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<?xml version="1.0"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd" [
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<!ENTITY ppolicy PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml3/reference.I-D.behera-ldap-password-policy.xml'>
<!ENTITY kdcmodel PUBLIC '' 'http://xml.resource.org/public/rfc/bibxml3/reference.I-D.draft-ietf-krb-wg-kdc-model-06.xml'>
]>
<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
<?rfc symrefs="yes" ?>
<rfc
ipr="trust200902"
category="info"
docName="draft-chu-ldap-kdc-schema-01">
<front>
<title abbrev="LDAP KDC Schema">
An LDAP Schema for Kerberos KDC Information
</title>
<author initials="H." fullname="Howard Chu" surname="Chu">
<organization>Symas Corp.</organization>
<address>
<postal>
<street>18740 Oxnard Street, Suite 313A</street>
<city>Tarzana</city>
<region>California</region>
<code>91356</code>
<country>US</country>
</postal>
<phone>+1 818 757-7087</phone>
<email>hyc@symas.com</email>
<uri>http://www.symas.com</uri>
</address>
</author>
<author initials="S." fullname="Simo Sorce" surname="Sorce">
<organization>Red Hat, Inc.</organization>
<address>
<postal>
<street>140 Broadway, 24th Floor</street>
<city>New York</city>
<region>New York</region>
<code>10005</code>
<country>US</country>
</postal>
<phone>+1 212 344-2501</phone>
<email>ssorce@redhat.com</email>
<uri>http://www.redhat.com</uri>
</address>
</author>
<date month="October" year="2009"/>
<abstract>
<t>This document describes an <xref target="RFC4511">LDAP</xref> schema for implementing the
<xref target="RFC4120">Kerberos 5</xref>
<xref target="I-D.ietf-krb-wg-kdc-model">KDC Information Model</xref>.
It also defines additional elements which are not covered by the Information Model,
but are already in common use.
</t>
</abstract>
</front>
<middle>
<section anchor="background" title="Background and Motivation">
<t>Both Kerberos and LDAP are frequently used separately for
distributed authentication. They can also be used in combination,
but typically their user databases remained separate. This distinction
in databases causes unnecessary duplication of data and administration
overhead. As such it is desirable for both systems to share a single
database. Since the LDAP data model is more general it is most
appropriate to store the Kerberos data in LDAP.</t>
<t>A number of Kerberos implementations already have support for
using LDAP as their KDC backing store. However, each implementation
uses its own schema, and the multiple schemas are mutually
incompatible. For the sake of interoperability and administrative
ease, it is important to define a single standard schema that can
be used uniformly by all Kerberos KDC implementations and interoperates
with existing LDAP specifications.</t>
</section>
<section anchor="general" title="General Issues">
<section anchor="genera.terms" title="Terminology">
<t>The key words "MUST", "SHOULD", and "MAY" used in this document
are to be interpreted as described in
<xref target="RFC2119"/>.</t>
<t>The OIDs defined below are derived from
<!-- joint-iso-ccitt(2) country(16) us(840) organization(1) Novell(113719) applications(1) kerberos(301)
and
iso(1) member-body(2) United States(840) mit (113554) infosys(1) ldap(4) attributeTypes(1) Kerberos(6) -->
TBD.OID:<vspace blankLines="0"/>
KRBSYN = TBD.OID.0<vspace blankLines="0"/>
KRBATTR = TBD.OID.1<vspace blankLines="0"/>
KRBOC = TBD.OID.2<vspace blankLines="0"/>
</t>
</section>
<section title="Schema">
<t>The attributes and classes defined in this document are summarized
below.</t>
<section anchor="general.attrs" title="Attributes">
<t>The following attributes are defined in this document:
<list style="empty">
<t>
krbPrincipalName<vspace blankLines="0"/>
krbPrincipalAliases<vspace blankLines="0"/>
krbPrincStartTime<vspace blankLines="0"/>
krbPrincEndTime<vspace blankLines="0"/>
krbTicketMaxLife<vspace blankLines="0"/>
krbTicketMaxRenewal<vspace blankLines="0"/>
krbEncSaltTypes<vspace blankLines="0"/>
krbRealmName<vspace blankLines="0"/>
krbPrincipalRealm<vspace blankLines="0"/>
krbKeySet<vspace blankLines="0"/>
krbKeyVersion<vspace blankLines="0"/>
krbTicketPolicy<vspace blankLines="0"/>
krbExtraData<vspace blankLines="0"/>
krbPrincNamingAttr<vspace blankLines="0"/>
krbPrincContainer<vspace blankLines="0"/>
krbPwdPolicy<vspace blankLines="0"/>
krbLDAPURI<vspace blankLines="0"/>
</t>
</list>
Additionally, some of the attributes defined in
<xref target="I-D.behera-ldap-password-policy">LDAP Password Policy
</xref> are required.
</t>
<t>Note: The MIT/Novell schema includes a number of elements for storing the KDC configuration
in LDAP. The Information Model doesn't cover these aspects, so I've omitted them for now.
Do we need to add them?</t>
</section>
<section anchor="general.classes" title="Object Classes">
<t>The following object classes are defined in this document:
<list style="empty">
<t>
krbKDCInfo<vspace blankLines="0"/>
krbPrincipal<vspace blankLines="0"/>
krbRealm<vspace blankLines="0"/>
</t>
</list>
</t>
</section>
</section>
</section>
<section anchor="attrdefs" title="Attribute Definitions">
<t>This section contains attribute definitions to be implemented
by KDCs supporting this schema:
<figure title="">
<artwork>
( KRBATTR.1
NAME 'krbPrincipalName'
DESC 'Canonical principal name'
EQUALITY caseExactIA5Match
SUBSTR caseExactSubstringsMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.26
SINGLE-VALUE )
</artwork>
</figure>
<figure>
<artwork>
( KRBATTR.2
NAME 'krbPrincipalAliases'
SUP krbPrincipalName )
</artwork>
</figure>
These attributes implement section 6.1.1.1 of the Information Model. The
krbPrincipalName attribute contains the canonical name of the principal.
Any aliases are stored in the krbPrincipalAliases attribute. Since the
krbPrincipalAliases attribute is a subtype of the krbPrincipalName
attribute, a search on krbPrincipalName will also search the aliases.
</t>
<t>
<figure>
<artwork>
( KRBATTR.3
NAME 'krbPrincStartTime'
EQUALITY generalizedTimeMatch
ORDERING generalizedTimeOrderingMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.24
SINGLE-VALUE )
</artwork></figure>
This attribute implements section 6.1.1.2 of the Information Model.
It holds the date the principal becomes valid.
</t>
<t>
<figure>
<artwork>
( KRBATTR.4
NAME 'krbPrincEndTime'
EQUALITY generalizedTimeMatch
ORDERING generalizedTimeOrderingMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.24
SINGLE-VALUE )
</artwork></figure>
This attribute implements section 6.1.1.3 of the Information Model.
It holds the date the principal becomes invalid.
</t>
<t>
<figure>
<artwork>
( KRBATTR.5
NAME 'krbTicketMaxLife'
EQUALITY integerMatch
ORDERING integerOrderingMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
SINGLE-VALUE )
</artwork>
</figure>
This attribute implements section 6.1.1.11 of the Information Model.
It holds the maximum ticket lifetime in seconds for a principal.
</t>
<t>
<figure>
<artwork>
( KRBATTR.6
NAME 'krbTicketMaxRenewal'
EQUALITY integerMatch
ORDERING integerOrderingMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
SINGLE-VALUE )
</artwork>
</figure>
This attribute implements section 6.1.1.12 of the Information Model.
It holds the maximum time in seconds a ticket may be renewed for.
</t>
<t>
<figure>
<artwork>
( KRBATTR.7
NAME 'krbEncSaltTypes'
EQUALITY caseIgnoreMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.15 )
</artwork>
</figure>
This attribute implements section 6.1.1.13 of the Information Model.
Holds the allowed encryption/salt type combinations for this principal.
If empty or absent any combination supported by the implementation is allowed.
Values are stored in the form of key:salt strings.
The supported encryption types are mentioned in
<xref target="RFC3961"/>. The supported salt types are:
<list style='empty'>
<t>NORMAL</t>
<t>V4</t>
<t>NOREALM</t>
<t>ONLYREALM</t>
<t>SPECIAL</t>
<t>AFS3</t>
</list>
Example: <spanx style='verb'>des-cbc-crc:normal</spanx>
<vspace blankLines='1'/>
Note that sections 6.1.1.4 thru 6.1.1.10 of the Information Model
are implemented using the LDAP Password Policy schema.
</t>
<t>
<figure>
<artwork>
( KRBATTR.8
NAME 'krbRealmName'
EQUALITY octetStringMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.40 )
</artwork>
</figure>
<figure>
<artwork>
( KRBATTR.9
NAME 'krbPrincipalRealm'
DESC 'DN of krbRealm entry'
SUP distinguishedName )
</artwork>
</figure>
These attributes provide information about the current realm. They provide
the minimal set of information required to implement section 6.1.3 of the
Information Model.
</t>
<t>
<figure>
<artwork>
( KRBATTR.10
NAME 'krbKeyVersion'
EQUALITY integerMatch
ORDERING integerOrderingMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
SINGLE-VALUE )
</artwork>
</figure>
This attribute implements section 6.2.1.1 of the Information Model.
It stores the version number of the current key.
</t>
<t>
<figure>
<artwork>
( KRBATTR.11
NAME 'krbKeySet'
EQUALITY octetStringMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.40 )
</artwork>
</figure>
This attribute implements sections 6.3.1.1 thru 6.3.1.4 of the Information Model.
Sections 6.3.1.5 thru 6.3.1.7 are implemented using the LDAP Password Policy schema.
This attribute holds the principal's keys optionally encrypted with the
Master Key. The attribute is encoded using <xref target="X.680">ASN.1</xref>
<xref target="X.690">DER</xref>.
<figure><artwork>
##### The format of the value for this attribute is explained below,
##### KrbKeySet ::= SEQUENCE {
##### kvno [0] UInt32,
##### mkvno [1] UInt32 OPTIONAL,
##### keys [2] SEQUENCE OF KrbKey,
##### ...
##### }
#####
##### KrbKey ::= SEQUENCE {
##### salt [0] KrbSalt OPTIONAL,
##### key [1] EncryptionKey,
##### s2kparams [2] OCTET STRING OPTIONAL,
##### ...
##### }
#####
##### KrbSalt ::= SEQUENCE {
##### type [0] Int32,
##### salt [1] OCTET STRING OPTIONAL
##### }
#####
##### EncryptionKey ::= SEQUENCE {
##### keytype [0] Int32,
##### keyvalue [1] OCTET STRING
##### }
</artwork></figure></t>
<t>
<figure>
<artwork>
( KRBATTR.12
NAME 'krbTicketPolicy'
EQUALITY integerMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
SINGLE-VALUE )
</artwork>
</figure>
This attribute is related to section 6.4 of the Information Model. It
defines the flags that a user is allowed or required to use in a ticket
request.
<figure><artwork>
#krb5KDCFlagsSyntax SYNTAX ::= {
# WITH SYNTAX INTEGER
#-- initial(0), -- require as-req
#-- forwardable(1), -- may issue forwardable
#-- proxiable(2), -- may issue proxiable
#-- renewable(3), -- may issue renewable
#-- postdate(4), -- may issue postdatable
#-- server(5), -- may be server
#-- client(6), -- may be client
#-- invalid(7), -- entry is invalid
#-- require-preauth(8), -- must use preauth
#-- change-pw(9), -- change password service
#-- require-hwauth(10), -- must use hwauth
#-- ok-as-delegate(11), -- as in TicketFlags
#-- user-to-user(12), -- may use user-to-user auth
#-- immutable(13) -- may not be deleted
# ID { 1.3.6.1.4.1.5322.10.0.1 }
#}
</artwork></figure>
</t>
<t>
<figure>
<artwork>
( KRBATTR.13
NAME 'krbExtraData'
EQUALITY octetStringMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.40 )
</artwork></figure>
This attribute holds arbitrary data that may be needed by a particular
implementation. The values are encoded in ASN.1 DER.
<figure><artwork>
##### The format of the values for this attribute is explained below,
##### ExtraData ::= SEQUENCE {
##### tag [0] OCTET STRING,
##### data [1] OCTET STRING
##### }
</artwork></figure>
</t>
<t>
The following four attributes are outside the scope of the Information Model
but may be useful in some deployments.
<figure>
<artwork>
( KRBATTR.14
NAME 'krbPrincNamingAttr'
EQUALITY objectIdentifierMatch
SYNTAX 1.3.6.1.4.1.1466.115.121.1.38
SINGLE-VALUE )
</artwork></figure>
This attribute records what attribute will be used to name
newly created principal entries.
<figure>
<artwork>
( KRBATTR.15
NAME 'krbPrincContainer'
DESC 'DN of container entry for principals'
SUP distinguishedName
SINGLE-VALUE )
</artwork></figure>
This attribute points to the container entry under which
new principal entries will be created.
<figure>
<artwork>
( KRBATTR.16
NAME 'krbPwdPolicy'
DESC 'DN of password policy subentry'
SUP distinguishedName
SINGLE-VALUE )
</artwork></figure>
This attribute points to the LDAP password policy subentry
containing the policy that should be applied to Kerberos principals.
Note that in LDAP servers with full subentry support, the subentry's
subtree search specification defines what entries the subentry applies
to, so this attribute is unnecessary; it is provided merely for
informational purposes.
<figure>
<artwork>
( KRBATTR.17
NAME 'krbLDAPURI'
DESC 'LDAP search parameters for locating principals'
SUP labeledURI )
</artwork></figure>
This attribute contains LDAP URIs that the KDC will search when
locating principals. The URI values must conform to the syntax
defined in <xref target="RFC4516"/>. As a special case, the URI
prefix "ldap:///" is taken to mean the current LDAP server.
</t>
</section>
<section anchor="classdefs" title="Class Definitions">
<t>This section contains class definitions to be implemented by KDCs
supporting the schema.</t>
<t>
<figure>
<artwork>
( KRBOC.1 NAME 'krbKDCInfo' SUP top AUXILIARY
MAY ( krbTicketMaxLife $ krbTicketMaxRenewal $
krbEncSaltTypes $ krbTicketPolicy $
krbKeySet $ krbKeyVersion ) )
</artwork>
</figure>
<figure>
<artwork>
( KRBOC.2 NAME 'krbPrincipal' SUP krbKDCInfo AUXILIARY
MUST ( krbPrincipalName )
MAY ( krbPrincipalAliases $ krbPrincipalRealm $
krbPrincStartTime $ krbPrincEndTime $
krbExtraData ) )
</artwork>
</figure>
</t>
<t>
<figure>
<artwork>
( KRBOC.3 NAME 'krbRealm' SUP krbKDCInfo AUXILIARY
MUST ( krbRealmName )
MAY ( krbPrincNamingAttr $ krbPrincContainer $
krbPwdPolicy $ krbLDAPURI ) )
</artwork>
</figure>
Note that in a krbRealm object the krbKeySet and krbKeyVersion
attributes actually reflect the Master key for the realm. In this
case the krbKeySet's mkvno field and all other optional fields
are omitted.
</t>
</section>
<section anchor="impl" title="Implementation Details">
<t>Since the LDAP Password Policy is intimately involved in the
security mechanisms of this proposal, the directory should be treated
as more than just a passive data store. (The KDC can certainly read
the policy attributes and evaluate them itself, but that would mean
needlessly duplicating all of the functionality that is already
implemented in the directory server.) This means that for every
Kerberos authentication being serviced, a corresponding LDAP
operation must also be performed, in order to allow the password
policy mechanisms to operate.</t>
<t>The mechanism outlined here assumes that the plain LDAP credentials
and the Kerberos credentials are unified (or at least synchronized). In
that case, for every incoming Kerberos authentication request, the KDC
can issue an LDAP Compare request using the known credentials of
the user and the LDAP Password Policy control. The result of the request
will carry any relevant error codes if the account is disabled, the
password is expired, or various other failures. If preauthentication is
in use and the request is invalid, a Compare with known invalid
credentials may be used to update the password policy state.</t>
<section title="Model Details">
<t>A number of data elements described in the Information Model are
delegated to the LDAP DSA for management. Details of their
usage are described here.</t>
<section title="principalIsDisabled">
<t>Section 6.1.1.4 of the Information Model.
If the KDC is using LDAP requests to operate the
Password Policy mechanism then it does not need to reference or manipulate
this attribute directly. Otherwise, this effect is controlled by setting
the krbPrincStartTime attribute to a value greater than or equal to the
krbPrincEndTime attribute.</t>
</section>
<section title="principalNumberOfFailedAuthenticationAttempts">
<t>Section 6.1.1.5 of the Information Model.
If the KDC is using LDAP requests to operate the
Password Policy mechanism then it does not need to reference or manipulate
this attribute directly. Otherwise, this value is obtained by counting the
number of values stored in the pwdFailureTime attribute.</t>
</section>
<section title="principalLastFailedAuthentication">
<t>Section 6.1.1.6 of the Information Model.
If the KDC is using LDAP requests to operate the
Password Policy mechanism then it does not need to reference or manipulate
this attribute directly. Otherwise, this value is obtained by retrieving the
values stored in the pwdFailureTime attribute and selecting the most recent value.</t>
</section>
<section title="principalLastSuccessfulAuthentication">
<t>Section 6.1.1.7 of the Information Model. This corresponds to the
pwdLastSuccess attribute.
If the KDC is using LDAP requests to operate the
Password Policy mechanism then it does not need to reference or manipulate
this attribute directly.</t>
</section>
<section title="principalLastCredentialChangeTime">
<t>Section 6.1.1.8 of the Information Model. This corresponds to the
pwdChangedTime attribute.
If the KDC uses the LDAP <xref target="RFC3062">Password Modify</xref> request
then it does not need to reference or manipulate
this attribute directly.</t>
</section>
<section title="principalCreateTime">
<t>Section 6.1.1.9 of the Information Model. This corresponds to the
createTimestamp attribute.
The KDC does not need to reference or manipulate this attribute directly.</t>
</section>
<section title="principalModifyTime">
<t>Section 6.1.1.10 of the Information Model. This corresponds to the
modifyTimestamp attribute.
The KDC does not need to reference or manipulate this attribute directly.</t>
</section>
<section title="keyNotUsedAfter">
<t>Section 6.3.1.5 of the Information Model. This corresponds to the
pwdEndTime attribute. If the KDC is using LDAP requests to operate the
Password Policy mechanism then it does not need to reference or manipulate
this attribute directly.</t>
</section>
<section title="keyNotUsedBefore">
<t>Section 6.3.1.6 of the Information Model. This corresponds to the
pwdStartTime attribute. If the KDC is using LDAP requests to operate the
Password Policy mechanism then it does not need to reference or manipulate
this attribute directly.</t>
</section>
<section title="keyIsDisabled">
<t>Section 6.3.1.7 of the Information Model.
If the KDC is using LDAP requests to operate the
Password Policy mechanism then it does not need to reference or manipulate
this attribute directly. Otherwise, this effect is controlled by setting
the pwdStartTime attribute to a value greater than or equal to the
pwdEndTime attribute.</t>
</section>
</section>
<section title="KeySet details">
<t>The krbKeySet attribute is multi-valued but it is expected that
it will usually only contain one value. During a password change operation
the KDC may choose to keep one previous value present to allow currently
active clients to continue to operate using the previous key. How long to
retain this old password is unspecified here. Note also that the LDAP
Password Policy mechanism already has provisions for password history
management, so the krbKeySet attribute should not be used for
long-term password history tracking.</t>
</section>
</section>
<section anchor="security" title="Security Considerations">
<t>This entire document is concerned with an implementation of a secure
distributed authentication mechanism. It should be understood that
the various keys used here are all sensitive pieces of data and must
be adequately protected using access controls and other mechanisms.
Likewise all communications between the KDC and DSA must be protected
whenever sensitive data is being referenced.</t>
<t>In common practice the KDC and DSA have been colocated on a
single host and communicated over a local
<xref target="I-D.chu-ldap-ldapi">LDAP IPC</xref> session. As such it was
implied that the host security was equivalent for both. If a KDC is
configured to use a remote DSA, the remote host should be
configured with at least the same level of security as the KDC host,
and a secure channel MUST be used for the LDAP session.</t>
<t>Storing the Master Key in the DSA makes it even more
crucial that the LDAP host, service, and data files be adequately
protected. Backups of the LDAP database should also be encrypted to
protect the integrity of any keys contained therein.</t>
</section>
<section title="IANA Considerations">
<t>In accordance with <xref target="RFC4520"/> the following registrations
are requested.</t>
<section title="Object Identifiers">
<t>[[List of OIDs, registration template goes here...]]</t>
</section>
<section title="LDAP Descriptors">
<t>[[List of Attribute and ObjectClass descriptors, template goes here...]]</t>
</section>
</section>
<section anchor="acks" title="Acknowledgements">
<t>Thanks to Love H&ouml;rnquist &Aring;strand
from Apple Corp. for the initial feedback on this document.</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119;
&rfc3062;
&rfc3961;
&rfc4120;
&rfc4511;
&rfc4516;
&rfc4520;
&ppolicy;
&kdcmodel;
<reference anchor="X.680">
<front>
<title>Abstract Syntax Notation One (ASN.1): Specification of basic notation</title>
<author>
<organization abbrev="ITU-T">
International Telecommunications Union</organization>
</author>
<date month="July" year="2002" />
</front>
<seriesInfo name="ITU-T Recommendation" value="X.680" />
</reference>
<reference anchor="X.690">
<front>
<title>Information Technology - ASN.1 encoding rules: Specification of Basic
Encoding Rules (BER), Canonical Encoding Rules (CER) and
Distinguished Encoding Rules (DER)</title>
<author>
<organization abbrev="ITU-T">
International Telecommunications Union</organization>
</author>
<date month="July" year="2002" />
</front>
<seriesInfo name="ITU-T Recommendation" value="X.690" />
</reference>
</references>
<references title="Informative References">
&ldapi;
</references>
</back>
</rfc>
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