An LDAP Schema for Kerberos KDC Information

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. 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.

The key words "MUST", "SHOULD", and "MAY" used in this document are to be interpreted as described in . The OIDs defined below are derived from TBD.OID: KRBSYN = TBD.OID.0 KRBATTR = TBD.OID.1 KRBOC = TBD.OID.2

The attributes and classes defined in this document are summarized below.

The following attributes are defined in this document: krbPrincipalName krbPrincipalAliases krbTicketMaxLife krbTicketMaxRenewal krbEncSaltTypes krbRealmName krbPrincipalRealm krbKeySet krbKeyVersion krbTicketPolicy krbExtraData krbPrincNamingAttr krbPrincContainer krbPwdPolicy krbLDAPURI Additionally, some of the attributes defined in LDAP Password Policy are required. 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?

The following object classes are defined in this document: krbKDCInfo krbPrincipal krbRealm

This section contains attribute definitions to be implemented by KDCs supporting this schema:

( 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 )

( KRBATTR.2 NAME 'krbPrincipalAliases' SUP krbPrincipalName ) 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.

( KRBATTR.3 NAME 'krbPrincStartTime' EQUALITY generalizedTimeMatch ORDERING generalizedTimeOrderingMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.24 SINGLE-VALUE ) This attribute impelents section 6.1.1.2 of the Information Model. It holds the date the principal becomes valid.

( KRBATTR.4 NAME 'krbPrincEndTime' EQUALITY generalizedTimeMatch ORDERING generalizedTimeOrderingMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.24 SINGLE-VALUE ) This attribute impelents section 6.1.1.3 of the Information Model. It holds the date the principal becomes invalid.

( KRBATTR.5 NAME 'krbTicketMaxLife' EQUALITY integerMatch ORDERING integerOrderingMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE ) This attribute implements section 6.1.1.11 of the Information Model. It holds the maximum ticket lifetime in seconds for a principal.

( KRBATTR.6 NAME 'krbTicketMaxRenewal' EQUALITY integerMatch ORDERING integerOrderingMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE ) 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.

( KRBATTR.7 NAME 'krbEncSaltTypes' EQUALITY caseIgnoreMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.15 ) 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. Note that sections 6.1.1.4 thru 6.1.1.10 are implemented using the LDAP Password Policy schema.

( KRBATTR.8 NAME 'krbRealmName' EQUALITY octetStringMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.40 )

( KRBATTR.9 NAME 'krbPrincipalRealm' DESC 'DN of krbRealm entry' SUP distinguishedName ) 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.

( KRBATTR.10 NAME 'krbKeyVersion' EQUALITY integerMatch ORDERING integerOrderingMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE ) This attribute implements section 6.2.1.1 of the Information Model. It stores the version number of the current key.

( KRBATTR.11 NAME 'krbKeySet' EQUALITY octetStringMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.40 ) 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 ASN.1 DER.

##### 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 ##### }

( KRBATTR.12 NAME 'krbTicketPolicy' EQUALITY integerMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.27 SINGLE-VALUE ) 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.

#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 } #}

( KRBATTR.13 NAME 'krbExtraData' EQUALITY octetStringMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.40 ) This attribute holds arbitrary data that may be needed by a particular implementation. The values are encoded in ASN.1 DER.

##### The format of the values for this attribute is explained below, ##### ExtraData ::= SEQUENCE { ##### tag [0] OCTET STRING, ##### data [1] OCTET STRING ##### } The following four attributes are outside the scope of the Information Model but may be useful in some deployments.

( KRBATTR.14 NAME 'krbPrincNamingAttr' EQUALITY objectIdentifierMatch SYNTAX 1.3.6.1.4.1.1466.115.121.1.38 SINGLE-VALUE ) This attribute records what attribute will be used to name newly created principal entries.

( KRBATTR.15 NAME 'krbPrincContainer' DESC 'DN of container entry for principals' SUP distinguishedName SINGLE-VALUE ) This attribute points to the container entry under which new principal entries will be created.

( KRBATTR.16 NAME 'krbPwdPolicy' DESC 'DN of password policy subentry' SUP distinguishedName SINGLE-VALUE ) 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.

( KRBATTR.17 NAME 'krbLDAPURI' DESC 'LDAP search parameters for locating principals' SUP labeledURI ) This attribute contains LDAP URIs that the KDC will search when locating principals. The URI values must conform to the syntax defined in . As a special case, the URI prefix "ldap:///" is taken to mean the current LDAP server.

This section contains class definitions to be implemented by KDCs supporting the schema.

( KRBOC.1 NAME 'krbKDCInfo' SUP top AUXILIARY MAY ( krbTicketMaxLife $ krbTicketMaxRenewal $ krbEncSaltTypes $ krbTicketPolicy $ krbKeySet $ krbKeyVersion ) )

( KRBOC.2 NAME 'krbPrincipal' SUP krbKDCInfo AUXILIARY MUST ( krbPrincipalName ) MAY ( krbPrincipalAliases $ krbPrincipalRealm $ krbPrincStartTime $ krbPrincEndTime $ krbExtraData ) )

( KRBOC.3 NAME 'krbRealm' SUP krbKDCInfo AUXILIARY MUST ( krbRealmName ) MAY ( krbPrincNamingAttr $ krbPrincContainer $ krbPwdPolicy $ krbLDAPURI ) ) 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.

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. 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.

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.

Section 6.1.1.2 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.

Section 6.1.1.3 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.

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 pwdStartTime attribute to a value greater than or equal to the pwdEndTime attribute.

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.

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.

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.

Section 6.1.1.8 of the Information Model. This corresponds to the pwdChangedTime attribute. If the KDC uses the LDAP Password Modify request then it does not need to reference or manipulate this attribute directly.

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.

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.

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.

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. In common practice the KDC and DSA have been colocated on a single host and communicated over a local LDAP IPC 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. 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.

In accordance with the following registrations are requested.

[[List of OIDs, registration template goes here...]]

[[List of Attribute and ObjectClass descriptors, template goes here...]]

Thanks to Love Hörnquist Åstrand from Apple Corp. for the initial feedback on this document.