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Kerberos Working Group M. Swift
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Internet Draft University of WA
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Document: draft-ietf-krb-wg-kerberos-referrals-00.txt J. Brezak
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Category: Standards Track Microsoft
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Generating KDC Referrals to locate Kerberos realms
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This document is an Internet-Draft and is in full conformance with
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all provisions of Section 10 of RFC2026 [1].
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Internet-Drafts are working documents of the Internet Engineering
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Task Force (IETF), its areas, and its working groups. Note that
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other groups may also distribute working documents as Internet-
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Drafts. Internet-Drafts are draft documents valid for a maximum of
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six months and may be updated, replaced, or obsoleted by other
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documents at any time. It is inappropriate to use Internet- Drafts
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as reference material or to cite them other than as "work in
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The list of current Internet-Drafts can be accessed at
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http://www.ietf.org/ietf/1id-abstracts.txt
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The list of Internet-Draft Shadow Directories can be accessed at
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http://www.ietf.org/shadow.html.
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The draft documents a new method for a Kerberos Key Distribution
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Center (KDC) to respond to client requests for kerberos tickets when
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the client does not have detailed configuration information on the
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realms of users or services. The KDC will handle requests for
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principals in other realms by returning either a referral error or a
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cross-realm TGT to another realm on the referral path. The clients
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will use this referral information to reach the realm of the target
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principal and then receive the ticket.
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2. Conventions used in this document
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
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this document are to be interpreted as described in RFC-2119 [2].
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Swift Category - Standards Track 1
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KDC Referrals February 2001
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Current implementations of the Kerberos AS and TGS protocols, as
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defined in RFC 1510 [3], use principal names constructed from a
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known user or service name and realm. A service name is typically
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constructed from a name of the service and the DNS host name of the
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computer that is providing the service. Many existing deployments of
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Kerberos use a single Kerberos realm where all users and services
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would be using the same realm. However in an environment where there
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are multiple trusted Kerberos realms, the client needs to be able to
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determine what realm a particular user or service is in before
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making an AS or TGS request. Traditionally this requires client
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configuration to make this possible.
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When having to deal with multiple trusted realms, users are forced
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to know what realm they are in before they can obtain a ticket
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granting ticket (TGT) with an AS request. However, in many cases the
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user would like to use a more familiar name that is not directly
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related to the realm of their Kerberos principal name. A good
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example of this is an RFC-822 style email name. This document
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describes a mechanism that would allow a user to specify a user
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principal name that is an alias for the user's Kerberos principal
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name. In practice this would be the name that the user specifies to
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obtain a TGT from a Kerberos KDC. The user principal name no longer
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has a direct relationship with the Kerberos principal or realm. Thus
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the administrator is able to move the user's principal to other
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realms without the user having to know that it happened.
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Once a user has a TGT, they would like to be able to access services
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in any trusted Kerberos realm. To do this requires that the client
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be able to determine what realm the target service's host is in
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before making the TGS request. Current implementations of Kerberos
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typically have a table that maps DNS host names to corresponding
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Kerberos realms. In order for this to work on the client, each
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application canonicalizes the host name of the service by doing a
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DNS lookup followed by a reverse lookup using the returned IP
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address. The returned primary host name is then used in the
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construction of the principal name for the target service. In order
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for the correct realm to be added for the target host, the mapping
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table [domain_to_realm] is consulted for the realm corresponding to
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the DNS host name. The corresponding realm is then used to complete
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the target service principal name.
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This traditional mechanism requires that each client have very
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detailed configuration information about the hosts that are
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providing services and their corresponding realms. Having client
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side configuration information can be very costly from an
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administration point of view - especially if there are many realms
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and computers in the environment.
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Current implementations of Kerberos also have difficulty with
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services on hosts that can have multiple host names (multi-homed
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hosts). Traditionally, each host name would need to have a distinct
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principal and a corresponding key. An extreme example of this would
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be a Web server with multiple host names for each domain that it is
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Swift Category - Standards Track 2
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KDC Referrals February 2001
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supporting. Principal aliases allow multi-homed hosts to have a
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single Kerberos principal (with a single key) that can have
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identities for each distinct host name. This mechanism allows the
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Kerberos client to request a service ticket for the distinct
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hostname and allows the KDC to return a ticket for the single
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principal that the host is using. This canonical principal name
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allows the host to only have to manage a single key for all of the
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identities that it supports. In addition, the client only needs to
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know the realm of the canonical service name, not all of the
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This draft proposes a solution for these problems and simplifies
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administration by minimizing the configuration information needed on
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each computer using Kerberos. Specifically it describes a mechanism
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to allow the KDC to handle Canonicalization of names, provide for
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principal aliases for users and services and provide a mechanism for
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the KDC to determine the trusted realm authentication path by being
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able to generate referrals to other realms in order to locate
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To rectify these problems, this draft introduces three new kinds of
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1. AS ticket referrals, in which the client doesn't know which realm
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contains a user account.
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2. TGS ticket referrals, in which the client doesn't know which
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realm contains a server account.
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3. Cross realm shortcut referrals, in which the KDC chooses the next
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path on a referral chain
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4. Realm Organization Model
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This draft assumes that the world of principals is arranged on
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multiple levels: the realm, the enterprise, and the world. A KDC may
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issue tickets for any principal in its realm or cross-realm tickets
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for realms with which it has a direct trust relationship. The KDC
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also has access to a trusted name service that can resolve any name
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from within its enterprise into a realm. This trusted name service
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removes the need to use an untrusted DNS lookup for name resolution.
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For example, consider the following configuration, where lines
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indicate trust relationships:
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OFFICE.MS.COM NT.MS.COM
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In this configuration, all users in the MS.COM enterprise could have
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a principal name such as alice@MS.COM, with the same realm portion.
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In addition, servers at MS.COM should be able to have DNS host names
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from any DNS domain independent of what Kerberos realm their
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principal resides in.
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5.1 Service Principal Names
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The standard Kerberos model in RFC 1510 [3] gives each Kerberos
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principal a single name. However, if a service is reachable by
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several addresses, it is useful for a principal to have multiple
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names. Consider a service running on a multi-homed machine. Rather
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than requiring a separate principal and password for each name it
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exports, a single account with multiple names could be used.
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Multiple names are also useful for services in that clients need not
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perform DNS lookups to resolve a host name into a full DNS address.
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Instead, the service may have a name for each of its supported host
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names, including its IP address. Nonetheless, it is still convenient
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for the service to not have to be aware of all these names. Thus a
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new name may be added to DNS for a service by updating DNS and the
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KDC database without having to notify the service. In addition, it
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implies that these aliases are globally unique: they do not include
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a specifier dictating what realm contains the principal. Thus, an
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alias for a server is of the form "class/instance/name" and may be
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transmitted as any name type.
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5.2 Client Principal Names
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Similarly, a client account may also have multiple principal names.
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More useful, though, is a globally unique name that allows
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unification of email and security principal names. For example, all
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users at MS may have a client principal name of the form
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"joe@MS.COM" even though the principals are contained in multiple
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realms. This global name is again an alias for the true client
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principal name, which is indicates what realm contains the
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principal. Thus, accounts "alice" in the realm ntdev.MS.COM and
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"bob" in office.MS.COM may logon as "alice@MS.COM" and "bob@MS.COM".
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This requires a new client principal name type, as the AS-REQ
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message only contains a single realm field, and the realm portion of
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this name doesn't correspond to any Kerberos realm. Thus, the entire
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name "alice@MS.COM" is transmitted in the client name field of the
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AS-REQ message, with a name type of KRB-NT-ENTERPRISE-PRINCIPAL.
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KRB-NT-ENTERPRISE-PRINCIPAL 10
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5.3 Name Canonicalization
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In order to support name aliases, the Kerberos client must
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explicitly request the name-canonicalization KDC option (bit 15) in
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the ticket flags for the TGS-REQ. This flag indicates to the KDC
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that the client is prepared to receive a reply with a different
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client or server principal name than the request. Thus, the
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KDCOptions types is redefined as:
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KDCOptions ::= BIT STRING {
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name-canonicalize(15),
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The simplest form of ticket referral is for a user requesting a
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ticket using an AS-REQ. In this case, the client machine will send
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the AS request to a convenient trusted realm, either the realm of
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the client machine or the realm of the client name. In the case of
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the name Alice@MS.COM, the client may optimistically choose to send
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the request to MS.COM.
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The client will send the string "alice@MS.COM" in the client
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principal name field using the KRB-NT-ENTERPRISE-PRINCIPAL name type
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with the crealm set to MS.COM. The KDC will try to lookup the name
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in its local account database. If the account is present in the
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crealm of the request, it MUST return a KDC reply structure with the
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appropriate ticket. If the account is not present in the crealm
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specified in the request and the name-canonicalize flag in the
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KDCoptions is set, the KDC will try to lookup the entire name,
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Alice@MS.COM, using a name service. If this lookup is unsuccessful,
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it MUST return the error KDC_ERR_C_PRINCIPAL_UNKNOWN. If the lookup
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is successful, it MUST return an error KDC_ERR_WRONG_REALM (0x44)
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and in the error message the cname and crealm field MUST contain the
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client name and the true realm of the client. If the KDC contains
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the account locally, it MUST return a normal ticket. The client name
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and realm portions of the ticket and KDC reply message MUST be the
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client's true name in the realm, not the globally unique name.
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If the client receives a KDC_ERR_WRONG_REALM error, it will issue a
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new AS request with the same client principal name used to generate
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the first referral to the realm specified by the crealm field of the
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kerberos error message from the first request. This request MUST
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produce a valid AS response with a ticket for the canonical user
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name. The ticket MUST also include the ticket extension containing
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the TE-REFERRAL-DATA with the referred-names set to the name from
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Swift Category - Standards Track 5
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KDC Referrals February 2001
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the AS request. Any other error or referral will terminate the
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request and result in a failed AS request.
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The server referral mechanism is a bit more complex than the client
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referral mechanism. The primary problem is that the KDC must return
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a referral ticket rather than an error message, so it will include
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in the TGS response information about what realm contains the
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service. This is done by returning information about the server name
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in the pre-auth data field of the KDC reply.
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If the KDC resolves the server principal name into a principal in
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its realm, it may return a normal ticket. If the name-canonicalize
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flag in the KDCoptions is not set, then the KDC MUST only look up
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the name as a normal principal name. Otherwise, it MUST search all
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aliases as well. The server principal name in both the ticket and
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the KDC reply MUST be the true server principal name instead of one
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of the aliases. This frees the application server from needing to
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know about all its aliases.
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If the name-canonicalize flag in the KDCoptions is set and the KDC
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doesn't find the principal locally, the KDC can return a cross-realm
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ticket granting ticket to the next hop on the trust path towards a
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realm that may be able to resolve the principal name.
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If the KDC can determine the service principal's realm, it can
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return the server realm as ticket extension data. The ticket
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extension MUST be encrypted using the session key from the ticket,
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and the same etype as is used to protect the TGS reply body.
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The data itself is an ASN.1 encoded structure containing the
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server's realm, and if known, canonical principal name and alias
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names. The first name in the sequence is the canonical principal
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TE-REFERRAL-DATA ::= SEQUENCE {
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referred-server-realm[0] KERB-REALM
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referred-names[1] SEQUENCE OF
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PrincipalNames OPTIONAL
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The client can use this information to request a chain of cross-
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realm ticket granting tickets until it reaches the realm of the
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server, and can then expect to receive a valid service ticket.
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In order to facilitate cross-realm interoperability, a client SHOULD
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NOT send short names in TGS requests to the KDC. A short name is
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defined as a Kerberos name that includes a DNS name that is not
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fully qualified. The client MAY use forward DNS lookups to obtain
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Swift Category - Standards Track 6
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KDC Referrals February 2001
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the long name that corresponds to the user entered short name (the
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short name will be a prefix of the corresponding long name).
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The client may use the referred-names field to tell if it already
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has a ticket to the server in its ticket cache.
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The client can use this information to request a chain of cross-
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realm ticket granting tickets until it reaches the realm of the
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server, and can then expect to receive a valid service ticket.
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However an implementation should limit the number of referrals that
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it processes to avoid infinite referral loops. A suggested limit is
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5 referrals before giving up.
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8. Cross Realm Routing
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The current Kerberos protocol requires the client to explicitly
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request a cross-realm TGT for each pair of realms on a referral
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chain. As a result, the client machines need to be aware of the
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trust hierarchy and of any short-cut trusts (those that aren't
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parent-child trusts). This requires more configurations on the
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client. Instead, the client should be able to request a TGT to the
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target realm from each realm on the route. The KDC will determine
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the best path for the client and return a cross-realm TGT. The
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client has to be aware that a request for a cross-realm TGT may
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return a TGT for a realm different from the one requested.
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9. Security Considerations
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The original Kerberos specification stated that the server principal
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name in the KDC reply was the same as the server name in the
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request. These protocol changes break that assumption, so the client
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may be vulnerable to a denial of service attack by an attacker that
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replays replies from previous requests. It can verify that the
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request was one of its own by checking the client-address field or
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authtime field, though, so the damage is limited and detectable.
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For the AS exchange case, it is important that the logon mechanism
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not trust a name that has not been used to authenticate the user.
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For example, the name that the user enters as part of a logon
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exchange may not be the name that the user authenticates as, given
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that the KDC_ERR_WRONG_REALM error may have been returned. The
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relevant Kerberos naming information for logon (if any), is the
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client name and client realm in the service ticket targeted at the
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workstation that was obtained using the user's initial TGT.
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How the client name and client realm is mapped into a local account
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for logon is a local matter, but the client logon mechanism MUST use
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additional information such as the client realm and/or authorization
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attributes from the service ticket presented to the workstation by
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the user, when mapping the logon credentials to a local account on
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Swift Category - Standards Track 7
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This section contains issues and suggestions that need to be
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incorporated into this draft. From Ken Raeburn [raeburn@mit.edu]:
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1) No means to do name canonicalization if you're not
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authenticating. Is it okay to require credentials in order to do
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canonicalization? If so, how about this: Send a TGS_REQ for the
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service name you have. If you get back a TGS_REP for a service,
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great; pull out the name and throw out the credentials. If you
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get back a TGS_REP for a TGT service, ask again in the specified
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realm. If you get back a KRB_ERROR because policy prohibits you
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from authenticating to that service, we can add to the
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specification that the {realm,sname} in the KRB_ERROR must be the
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canonical name, and the checksum must be used. As long as the
480
checksum is present, it's still a secure exchange with the KDC.
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If we have to be able to do name canonicalization without any
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sort of credentials, either client-side (tickets) or server-side
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(tickets automatically acquired via service key), I think we just
485
lose. But maybe GSSAPI should be changed if that's the case.
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2) Can't refer to another realm and specify a different service name
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to give to that realm's KDC. The local KDC can tell you a
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different service name or a different realm name, but not both.
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This comes up in the "gnuftp.raeburn.org CNAME ftp.gnu.org" type
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of case I've mentioned.
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Except ... the KDC-REP structure includes padata and ticket
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extensions fields that are extensible. We could add a required
495
value to one of them -- perhaps only in the case where you return
496
a TGT when not asked -- that contains signed information about
497
the principal name to ask for in the other realm. (It would have
498
to be required, otherwise a man-in-the-middle could make it go
499
away.) Signing would be done using the session key for the TGS.
501
3) Secure canonicalization of service name in AS_REQ. If the
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response is an AS_REP, we need a way to tell that the altered
503
server name wasn't a result of a MITM attack on the AS_REQ
504
message. Again, the KDC-REP extensible fields could have a new
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required value added when name canonicalization happens,
506
indicating what the original principal name (in the AS_REQ
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message) was, and signed using the same key as protects the
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AS_REP. If it doesn't match what the client requested, the
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messages were altered in transit.
511
4) Client name needs referral to another realm, and server name
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needs canonicalization of some sort. The above fixes wouldn't
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work for this case, and I'm not even sure which KDC should be
514
doing the canonicalization anyways.
517
The other-principal-name datum would probably look something like:
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Swift Category - Standards Track 8
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KDC Referrals February 2001
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PrincipalAndNonce ::= SEQUENCE {
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name[0] PrincipalName,
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nonce[1] INTEGER -- copied from KDC_REQ
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SignedPrincipal ::= SEQUENCE {
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name-and-nonce[0] PrincipalAndNonce,
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{PA,TE}-ORIGINAL-SERVER-PRINCIPAL ::= SignedPrincipal
541
{PA,TE}-REMOTE-SERVER-PRINCIPAL ::= SignedPrincipal
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with the checksum computed over the encoding of the 'name-and-nonce'
544
field, and appropriate PA- or TE- numbers assigned. I don't have a
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strong opinion on whether it'd be a pa-data or ticket extension;
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conceptually it seems like an abuse of either, but, well, I think
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I'd rather abuse them than leave the facility both in and
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The nonce is needed because multiple exchanges may be made with the
551
same key, and these extension fields aren't packed in with the other
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encrypted data in the same response, so a MITM could pick apart
553
multiple messages and mix-and-match components. (In a TGS_REQ
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exchange, a subsession key would help, but it's not required.)
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The extension field would be required to prevent a MITM from
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discarding the field from a response; a flag bit in a protected part
558
of the message (probably in 'flags' in EncKDCRepPart) could also let
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us know of a cases where the information can be omitted, namely,
560
when no name change is done. Perhaps the bit should be set to
561
indicate that a name change *was* done, and clear if it wasn't,
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making the no-change case more directly compatible with RFC1510.
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1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
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9, RFC 2026, October 1996.
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2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
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Levels", BCP 14, RFC 2119, March 1997
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3 Kohl, J., Neuman, C., "The Kerberos Network Authentication
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Service (V5)", RFC 1510, September 1993
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12. Author's Addresses
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University of Washington
582
Email: mikesw@cs.washington.edu
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Swift Category - Standards Track 9
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KDC Referrals February 2001
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Email: jbrezak@Microsoft.com
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Email: jtrostle@cisco.com
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Massachusetts Institute of Technology 77
612
Cambridge, Massachusetts 02139
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Email: raeburn@mit.edu
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Swift Category - Standards Track 10
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KDC Referrals February 2001
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Full Copyright Statement
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Copyright (C) The Internet Society (1999). All Rights Reserved.
668
This document and translations of it may be copied and furnished to
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others, and derivative works that comment on or otherwise explain it
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or assist in its implementation may be prepared, copied, published
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and distributed, in whole or in part, without restriction of any
672
kind, provided that the above copyright notice and this paragraph
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are included on all such copies and derivative works. However, this
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document itself may not be modified in any way, such as by removing
675
the copyright notice or references to the Internet Society or other
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Internet organizations, except as needed for the purpose of
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developing Internet standards in which case the procedures for
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copyrights defined in the Internet Standards process must be
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followed, or as required to translate it into languages other than
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The limited permissions granted above are perpetual and will not be
683
revoked by the Internet Society or its successors or assigns.
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This document and the information contained herein is provided on an
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
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Swift Category - Standards Track 11