Network Working Group G. Zorn
Internet-Draft Cisco Systems
Updates: 2865 (if approved) May 13, 2004
Expires: November 11, 2004
User Session Tracking in RADIUS
draft-zorn-radius-logoff-02.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance
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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.
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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 . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1 Normative References . . . . . . . . . . . . . . . . . . . . 10
8.2 Informative References . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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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.
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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
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to authenticate the reply from the RADIUS server.
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+NotificationAuth+Attributes+Secret)
where '+' denotes concatenation.
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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 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
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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
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.
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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.
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
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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
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.
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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].
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.
8. References
8.1 Normative References
[ERRMSG] Zorn, G., "RADIUS Error Messages",
draft-zorn-radius-err-msg-00.txt (work in progress),
May 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.
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[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.
8.2 Informative References
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June
2000.
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
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