Kerberos Working Group Karthik
Jaganathan
Internet Draft Larry Zhu
Document: draft-ietf-krb-wg-kerberos-referrals-03.txt John Brezak
Category: Standards Track Microsoft
Mike Swift
University of
Washington
Jonathan Trostle
Cisco Systems
Expires: August
2004
Generating KDC Referrals to locate Kerberos realms
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
Internet-Drafts are working documents of the Internet Engineering
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1. Abstract
The draft documents a new method for a Kerberos Key Distribution
Center (KDC) to respond to client requests for kerberos tickets when
the client does not have detailed configuration information on the
realms of users or services. The KDC will handle requests for
principals in other realms by returning either a referral error or a
cross-realm TGT to another realm on the referral path. The clients
will use this referral information to reach the realm of the target
principal and then receive the ticket.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC-2119 [2].
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3. Introduction
Current implementations of the Kerberos AS and TGS protocols, as
defined in [3], use principal names constructed from a known user or
service name and realm. A service name is typically constructed from
a name of the service and the DNS host name of the computer that is
providing the service. Many existing deployments of Kerberos use a
single Kerberos realm where all users and services would be using
the same realm. However in an environment where there are multiple
trusted Kerberos realms, the client needs to be able to determine
what realm a particular user or service is in before making an AS or
TGS request. Traditionally this requires client configuration to
make this possible.
When having to deal with multiple trusted realms, users are forced
to know what realm they are in before they can obtain a ticket
granting ticket (TGT) with an AS request. However, in many cases the
user would like to use a more familiar name that is not directly
related to the realm of their Kerberos principal name. A good
example of this is an RFC-822 style email name. This document
describes a mechanism that would allow a user to specify a user
principal name that is an alias for the user's Kerberos principal
name. In practice this would be the name that the user specifies to
obtain a TGT from a Kerberos KDC. The user principal name no longer
has a direct relationship with the Kerberos principal or realm. Thus
the administrator is able to move the user's principal to other
realms without the user having to know that it happened.
Once a user has a TGT, they would like to be able to access services
in any trusted Kerberos realm. To do this requires that the client
be able to determine what realm the target service's host is in
before making the TGS request. Current implementations of Kerberos
typically have a table that maps DNS host names to corresponding
Kerberos realms. In order for this to work on the client, each
application canonicalizes the host name of the service by doing a
DNS lookup followed by a reverse lookup using the returned IP
address. The returned primary host name is then used in the
construction of the principal name for the target service. In order
for the correct realm to be added for the target host, the mapping
table [domain_to_realm] is consulted for the realm corresponding to
the DNS host name. The corresponding realm is then used to complete
the target service principal name.
This traditional mechanism requires that each client have very
detailed configuration information about the hosts that are
providing services and their corresponding realms. Having client
side configuration information can be very costly from an
administration point of view - especially if there are many realms
and computers in the environment.
There are also cases where specific DNS aliases (local names) have
been setup in an organization to refer to a server in another
organization (remote server). The server has different DNS names in
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each organization and each organization has a Kerberos realm that is
configured to service DNS names within that organization. Ideally
users are able to authenticate to the server in the other
organization using the local server name. This would mean that the
local realm be able to produce a ticket to the remote server under
its name. You could give that remote server an identity in the local
realm and then have that remote server maintain a separate secret
for each alias it is known as. Alternatively you could arrange to
have the local realm issue a referral to the remote realm and notify
the requesting client of the server's remote name that should be
used in order to request a ticket.
This draft proposes a solution for these problems and simplifies
administration by minimizing the configuration information needed on
each computer using Kerberos. Specifically it describes a mechanism
to allow the KDC to handle Canonicalization of names, provide for
principal aliases for users and services and provide a mechanism for
the KDC to determine the trusted realm authentication path by being
able to generate referrals to other realms in order to locate
principals.
To rectify these problems, this draft introduces three new kinds of
KDC referrals:
1. AS ticket referrals, in which the client doesn't know which realm
contains a user account.
2. TGS ticket referrals, in which the client doesn't know which
realm contains a server account.
3. Cross realm shortcut referrals, in which the KDC chooses the next
path on a referral chain
4. Realm Organization Model
This draft assumes that the world of principals is arranged on
multiple levels: the realm, the enterprise, and the world. A KDC may
issue tickets for any principal in its realm or cross-realm tickets
for realms with which it has a direct trust relationship. The KDC
also has access to a trusted name service that can resolve any name
from within its enterprise into a realm. This trusted name service
removes the need to use an untrusted DNS lookup for name resolution.
For example, consider the following configuration, where lines
indicate trust relationships:
MS.COM
/ \
/ \
OFFICE.MS.COM NT.MS.COM
In this configuration, all users in the MS.COM enterprise could have
a principal name such as alice@MS.COM, with the same realm portion.
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
principal resides in.
5. Client Name Canonicalization
A client account may have multiple principal names. More useful,
though, is a globally unique name that allows unification of email
and security principal names. For example, all users at MS may have
a client principal name of the form "joe@MS.COM" even though the
principals are contained in multiple realms. This global name is
again an alias for the true client principal name, which indicates
what realm contains the principal. Thus, accounts "alice" in the
realm NT.MS.COM and "bob" in OFFICE.MS.COM may logon as
"alice@MS.COM" and "bob@MS.COM".
This utilizes a new client principal name type, as the AS-REQ
message only contains a single realm field, and the realm portion of
this name doesn't correspond to any Kerberos realm. Thus, the entire
name "alice@MS.COM" is transmitted in the client name field of the
AS-REQ message, with a name type of KRB-NT-ENTERPRISE-PRINCIPAL.
KRB-NT-ENTERPRISE-PRINCIPAL 10
The KDC will recognize this name type and then transform the
requested name into the true principal name. The true principal name
can be using a name type different from the requested name type.
Typically the returned principal name will be a KRB-NT-PRINCIPAL.
The returned name will be the same in the AS response and in the
ticket. The KDC will always return a different name type than KRB-
NT-ENTERPRISE-PRINCIPAL. This is regardless of the presence of the
"canonicalize" KDC option.
If the "canonicalize" KDC option is set, then the KDC MAY change the
client principal name and type in the AS response and ticket
regardless of the name type of the client name in the request. For
example the AS request may specify a client name of "fred@MS.COM" as
an KRB-NT-PRINCIPAL with the "canonicalize" KDC option set and the
KDC will return with a client name of "104567" as a KRB-NT-UID.
6. Requesting a referral
In order to request referrals, the Kerberos client must explicitly
request the canonicalize KDC option (bit 15) in the KDC options for
the TGS-REQ. This flag indicates to the KDC that the client is
prepared to receive a reply that contains a principal name other
than the one requested. Thus, the KDCOptions types is redefined as:
KDCOptions ::= BIT STRING {
reserved(0),
forwardable(1),
forwarded(2),
proxiable(3),
proxy(4),
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allow-postdate(5),
postdated(6),
unused7(7),
renewable(8),
unused9(9),
unused10(10),
unused11(11),
canonicalize(15),
renewable-ok(27),
enc-tkt-in-skey(28),
renew(30),
validate(31)
}
The client should expect, when sending names with the "canonicalize"
KDC option, that names in the KDC's reply will be different than the
name in the request.
6.1 Client Referrals
The simplest form of ticket referral is for a user requesting a
ticket using an AS-REQ. In this case, the client machine will send
the AS request to a convenient trusted realm, either the realm of
the client machine or the realm of the client name. In the case of
the name Alice@MS.COM, the client may optimistically choose to send
the request to MS.COM. The realm in the AS request is always the
name of the realm that the request is for as specified in [3].
The client will send the string "alice@MS.COM" in the client
principal name field using the KRB-NT-ENTERPRISE-PRINCIPAL name type
with the crealm set to MS.COM. The KDC will try to lookup the name
in its local account database. If the account is present in the
realm of the request, it MUST return a KDC reply structure with the
appropriate ticket.
If the account is not present in the realm specified in the request
and the "canonicalize" KDC option is set, the KDC will try to lookup
the entire name, Alice@MS.COM, using a name service. If this lookup
is unsuccessful, it MUST return the error
KDC_ERR_C_PRINCIPAL_UNKNOWN. If the lookup is successful, it MUST
return an error KDC_ERR_WRONG_REALM (0x44) and in the error message
the crealm field will contain the the true realm of the client or
another realm that has better information about the client's true
realm. The client MUST NOT use a cname returned from a referral.
If the KDC contains the account locally and "canonicalize" KDC
option is not set, it MUST return a normal ticket. The client name
and realm portions of the ticket and KDC reply message MUST be the
client's true name in the realm, not the globally unique name.
If the client receives a KDC_ERR_WRONG_REALM error, it will issue a
new AS request with the same client principal name used to generate
the first referral to the realm specified by the realm field of the
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kerberos error message from the first request. This request MUST
produce a valid AS response with a ticket for the canonical user
name.
An implementation should limit the number of referrals that it
processes to avoid infinite referral loops. A suggested limit is 5
referrals before giving up. In MicrosoftÆs implementation the
default limit is 3 since through the use of the global catalog any
domain in one forest is reachable from any other domain in another
trusting forest with 3 or less referrals.
6.2 Service Referrals
The primary problem is that the KDC must return a referral ticket
rather than an error message as is done in AS request referrals.
There needs to be a place to include in the TGS response information
about what realm contains the service. This is done by returning
information about the service name in the pre-auth data field of the
KDC reply.
If the KDC resolves the service principal name into a principal in
the realm specified by the service realm name, it will return a
normal ticket. When using canonicalization, the client can omit the
service realm name. If it is supplied, it is used as a hint by the
KDC, but the service principal lookup is not constrained to locating
the service principal name in that specified realm. If the
"canonicalize" flag in the KDC options is not set, then the KDC MUST
only look up the name as a normal principal name in the specified
service realm.
If the "canonicalize" flag in the KDC options is set and the KDC
doesn't find the principal locally, the KDC can return a cross-realm
ticket granting ticket to the next hop on the trust path towards a
realm that may be able to resolve the principal name.
If the KDC can determine the service principal's realm, it SHOULD
return the service realm as KDC supplied pre-authentication data
element. The preauth data MUST be encrypted using the sub-session
key from the authenticator if present or the session key from the
ticket.
The data itself is an ASN.1 encoded structure containing the
server's realm, and if known, the real principal name.
PA-SERVER-REFERRAL-INFO 25
PA-SERVER-REFERRAL :: = KERB-ENCRYPTED-DATA
-- PA-SERVER-REFERRAL-DATA
PA-SERVER-REFERRAL-DATA ::= SEQUENCE {
referred-server-realm[0] KERB-REALM
referred-name[1] PrincipalName OPTIONAL
...
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}
If applicable to the encryption type, the key derivation value will
for the PA-SERVER-REFERRAL is 22.
If the referred-name field is present, the client MUST use that name
in a subsequent TGS request to the service realm when following the
referral.
The client will use this information to request a chain of cross-
realm ticket granting tickets until it reaches the realm of the
service, and can then expect to receive a valid service ticket.
However an implementation should limit the number of referrals that
it processes to avoid infinite referral loops. A suggested limit is
5 referrals before giving up.
This is an example of a client requesting a service ticket for a
service in realm NT.MS.COM where the client is in OFFICE.MS.COM.
+NC = Canonicalize KDCOption set
+PA-REFERRAL = returned PA-SERVER-REFERRAL-INFO
C: TGS-REQ sname=server/foo.nt.ms.com srealm=NULL +NC to
OFFICE.MS.COM
S: TGS-REP sname=krbtgt/MS.COM@OFFICE.MS.COM +PA-REFERRAL
containing NT.MS.COM
C: TGS-REQ sname=krbtgt/NT.MS.COM@MS.COM +NC to MS.COM
S: TGS-REP sname=krbtgt/NT.MS.COM@MS.COM
C: TGS-REQ sname=server/foo.nt.ms.com srealm=NT.MS.COM +NC to
NT.MS.COM
S: TGS-REP sname=server/foo.nt.ms.com@NT.MS.COM
Notice that the client only specifies the service name in the
initial and final TGS request.
7. Cross Realm Routing
The current Kerberos protocol requires the client to explicitly
request a cross-realm TGT for each pair of realms on a referral
chain. As a result, the client need to be aware of the trust
hierarchy and of any short-cut trusts (those that aren't parent-
child trusts). Instead, the client should be able to request a TGT
to the target realm from each realm on the route. The KDC will
determine the best path for the client and return a cross-realm TGT.
The client has to be aware that a request for a cross-realm TGT may
return a TGT for a realm different from the one requested.
For compatibility, the client MUST use the "canonicalize" KDC option
if it is able to use cross-realm routing from the KDC.
8. Compatibility with earlier implementations of name canonicalization
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The Microsoft Windows 2000 release included an earlier form of name-
canonicalization [4]. It has these differences:
1) The TGS referral data was returned inside of the KDC message as
"encrypted pre auth data".
KERB-ENCRYPTED-KDC-REPLY ::= SEQUENCE {
session-key[0] KERB-ENCRYPTION-KEY,
last-request[1] PKERB-LAST-REQUEST,
nonce[2] INTEGER,
key-expiration[3] KERB-TIME OPTIONAL,
flags[4] KERB-TICKET-FLAGS,
authtime[5] KERB-TIME,
starttime[6] KERB-TIME OPTIONAL,
endtime[7] KERB-TIME,
renew-until[8] KERB-TIME OPTIONAL,
server-realm[9] KERB-REALM,
server-name[10] KERB-PRINCIPAL-NAME,
client-addresses[11] PKERB-HOST-ADDRESSES
OPTIONAL,
encrypted-pa-data[12] SEQUENCE OF KERB-PA-DATA
OPTIONAL
}
2) The preauth data type definition in the encrypted preauth data is
as follows:
PA-SVR-REFERRAL-INFO 20
PA-SVR-REFERRAL-DATA ::= SEQUENCE {
referred-server-name[1] PrincipalName OPTIONAL
referred-server-realm[0] KERB-REALM
}
9. Security Considerations
In the case of TGS requests the client may be vulnerable to a denial
of service attack by an attacker that replays replies from previous
requests. The client can verify that the request was one of its own
by checking the client-address field or authtime field, though, so
the damage is limited and detectable. Clients MUST NOT process cross
realm referral TGTs if the KDC reply does not include the encrypted
PA-SERVER-REFERRAL-INFO.
For the AS exchange case, it is important that the logon mechanism
not trust a name that has not been used to authenticate the user.
For example, the name that the user enters as part of a logon
exchange may not be the name that the user authenticates as, given
that the KDC_ERR_WRONG_REALM error may have been returned. The
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
workstation that was obtained using the user's initial TGT.
How the client name and client realm is mapped into a local account
for logon is a local matter, but the client logon mechanism MUST use
additional information such as the client realm and/or authorization
attributes from the service ticket presented to the workstation by
the user, when mapping the logon credentials to a local account on
the workstation.
10. Acknowledgements
The authors wish to thank Ken Raeburn for his comments and
suggestions.
11.1 Normative References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
3 Neuman, C., Kohl, J., Ts'o, T., Yu, T., Hartman, S., and K.
Raeburn, "The Kerberos Network Authentication Service (V5)",
draft-ietf-krb-wg-kerberos-clarifications-00.txt, February 22,
2002. Work in progress.
11.2 Informative References
4 J. Trostle, I. Kosinovsky, and M. Swift,"Implementation of
Crossrealm Referral Handling in the MIT Kerberos Client", In
Network and Distributed System Security Symposium, February 2001.
12. Author's Addresses
Karthik Jaganathan
Microsoft
One Microsoft Way
Redmond, Washington
Email: karthikj@Microsoft.com
Larry Zhu
Microsoft
One Microsoft Way
Redmond, Washington
Email: lzhu@Microsoft.com
Michael Swift
University of Washington
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Seattle, Washington
Email: mikesw@cs.washington.edu
John Brezak
Microsoft
One Microsoft Way
Redmond, Washington
Email: jbrezak@Microsoft.com
Jonathan Trostle
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Email: jtrostle@cisco.com
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Jaganathan Category - Standards Track 11
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