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)
and securing the DNS itself. is in use, it is possible for someone to enumerate all employees of
the organization. This is usually not considered desirable, for the
same reason that enterprise phone listings are not often publicly
published and are even marked confidential.
References Using the URI type introduces another level of indirection that may
open a new vulnerability. One method of securing that indirection is
to include a hash of the certificate in the URI itself.
RFC 1034 Mockapetris, P., "Domain Names - Concepts and Facilities", If DNSSEC is used, then the non-existence of a CERT RR and,
STD 13, RFC 1034, November 1987. consequently, certificates or revocation lists can be securely
asserted. Without DNSSEC, this is not possible.
RFC 1035 Mockapetris, P., "Domain Names - Implementation and 8. IANA Considerations
Specifications", STD 13, RFC 1035, November 1987.
RFC 2119 Bradner, S., "Key words for use in RFCs to Indicate The IANA has created a new registry for CERT RR: certificate types.
Requirement Levels", BCP 14, RFC 2119, March 1997. The initial contents of this registry is:
RFC 2247 Kille, S., Wahl, M., Grimstad, A., Huber, R. and S. Decimal Type Meaning Reference
Sataluri, "Using Domains in LDAP/X.500 Distinguished ------- ---- ------- ---------
Names", RFC 2247, January 1998. 0 Reserved RFC 4398
1 PKIX X.509 as per PKIX RFC 4398
2 SPKI SPKI certificate RFC 4398
3 PGP OpenPGP packet RFC 4398
4 IPKIX The URL of an X.509 data object RFC 4398
5 ISPKI The URL of an SPKI certificate RFC 4398
6 IPGP The fingerprint and URL RFC 4398
of an OpenPGP packet
7 ACPKIX Attribute Certificate RFC 4398
8 IACPKIX The URL of an Attribute RFC 4398
Certificate
9-252 Available for IANA assignment
by IETF Standards action
253 URI URI private RFC 4398
254 OID OID private RFC 4398
255 Reserved RFC 4398
256-65279 Available for IANA assignment
by IETF Consensus
65280-65534 Experimental RFC 4398
65535 Reserved RFC 4398
RFC 2396 Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can
Resource Identifiers (URI): Generic Syntax", RFC 2396, only be assigned by an IETF standards action [6]. This document
August 1998. assigns 0x0001 through 0x0008 and 0x00FD and 0x00FE. Certificate
types 0x0100 through 0xFEFF are assigned through IETF Consensus [6]
based on RFC documentation of the certificate type. The availability
of private types under 0x00FD and 0x00FE ought to satisfy most
requirements for proprietary or private types.
RFC 2440 Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, The CERT RR reuses the DNS Security Algorithm Numbers registry. In
"OpenPGP Message Format", RFC 2240, November 1998. particular, the CERT RR requires that algorithm number 0 remain
reserved, as described in Section 2. The IANA will reference the
CERT RR as a user of this registry and value 0, in particular.
RFC 2434 Narten, T. and H. Alvestrand, "Guidelines for Writing an 9. Changes since RFC 2538
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
1. Editorial changes to conform with new document requirements,
including splitting reference section into two parts and
updating the references to point at latest versions, and to add
some additional references.
2. Improve terminology. For example replace "PGP" with "OpenPGP",
to align with RFC 2440.
3. In Section 2.1, clarify that OpenPGP public key data are binary,
not the ASCII armored format, and reference 10.1 in RFC 2440 on
how to deal with OpenPGP keys, and acknowledge that
implementations may handle additional packet types.
4. Clarify that integers in the representation format are decimal.
5. Replace KEY/SIG with DNSKEY/RRSIG etc, to align with DNSSECbis
terminology. Improve reference for Key Tag Algorithm
calculations.
6. Add examples that suggest use of CNAME to reduce bandwidth.
7. In Section 3, appended the last paragraphs that discuss
"content-based" vs "purpose-based" owner names. Add Section 3.2
for purpose-based X.509 CERT owner names, and Section 3.4 for
purpose-based OpenPGP CERT owner names.
8. Added size considerations.
9. The SPKI types has been reserved, until RFC 2692/2693 is moved
from the experimental status.
10. Added indirect types IPKIX, ISPKI, IPGP, and IACPKIX.
11. An IANA registry of CERT type values was created.
10. References
10.1. Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Kille, S., Wahl, M., Grimstad, A., Huber, R., and S. Sataluri,
"Using Domains in LDAP/X.500 Distinguished Names", RFC 2247,
January 1998.
[5] Callas, J., Donnerhacke, L., Finney, H., and R. Thayer,
"OpenPGP Message Format", RFC 2440, November 1998.
[6] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998. October 1998.
RFC 2535 Eastlake, D., "Domain Name System (DNS) Security [7] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
Extensions", RFC 2535, March 1999.
RFC 2459 Housley, R., Ford, W., Polk, W. and D. Solo, "Internet [8] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
X.509 Public Key Infrastructure Certificate and CRL Public Key Infrastructure Certificate and Certificate
Profile", RFC 2459, January 1999. Revocation List (CRL) Profile", RFC 3280, April 2002.
Authors' Addresses [9] Farrell, S. and R. Housley, "An Internet Attribute Certificate
Profile for Authorization", RFC 3281, April 2002.
Donald E. Eastlake 3rd [10] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
IBM Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
65 Shindegan Hill Road January 2005.
RR#1
Carmel, NY 10512 USA
Phone: +1-914-784-7913 (w) [11] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
+1-914-276-2668 (h) "DNS Security Introduction and Requirements", RFC 4033,
Fax: +1-914-784-3833 (w-fax) March 2005.
EMail: dee3@us.ibm.com
Olafur Gudmundsson [12] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
TIS Labs at Network Associates "Resource Records for the DNS Security Extensions", RFC 4034,
3060 Washington Rd, Route 97 March 2005.
Glenwood MD 21738
Phone: +1 443-259-2389 10.2. Informative References
EMail: ogud@tislabs.com
[13] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[14] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
[15] Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas, B.,
and T. Ylonen, "SPKI Certificate Theory", RFC 2693,
September 1999.
[16] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[17] Ramsdell, B., "Secure/Multipurpose Internet Mail Extensions
(S/MIME) Version 3.1 Message Specification", RFC 3851,
July 2004.
[18] Richardson, M., "A Method for Storing IPsec Keying Material in
DNS", RFC 4025, March 2005.
Appendix A. Copying Conditions
Regarding the portion of this document that was written by Simon
Josefsson ("the author", for the remainder of this section), the
author makes no guarantees and is not responsible for any damage
resulting from its use. The author grants irrevocable permission to
anyone to use, modify, and distribute it in any way that does not
diminish the rights of anyone else to use, modify, and distribute it,
provided that redistributed derivative works do not contain
misleading author or version information. Derivative works need not
be licensed under similar terms.
Author's Address
Simon Josefsson
EMail: simon@josefsson.org
Full Copyright Statement Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved. Copyright (C) The Internet Society (2006).
This document and translations of it may be copied and furnished to This document is subject to the rights, licenses and restrictions
others, and derivative works that comment on or otherwise explain it contained in BCP 78, and except as set forth therein, the authors
or assist in its implementation may be prepared, copied, published retain all their rights.
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 This document and the information contained herein are provided on an
revoked by the Internet Society or its successors or assigns. "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM 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.
This document and the information contained herein is provided on an Intellectual Property
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION Intellectual Property Rights or other rights that might be claimed to
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Acknowledgement
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Administrative Support Activity (IASA).
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