One document matched: draft-ietf-cat-iakerb-05.txt
Differences from draft-ietf-cat-iakerb-04.txt
INTERNET-DRAFT Mike Swift
draft-ietf-cat-iakerb-05.txt University of WA
Updates: RFC 1510 Jonathan Trostle
November 2000 Cisco Systems
Bernard Aboba
Microsoft
Glen Zorn
Cisco Systems
Initial Authentication and Pass Through Authentication
Using Kerberos V5 and the GSS-API (IAKERB)
0. Status Of This Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026 [6].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as
"work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This draft expires on May 31st, 2001. Please send comments to the
authors.
1. Abstract
This document defines an extension to the Kerberos protocol
specification (RFC 1510 [1]) and GSSAPI Kerberos mechanism (RFC
1964 [2]) that enables a client to obtain Kerberos tickets for
services where the KDC is not accessible. Some common scenarios
where lack of accessibility would occur are when the client does
not have an IP address prior to authenticating to an access point,
or a KDC is behind a firewall. The document specifies two
protocols to allow a client to exchange KDC messages with an
IAKERB proxy instead of a KDC.
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 RFC2119 [7].
3. Motivation
When authenticating using Kerberos V5, clients obtain tickets from
a KDC and present them to services. This method of operation works
well in many situations, but is not always applicable. The
following is a list of scenarios that this proposal addresses:
(1) The client must initially authenticate to an access point in
order to gain full access to the network. Here the client may be
unable to directly contact the KDC either because it does not have
an IP address or it is unable to send packets to the KDC, before
it authenticates to the access point. Two protocols in this proposal
address this problem: the IAKERB proxy option and the IAKERB
minimal messages option. In the IAKERB proxy option (see
Figure 1) an application server called the IAKERB proxy acts as a
protocol gateway and proxies Kerberos messages back and forth
between the client and the KDC. The IAKERB proxy is also
responsible for locating the KDC and may additionally perform
other application proxy level functions such as auditing.
Client <---------> IAKERB proxy <----------> KDC
Figure 1: IAKERB proxying
The second protocol is the minimal messages protocol that extends
the technique in [5]; this protocol should be considered for message
constrained environments. Here the client sends a ticket granting
ticket (TGT) to the IAKERB proxy which then includes the client's TGT
as an additional ticket in a TGS request to the KDC. The TGS reply will
provide the IAKERB proxy with a service ticket from itself targetted
at the client. An AP exchange between the IAKERB proxy and the client
is then used to complete mutual authentication (see Figure 2). Thus
mutual authentication is accomplished with three messages instead of
four (or possibly more in the crossrealm case) between the client and
the IAKERB proxy.
(2) A KDC is behind a firewall so the client will send Kerberos
messages to the IAKERB proxy which will proxy the KDC request and
reply messages (using the IAKERB proxy option).
(3) Optionally, IAKERB MAY be used in a scenario where the Kerberos
client does not know the server principal realm for a TGS request.
Here the client may send the TGS_REQ message to the application
server with a zero length realm name in the request body. The
application server MAY send a KDC_ERR_WRONG_REALM error back to the
client which will include the application server's realm in the
realm field of the KRB_ERROR message.
A compliant IAKERB proxy is not required to support the functionality
in (3), but is required to support the IAKERB proxy and IAKERB minimal
messages protocols. In general, the existing Kerberos paradigm where
clients contact the KDC to obtain service tickets should be preserved
where possible.
4. GSSAPI Encapsulation
The mechanism ID for IAKERB proxy GSS-API Kerberos, in
accordance with the mechanism proposed by SPNEGO for negotiating
protocol variations, is:
{iso(1) member-body(2) United States(840) mit(113554) infosys(1)
gssapi(2) krb5(2) initialauth(4)}.
The proposed mechanism ID for minimal messages IAKERB GSS-API
Kerberos, in accordance with the mechanism proposed by SPNEGO for
negotiating protocol variations, is:
{iso(1) member-body(2) United States(840) mit(113554) infosys(1)
gssapi(2) krb5(2) initialauthminmessages(5)}.
The AS request, AS reply, TGS request, and TGS reply messages are all
encapsulated using the format defined by RFC1964 [2]. This consists
of the GSS-API token framing defined in appendix B of RFC1508 [3]:
InitialContextToken ::=
[APPLICATION 0] IMPLICIT SEQUENCE {
thisMech MechType
-- MechType is OBJECT IDENTIFIER
-- representing "Kerberos V5"
innerContextToken ANY DEFINED BY thisMech
-- contents mechanism-specific;
-- ASN.1 usage within innerContextToken
-- is not required
}
The innerContextToken consists of a 2-byte TOK_ID field (defined
below), followed by the Kerberos V5 KRB-AS-REQ, KRB-AS-REP,
KRB-TGS-REQ, or KRB-TGS-REP messages, as appropriate. The TOK_ID field
shall be one of the following values, to denote that the message is
either a request to the KDC or a response from the KDC.
Message TOK_ID
KRB-KDC-REQ 00 03
KRB-KDC-REP 01 03
5. The IAKERB proxy protocol
The IAKERB proxy will proxy Kerberos KDC request, KDC reply, and KRB_ERROR
messages back and forth between the client and the KDC as illustrated in
Figure 1. Messages received from the client must first have the Kerberos
GSS header (RFC1964 [2]) stripped off. The unencapsulated message will
then be forwarded to a KDC. The IAKERB proxy is responsible for locating
an appropriate KDC using the realm information in the KDC request message
it received from the client. In addition, the IAKERB proxy SHOULD implement
the retry algorithm for KDC requests over UDP. For messages sent by the
KDC, the IAKERB proxy encapsulates them with a Kerberos GSS header
before sending them to the client.
6. The IAKERB minimal messages protocol
The IAKERB protocol consists of two sub-protocols: the proxy sub-protocol,
and the minimal messages sub-protocol. The client should initiate the
IAKERB minimal messages sub-protocol when the number of messages must be
minimized (the most significant reduction in the number of messages can
occur when the client and the IAKERB proxy are in different realms).
The determination of the sub-protocol to use is completely up to the
client, and a compliant IAKERB proxy server MUST support both protocols.
(a) AS_REQ case:
We extend the technique used in Hornstein [5]. The client indicates
that the minimal message sub-protocol will be used by using the
appropriate OID as described above.
The IAKERB proxy will proxy the returned message (AS_REP or KRB-ERROR)
from the KDC back to the client. The protocol is complete in the
KRB-ERROR case. In the AS_REP case, the IAKERB proxy will obtain the
client's TGT from the AS_REP message before forwarding it to the client.
The IAKERB proxy then sends a TGS_REQ message with the client's TGT in
the additional tickets field to the client's KDC (ENC-TKT-IN_SKEY
option).
The IAKERB proxy MAY handle returned KRB-ERROR messages and retry the
TGS request message. Ultimately, the IAKERB proxy either proxies a
KRB-ERROR message to the client, or it sends a GSS Initial Context
token containing an AP_REQ message to the client. The IAKERB proxy
MUST set the MUTUAL AUTH flag in the Initial Context token in order
to cause the client to authenticate as well. The client will reply
with the GSSAPI enscapsulated AP_REP message, if the IAKERB proxy's
authentication succeeds. If all goes well, then, in order to enable
subsequent efficient client authentications, the IAKERB proxy will
then send a final message of type KERB-TGT-REPLY containing a
Kerberos ticket that is the reverse ticket of the ticket that the
IAKERB proxy used to authenticate itself to the client:
KERB-TGT-REPLY :: = SEQUENCE {
pvno[0] INTEGER, -- 5
msg-type[1] INTEGER, -- 17
ticket[2] Ticket
}
The encryption key for the reverse ticket is the IAKERB proxy's long
term key. The fields are identical to the AP_REQ ticket, except the
client name will be switched with the server name, and the server
realm will be switched with the client realm. (The one other exception
is that addresses should not be copied unless the IAKERB proxy has
included the client's address in the TGS_REQ message to the KDC).
Sending the reverse ticket allows the client to efficiently initiate
subsequent reauthentication attempts with a RFC1964 AP_REQ message.
Note that the TGT-REPLY message is sent after mutual authentication
and key establishment are complete.
(b) TGS_REQ case:
ii. Minimal messages sub-protocol:
The client indicates that the minimal messages sub-protocol will be
used by using the appropriate OID as described above. The client
initially sends a KERB-TGT-REPLY message to the IAKERB proxy in
order to send it a TGT. The IAKERB proxy will obtain the client's
TGT from the KERB-TGT-REPLY message and then proceed to send a
TGS_REQ message to the appropriate KDC (in either the client realm
or its own realm) as in the AS_REQ case above. The protocol then
continues as in the minimal messages AS_REQ case described above
(see Figure 2):
Client --------> IAKERB proxy
TGT
Client IAKERB proxy --------------------> KDC
TGS_REQ with client
TGT as additional TGT
Client IAKERB proxy <-------------------- KDC
TGS_REP with service
ticket
Client <-------- IAKERB proxy KDC
AP_REQ
Client --------> IAKERB proxy KDC
AP_REP
Figure 2: IAKERB Minimal Messages Option: TGS case
7. Determining Application Server Realm from Error Message
As a last resort (with respect to determining the realm name for
the application server), the application client MAY send a TGS_REQ
message to the application server where the request body realm name
has length 0. The application server MAY reply with a KRB_ERROR
message with error code KDC_ERR_WRONG_REALM where the realm field
contains the application server realm.
Sending a TGS_REQ message to the application server without a realm
name in the request, followed by a TGS request using the returned
realm name and then sending an AP request with a mutual authentication
flag should be subject to a local policy decision (see security
considerations below).
8. Addresses in Tickets
In IAKERB, the machine sending requests to the KDC is the server and
not the client. As a result, the client should not include its
addresses in any KDC requests for two reasons. First, the KDC may
reject the forwarded request as being from the wrong client. Second,
in the case of initial authentication for a dial-up client, the client
machine may not yet possess a network address. Hence, as allowed by
RFC1510 [1], the addresses field of the AS and TGS requests SHOULD be
blank and the caddr field of the ticket SHOULD similarly be left blank.
9. Combining IAKERB with Other Kerberos Extensions
This protocol is usable with other proposed Kerberos extensions such as
PKINIT (Public Key Cryptography for Initial Authentication in Kerberos
[4]). In such cases, the messages which would normally be sent to the
KDC are instead sent by the client application to the server, which
then forwards them to a KDC.
10. Security Considerations
A principal is identified by its principal name and realm. A client
that sends a TGS request to an application server (in the IAKERB proxy
option) without the request realm name will only be able to mutually
authenticate the server up to its principal name. Thus when requesting
mutual authentication, it is preferable if clients can either determine
the server realm name beforehand, use the referral mechanism, or apply
some policy checks to the realm name obtained from the returned error
message.
11. Bibliography
[1] J. Kohl, C. Neuman. The Kerberos Network Authentication
Service (V5). Request for Comments 1510.
[2] J. Linn. The Kerberos Version 5 GSS-API Mechanism. Request
for Comments 1964
[3] J. Linn. Generic Security Service Application Program Interface.
Request for Comments 1508
[4] B. Tung, C. Neuman, M. Hur, A. Medvinsky, S. Medvinsky, J. Wray,
J. Trostle, "Public Key Cryptography for Initial Authentication in
Kerberos", Internet Draft draft-ietf-cat-kerberos-pkinit-12.txt.
[5] K. Hornstein, T. Lemon, B. Aboba, J. Trostle, DHCP Authentication
via Kerberos V, Internet Draft draft-hornstein-dhc-kerbauth-02.txt.
[6] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
12. This draft expires on May 31st, 2001.
13. Authors' Addresses
Michael Swift
University of Washington
Seattle, WA
Email: mikesw@cs.washington.edu
Jonathan Trostle
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134, U.S.A.
Email: jtrostle@cisco.com
Phone: (408) 527-6201
Bernard Aboba
Microsoft
One Microsoft Way
Redmond, Washington, 98052, U.S.A.
Email: bernarda@microsoft.com
Glen Zorn
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134, U.S.A.
Email: gwz@cisco.com
Phone: (425) 468-0955
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