Network Working Group G. Zorn
Internet-Draft Cisco Systems
Updates: 2865 (if approved) April 2004
Expires: September 30, 2004
User Session Tracking in RADIUS
draft-zorn-radius-logoff-01.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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 Internet-Draft will expire on September 30, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines a pair of new messages and a new attribute
designed to allow RADIUS servers to cleanly track user sessions.
Zorn Expires September 30, 2004 [Page 1]
Internet-Draft User Session Tracking in RADIUS April 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Specification of Requirements . . . . . . . . . . . . . . . . 3
3. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Packet Types . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1 User-Logoff-Notification . . . . . . . . . . . . . . . . . . . 6
4.2 User-Logoff-Acknowledgement . . . . . . . . . . . . . . . . . 8
5. Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1 Session-Id . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
Normative References . . . . . . . . . . . . . . . . . . . . . 10
Informative References . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
Zorn Expires September 30, 2004 [Page 2]
Internet-Draft User Session Tracking in RADIUS April 2004
1. Introduction
RFC 2865 defines a "session" as a service provided to a user with the
beginning of the session defined as the point where service is first
provided and the end of the session defined as the point where
service is ended. For authorization purposes, the beginning of a
session Many remote access deployments require the tracking/counting
of user sessions, for example to limit the number of simultaneous
logins. Note that this is explicitly an authorization issue.
Currently, however, the only way to track the number or even the
existence of user sessions is via RADIUS Accounting [RFC2866]. This
fact causes an unnecessarily tight binding between RADIUS [RFC2865]
and RADIUS Accounting, forcing implementers to combine both protocols
in a single server, devise a method to quickly search accounting
logs, etc. and service providers to implement RADIUS Accounting even
if they would not otherwise do so.
This document defines a message exchange that can be used to notify a
RADIUS server that a user session has terminated.
Discussion of this draft may be directed to the author.
2. Specification of Requirements
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].
3. Packet Format
Exactly one RADIUS packet is encapsulated in the UDP Data field
[RFC0768] where the UDP Destination Port field indicates 1812
(decimal).
When a reply is generated, the source and destination ports are
reversed.
A summary of the RADIUS data format is shown below. The fields are
transmitted from left to right.
Zorn Expires September 30, 2004 [Page 3]
Internet-Draft User Session Tracking in RADIUS April 2004
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Authenticator |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
The Code field is one octet, and identifies the type of RADIUS
packet. When a packet is received with an invalid Code field, it
is silently discarded.
The RADIUS Codes (decimal) defined in this document are as
follows:
<MSG1> User-Logoff-Notification
<MSG2> User-Logoff-Acknowledgement
Identifier
The Identifier field is one octet, and aids in matching requests
and replies. The RADIUS server can detect a duplicate request if
it has the same client source IP address, source UDP port and
Identifier within a short span of time.
Length
The Length field is two octets. It indicates the length of the
packet including the Code, Identifier, Length, Authenticator and
Attribute fields. Octets outside the range of the Length field
MUST be treated as padding and ignored on reception. If the
packet is shorter than the Length field indicates, it MUST be
silently discarded. The minimum length is 20 and maximum length
is 4096.
Authenticator
The Authenticator field is sixteen (16) octets. The most
significant octet is transmitted first. This value is used to
authenticate the reply from the RADIUS server.
Zorn Expires September 30, 2004 [Page 4]
Internet-Draft User Session Tracking in RADIUS April 2004
Notification Authenticator
In User-Logoff-Notification packets, the Authenticator value is
a 16 octet random number, called the Notification
Authenticator. The value SHOULD be unpredictable and unique
over the lifetime of a secret (the password shared between the
client and the RADIUS server), since repetition of an
authenticator value in conjunction with the same secret would
permit an attacker to reply with a previously intercepted
response. Since it is expected that the same secret MAY be
used to authenticate with servers in disparate geographic
regions, the Notification Authenticator field SHOULD exhibit
global and temporal uniqueness.
The Authenticator value in an User-Logoff-Notification packet
SHOULD also be unpredictable, lest an attacker trick a server
into responding to a predicted future request, and then use the
response to masquerade as that server to a future notification
packet.
Although protocols such as RADIUS are incapable of protecting
against theft of an authenticated session via realtime active
wiretapping attacks, generation of unique unpredictable
requests can protect against a wide range of active attacks
against authentication.
Acknowledgement Authenticator
The value of the Authenticator field in the
User-Logoff-Acknowledgement packet is called the
Acknowledgement Authenticator, and contains a one-way MD5 hash
calculated over a stream of octets consisting of: the RADIUS
packet, beginning with the Code field, including the
Identifier, the Length, the Notification Authenticator field
from the User-Logoff-Notification packet, and the response
Attributes, followed by the shared secret. That is,
Acknowledgement Auth =
MD5(Code+ID+Length+RequestAuth+Attributes+Secret)
where '+' denotes concatenation.
Administrative Note
The secret shared between the client and the RADIUS server SHOULD
be at least as large and unguessable as a well- chosen password.
It is preferred that the secret be at least 16 octets. This is to
ensure a sufficiently large range for the secret to provide
protection against exhaustive search attacks. The secret MUST NOT
Zorn Expires September 30, 2004 [Page 5]
Internet-Draft User Session Tracking in RADIUS April 2004
be empty (length 0) since this would allow packets to be trivially
forged.
A RADIUS server MUST use the source IP address of the RADIUS UDP
packet to decide which shared secret to use, so that RADIUS
requests can be proxied.
When using a forwarding proxy, the proxy must be able to alter the
packet as it passes through in each direction - when the proxy
forwards the request, the proxy MAY add a Proxy-State Attribute,
and when the proxy forwards a response, it MUST remove its
Proxy-State Attribute if it added one. Proxy-State is always
added or removed after any other Proxy-States, but no other
assumptions regarding its location within the list of attributes
can be made. Since Access-Accept and Access-Reject replies are
authenticated on the entire packet contents, the stripping of the
Proxy-State attribute invalidates the signature in the packet - so
the proxy has to re-sign it.
Further details of RADIUS proxy implementation are outside the
scope of this document.
4. Packet Types
The RADIUS Packet type is determined by the Code field in the first
octet of the Packet.
4.1 User-Logoff-Notification
Description
User-Logoff-Notification packets are sent to a RADIUS server as an
indication that a previously authorized session has ended. A
RADIUS client wishing to indicate the end of a user session MUST
transmit a RADIUS packet with the Code field set to <MSG1>
(User-Logoff-Notification).
Upon receipt of an User-Logoff-Notification packet from a valid
client, the server MUST reply using either a
User-Logoff-Acknowledgement message or a Server-Error-Notification
message [ERRMSG].
A User-Logoff-Notification message MUST contain either a
NAS-IP-Address Attribute [RFC2865] or a NAS-Identifier Attribute
[RFC2865] or both.
A User-Logoff-Notification message MUST contain a Session-Id
Zorn Expires September 30, 2004 [Page 6]
Internet-Draft User Session Tracking in RADIUS April 2004
Attribute (see below) if one was returned from the server in the
Access-Accept message for the session; if no Session-Id Attribute
is included, the packet MUST contain a User-Name Attribute and
such additional Attributes as are necessary to positively identify
a given user session (e.g., Service-Type [RFC2865],
Calling-Station-Id [RFC2865], etc.).
To help avoid spoofing attacks, a User-Logoff-Notification message
SHOULD contain a Message-Authenticator Attribute [RFC2869].
A summary of the User-Logoff-Notification packet format is shown
below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Notification Authenticator |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
<MSG1> for User-Logoff-Notification
Identifier
The Identifier field MUST be changed whenever the content of the
Attributes field changes, and whenever a valid reply has been
received for a previous request. For retransmissions, the
Identifier MUST remain unchanged.
Notification Authenticator
The Notification Authenticator value MUST be changed each time a
new Identifier is used.
Attributes
The Attribute field is variable in length, and contains the list
of required Attributes, as well as any desired optional
Attributes.
Zorn Expires September 30, 2004 [Page 7]
Internet-Draft User Session Tracking in RADIUS April 2004
4.2 User-Logoff-Acknowledgement
Description
User-Logoff-Acknowledgement packets are sent by a RADIUS server as
an acknowldgement that a previously authorized session has ended.
A RADIUS server wishing to acknowledge the end of a user session
MUST transmit a RADIUS packet with the Code field set to <MSG2>
(User-Logoff-Acknowledgement).
No Attributes are required in a User-Logoff-Acknowledgement
packet.
A summary of the User-Logoff-Acknowledgement packet format is
shown below. The fields are transmitted from left to right.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Identifier | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Acknowledgement Authenticator |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
Code
<MSG2> for User-Logoff-Acknowledgement
Identifier
The Identifier field is a copy of the Identifier field of the
User-Logoff-Notification packet which caused this
User-Logoff-Acknowledgement packet to be created. .
Acknowledgement Authenticator
The Acknowledgement Authenticator value is calculated from the
User-Logoff-Notification packet, as described above.
Attributes
The Attribute field is variable in length, and contains any
Zorn Expires September 30, 2004 [Page 8]
Internet-Draft User Session Tracking in RADIUS April 2004
desired optional Attributes.
5. Attributes
5.1 Session-Id
Description
This attribute contains a unique identifier to make it possible
for a RADIUS server to uniquely identify and track user sessions.
In order to enable session tracking, this Attribute SHOULD be
included in both the Access-Accept packet and the associated
User-Logoff-Notification packet. The value of the Attribute in
both messages MUST be the same.
The Identifier field of the Session-Id Attribute SHOULD contain
UTF-8 encoded 10646 characters [RFC3629].
A summary of the Session-Id attribute format is shown below. The
fields are transmitted from left to right.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Identifier...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
<ATR> for Session-Id.
Length
>= 3
Identifier
The Identifier field SHOULD be a string of UTF-8 encoded 10646
characters [RFC3629].
6. IANA Considerations
The criteria to be used by the Internet Assigned Numbers Authority
(IANA) for assignment of numbers within namespaces defined within
this document are identical to those given in [RFC3575].
Zorn Expires September 30, 2004 [Page 9]
Internet-Draft User Session Tracking in RADIUS April 2004
7. Security Considerations
If the User-Logoff-Notification packet is unauthenticated or if the
shared secret is compromised, an attacker might be able to convince
the server that user sessions had completed when they had not. If
the server was limiting the number of simultaneous sessions, this
could enable one or more users to exceed their session quota and
possibly lead to service being denied to legitimate users.
Normative References
[ERRMSG] Zorn, G., "RADIUS Error Messages",
draft-zorn-radius-err-msg-00.txt (work in progress), April
2004.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)", RFC
2865, June 2000.
[RFC2869] Rigney, C., Willats, W. and P. Calhoun, "RADIUS
Extensions", RFC 2869, June 2000.
[RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote
Authentication Dial In User Service)", RFC 3575, July
2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
Informative References
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
Zorn Expires September 30, 2004 [Page 10]
Internet-Draft User Session Tracking in RADIUS April 2004
Author's Address
Glen Zorn
Cisco Systems
2901 Third Avenue, Suite 600
Seattle, WA 98121
US
Phone: +1 425 344 8113
EMail: gwz@cisco.com
Zorn Expires September 30, 2004 [Page 11]
Internet-Draft User Session Tracking in RADIUS April 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Zorn Expires September 30, 2004 [Page 12]
Internet-Draft User Session Tracking in RADIUS April 2004
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Zorn Expires September 30, 2004 [Page 13]