................3		2.2. Text Representation of CERT RRs ............................6
	2.2 Text Representation of CERT RRs........................4		2.3. X.509 OIDs .................................................6
	2.3 X.509 OIDs.............................................4		3. Appropriate Owner Names for CERT RRs ............................7
	3. Appropriate Owner Names for CERT RRs....................5		3.1. Content-Based X.509 CERT RR Names ..........................8
	3.1 X.509 CERT RR Names....................................5		3.2. Purpose-Based X.509 CERT RR Names ..........................9
	3.2 PGP CERT RR Names......................................6		3.3. Content-Based OpenPGP CERT RR Names ........................9
	4. Performance Considerations..............................6		3.4. Purpose-Based OpenPGP CERT RR Names .......................10
	5. IANA Considerations.....................................7		3.5. Owner Names for IPKIX, ISPKI, IPGP, and IACPKIX ...........10
	6. Security Considerations.................................7		4. Performance Considerations .....................................11
	References.................................................8		5. Contributors ...................................................11
	Authors' Addresses.........................................9		6. Acknowledgements ...............................................11
	Full Copyright Notice.....................................10		7. Security Considerations ........................................12
			8. IANA Considerations ............................................12
			9. Changes since RFC 2538 .........................................13
			10. References ....................................................14
			10.1. Normative References .....................................14
			10.2. Informative References ...................................15
			Appendix A. Copying Conditions ...................................16
	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 PGP		are X.509 certificates/CRLs in the X.500 directory system or OpenPGP
	certificates/revocations used by PGP 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.		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 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 [RFC2119].		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 define in section 2.1		The type field is the certificate type as defined in Section 2.1
	below.		below.
	The algorithm field has the same meaning as the algorithm field in		The key tag field is the 16-bit value computed for the key embedded
	KEY and SIG RRs [RFC 2535] except that a zero algorithm field		in the certificate, using the RRSIG Key Tag algorithm described in
	indicates the algorithm is unknown to a secure DNS, which may simply		Appendix B of [12]. This field is used as an efficiency measure to
	be the result of the algorithm not having been standardized for		pick which CERT RRs may be applicable to a particular key. The key
	secure DNS.		tag can be calculated for the key in question, and then only CERT RRs
			with the same key tag need to be examined. Note that two different
			keys can have the same key tag. However, the key MUST be transformed
			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
			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
			zero and the tag field is meaningless and SHOULD be zero.
	The key tag field is the 16 bit value computed for the key embedded		The algorithm field has the same meaning as the algorithm field in
	in the certificate as specified in the DNSSEC Standard [RFC 2535].		DNSKEY and RRSIG RRs [12], except that a zero algorithm field
	This field is used as an efficiency measure to pick which CERT RRs		indicates that the algorithm is unknown to a secure DNS, which may
	may be applicable to a particular key. The key tag can be calculated		simply be the result of the algorithm not having been standardized
	for the key in question and then only CERT RRs with the same key tag		for DNSSEC [11].
	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		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 cert		2 SPKI SPKI certificate
	3 PGP PGP cert		3 PGP OpenPGP packet
	4-252 available for IANA assignment		4 IPKIX The URL of an X.509 data object
			5 ISPKI The URL of an SPKI certificate
			6 IPGP The fingerprint and URL of an OpenPGP packet
			7 ACPKIX Attribute Certificate
			8 IACPKIX The URL of an Attribute Certificate
			9-252 Available for IANA assignment
	253 URI URI private		253 URI URI private
	254 OID OID private		254 OID OID private
	255-65534 available for IANA assignment		255 Reserved
	65535 reserved		256-65279 Available for IANA assignment
			65280-65534 Experimental
			65535 Reserved
			These values represent the initial content of the IANA registry; see
			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 being defined by the IETF PKIX working group. The		to the profile defined by the IETF PKIX working group [8]. The
	certificate section will start with a one byte 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
			OID as a prefix.)
	The SPKI type is reserved to indicate a certificate formated as to be		The SPKI and ISPKI types are reserved to indicate the SPKI
	specified by the IETF SPKI working group.		certificate format [15], for use when the SPKI documents are moved
			from experimental status. The format for these two CERT RR types
			will need to be specified later.
	The PGP type indicates a Pretty Good Privacy certificate as described		The PGP type indicates an OpenPGP packet as described in [5] and its
	in RFC 2440 and its extensions and successors.		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
			transferable public keys as described in Section 10.1 of [5], but it
			MAY handle additional OpenPGP packets.
			The ACPKIX type indicates an Attribute Certificate format [9].
			The IPKIX and IACPKIX types indicate a URL that will serve the
			content that would have been in the "certificate, CRL, or URL" field
			of the corresponding type (PKIX or ACPKIX, respectively).
			The IPGP type contains both an OpenPGP fingerprint for the key in
			question, as well as a URL. The certificate portion of the IPGP CERT
			RR is defined as a one-octet fingerprint length, followed by the
			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 [RFC 2396] and the data after the null is the		a null-terminated URI [10], and the data after the null is the
	private format certificate itself. The URI SHOULD be such that a		private format certificate itself. The URI SHOULD be such that a
	retrieval from it will lead to documentation on the format of the		retrieval from it will lead to documentation on the format of the
	certificate. Recognition of private certificate types need not be		certificate. Recognition of private certificate types need not be
	based on URI equality but can use various forms of pattern matching		based on URI equality but can use various forms of pattern matching
	so that, for example, subtype or version information can also be		so that, for example, subtype or version information can also be
	encoded into the URI.		encoded into the URI.
	The OID private type indicates a private format certificate specified		The OID private type indicates a private format certificate specified
	by a an ISO OID prefix. The certificate section will start with a		by an ISO OID prefix. The certificate section will start with a
	one byte unsigned OID length and then a BER encoded OID indicating		one-octet unsigned OID length and then a BER-encoded OID indicating
	the nature of the remainder of the certificate section. This can be		the nature of the remainder of the certificate section. This can be
	an X.509 certificate format or some other format. X.509 certificates		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		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
	integer or as a mnemonic symbol as listed in section 2.1 above.		decimal integer or as a mnemonic symbol as listed in Section 2.1,
			above.
	The key tag field is represented as an unsigned integer.		The key tag field is represented as an unsigned decimal integer.
	The algorithm field is represented as an unsigned integer or a		The algorithm field is represented as an unsigned decimal integer or
	mnemonic symbol as listed in [RFC 2535].		a mnemonic symbol as listed in [12].
	The certificate / CRL portion is represented in base 64 and may be		The certificate/CRL portion is represented in base 64 [16] and may be
	divided up into any number of white space separated substrings, down		divided into any number of white-space-separated substrings, down to
	to single base 64 digits, which are concatenated to obtain the full		single base-64 digits, which are concatenated to obtain the 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:
	id-at-userCertificate		id-at-userCertificate
	= { joint-iso-ccitt(2) ds(5) at(4) 36 }		= { joint-iso-ccitt(2) ds(5) at(4) 36 }
	== 0x 03 55 04 24		== 0x 03 55 04 24
	id-at-cACertificate		id-at-cACertificate
	= { joint-iso-ccitt(2) ds(5) at(4) 37 }		= { joint-iso-ccitt(2) ds(5) at(4) 37 }
	== 0x 03 55 04 25		== 0x 03 55 04 25
	id-at-authorityRevocationList		id-at-authorityRevocationList
	= { joint-iso-ccitt(2) ds(5) at(4) 38 }		= { joint-iso-ccitt(2) ds(5) at(4) 38 }
	== 0x 03 55 04 26		== 0x 03 55 04 26
	skipping to change at page 5, line 26		skipping to change at page 7, line 26
	== 0x 03 55 04 27		== 0x 03 55 04 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 the use in DNS		Following some of the guidelines below may result in DNS names with
	names of characters that require DNS quoting which is to use a		characters that require DNS quoting as per Section 5.1 of RFC 1035
	backslash followed by the octal representation of the ASCII code for		[2].
	the character such as \000 for NULL.
	3.1 X.509 CERT RR Names		The choice of name under which CERT RRs are stored is important to
			clients that perform CERT queries. In some situations, the clients
			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
			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
			that uses X.509 certificates or the Key ID of an OpenPGP key.
	Some X.509 versions permit multiple names to be associated with		Therefore, two owner name guidelines are defined: content-based owner
	subjects and issuers under "Subject Alternate Name" and "Issuer		names and purpose-based owner names. A content-based owner name is
	Alternate Name". For example, x.509v3 has such Alternate Names with		derived from the content of the CERT RR data; for example, the
	an ASN.1 specification as follows:		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
			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
			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
			an incoming email.
			Implementations SHOULD use the purpose-based owner name guidelines
			described in this document and MAY use CNAME RRs at content-based
			owner names (or other names), pointing to the purpose-based owner
			name.
			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 DNS does not infer any relationship amongst CERT resource records
			based on similarities or differences of the DNS owner name(s) of CERT
			resource records. For example, if multiple labels are used when
			mapping from a CERT identifier to a domain name, then care must be
			taken in understanding wildcard record synthesis.
			3.1. Content-Based X.509 CERT RR Names
			Some X.509 versions, such as the PKIX profile of X.509 [8], permit
			multiple names to be associated with subjects and issuers under
			"Subject Alternative Name" and "Issuer Alternative Name". For
			example, the PKIX profile has such Alternate Names with an ASN.1
			specification as follows:
	GeneralName ::= CHOICE {		GeneralName ::= CHOICE {
	otherName [0] INSTANCE OF OTHER-NAME,		otherName [0] OtherName,
	rfc822Name [1] IA5String,		rfc822Name [1] IA5String,
	dNSName [2] IA5String,		dNSName [2] IA5String,
	x400Address [3] EXPLICIT OR-ADDRESS.&Type,		x400Address [3] ORAddress,
	directoryName [4] EXPLICIT 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 should 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 should be used.		inverse domain name ought to be used.
	(3) If neither of the above it 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 should 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 should be treated as described for PGP names in		included, then it ought to be treated as described below for
	3.2 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)
	should be mapped into a domain name as specified in RFC 2247.		ought to be mapped into a domain name as specified in [4].
	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		Example 1: An X.509v3 certificate is issued to /CN=John Doe /DC=Doe/
	Hacker/L=Basingstoke/O=Widget Inc/C=GB/ with Subject Alternate names		DC=com/DC=xy/O=Doe Inc/C=XY/ with Subject Alternative Names of (a)
	of (a) domain name widget.foo.example, (b) IPv4 address		string "John (the Man) Doe", (b) domain name john-doe.com, and (c)
	10.251.13.201, and (c) string "James Hacker		URI <https://www.secure.john-doe.com:8080/>. The storage locations
	<hacker@mail.widget.foo.example>". Then the storage locations
	recommended, in priority order, would be		recommended, in priority order, would be
	(1) widget.foo.example,		1. john-doe.com,
	(2) 201.13.251.10.in-addr.arpa, and		2. www.secure.john-doe.com, and
	(3) hacker.mail.widget.foo.example.		3. Doe.com.xy.
	3.2 PGP CERT RR Names		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)
			domain name widget.foo.example, (b) IPv4 address 10.251.13.201, and
			(c) string "James Hacker <hacker@mail.widget.foo.example>". The
			storage locations recommended, in priority order, would be
	PGP signed keys (certificates) use a general character string User ID		1. widget.foo.example,
	[RFC 2440]. However, it is recommended by PGP that such names include		2. 201.13.251.10.in-addr.arpa, and
	the RFC 822 email address of the party, as in "Leslie Example		3. hacker.mail.widget.foo.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		3.2. Purpose-Based X.509 CERT RR Names
	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		Due to the difficulty for clients that do not already possess a
	recommended.		certificate to reconstruct the content-based owner name,
			purpose-based owner names are recommended in this section.
			Recommendations for purpose-based owner names vary per scenario. The
			following table summarizes the purpose-based X.509 CERT RR owner name
			guidelines for use with S/MIME [17], SSL/TLS [13], and IPsec [14]:
			Scenario Owner name
			------------------ ----------------------------------------------
			S/MIME Certificate Standard translation of an RFC 2822 email
			address. Example: An S/MIME certificate for
			"postmaster@example.org" will use a standard
			hostname translation of the owner name,
			"postmaster.example.org".
			TLS Certificate Hostname of the TLS server.
			IPsec Certificate Hostname of the IPsec machine and/or, for IPv4
			or IPv6 addresses, the fully qualified domain
			name in the appropriate reverse domain.
			An alternate approach for IPsec is to store raw public keys [18].
			3.3. Content-Based OpenPGP CERT RR Names
			OpenPGP signed keys (certificates) use a general character string
			User ID [5]. However, it is recommended by OpenPGP that such names
			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
			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
			no RFC 2822 name can be extracted from the string name, no specific
			domain name is recommended.
			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. For example:
			$ORIGIN example.org.
			smith IN CERT PGP 0 0 <OpenPGP binary>
			john.smith IN CNAME smith
			js IN CNAME smith
			3.4. Purpose-Based OpenPGP CERT RR Names
			Applications that receive an OpenPGP packet containing encrypted or
			signed data but do not know the email address of the sender will have
			difficulties constructing the correct owner name and cannot use the
			content-based owner name guidelines. However, these clients commonly
			know the key fingerprint or the Key ID. The key ID is 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
			name identical to the key fingerprint and the key ID expressed in
			hexadecimal [16] is recommended. For example:
			$ORIGIN example.org.
			0424D4EE81A0E3D119C6F835EDA21E94B565716F IN CERT PGP ...
			F835EDA21E94B565716F IN CERT PGP ...
			B565716F IN CERT PGP ...
			If the same key material is stored for several owner names, the use
			of CNAME may help avoid data duplication. Note that CNAME is not
			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
			Key ID are stored.
			3.5. Owner Names for IPKIX, ISPKI, IPGP, and IACPKIX
			These types are stored under the same owner names, both purpose- and
			content-based, as the PKIX, SPKI, PGP, and ACPKIX types.
	4. Performance Considerations		4. Performance Considerations
	Current Domain Name System (DNS) implementations are optimized for		The Domain Name System (DNS) protocol was designed for small
	small transfers, typically not more than 512 bytes including		transfers, typically below 512 octets. While larger transfers will
	overhead. While larger transfers will perform correctly and work is		perform correctly and work is underway to make larger transfers more
	underway to make larger transfers more efficient, it is still		efficient, it is still advisable at this time that every reasonable
	advisable at this time to make every reasonable effort to minimize		effort be made to minimize the size of certificates stored within the
	the size of certificates stored within the DNS. Steps that can be		DNS. Steps that can be taken may include using the fewest possible
	taken may include using the fewest possible optional or extensions		optional or extension fields and using short field values for
	fields and using short field values for variable length fields that		necessary variable-length fields.
	must be included.
	5. IANA Considerations		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
			more than 64kb of payload, even if the corresponding certificate or
			certificate revocation list is larger. This document addresses this
			by defining "indirect" data types for each normal type.
	Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can		Deploying CERT RRs to support digitally signed email changes the
	only be assigned by an IETF standards action [RFC 2434] (and this		access patterns of DNS lookups from per-domain to per-user. If
	document assigns 0x0001 through 0x0003 and 0x00FD and 0x00FE).		digitally signed email and a key/certificate lookup based on CERT RRs
	Certificate types 0x0100 through 0xFEFF are assigned through IETF		are deployed on a wide scale, this may lead to an increased DNS load,
	Consensus [RFC 2434] based on RFC documentation of the certificate		with potential performance and cache effectiveness consequences.
	type. The availability of private types under 0x00FD and 0x00FE		Whether or not this load increase will be noticeable is not known.
	should satisfy most requirements for proprietary or private types.
	6. Security Considerations		5. Contributors
			The majority of this document is copied verbatim from RFC 2538, by
			Donald Eastlake 3rd and Olafur Gudmundsson.
			6. Acknowledgements
			Thanks to David Shaw and Michael Graff for their contributions to
			earlier works that motivated, and served as inspiration for, this
			document.
			This document was improved by suggestions and comments from Olivier
			Dubuisson, Scott Hollenbeck, Russ Housley, Peter Koch, Olaf M.
			Kolkman, Ben Laurie, Edward Lewis, John Loughney, Allison Mankin,
			Douglas Otis, Marcos Sanz, Pekka Savola, Jason Sloderbeck, Samuel
			Weiler, and Florian Weimer. No doubt the list is incomplete. We
			apologize to anyone we left out.
			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-secure		signatures. Thus, it is reasonable to store certificates in
	DNS zones or to retrieve certificates from DNS with DNS security		non-secure DNS zones or to retrieve certificates from DNS with DNS
	checking not implemented or deferred for efficiency. The results MAY		security checking not implemented or deferred for efficiency. The
	be trusted if the certificate chain is verified back to a known		results may be trusted if the certificate chain is verified back to a
	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.
	CERT RRs are not used in connection with securing the DNS security		If an organization chooses to issue certificates for its employees,
	additions so there are no security considerations related to CERT RRs		placing CERT RRs in the DNS by owner name, and if DNSSEC (with NSEC)