Network Working Group G. Zorn
Internet-Draft Cisco Systems
Updates: 2865 (if approved) April 26, 2004
Expires: October 25, 2004
User Session Tracking in RADIUS
draft-zorn-radius-logoff-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
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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
Normative References . . . . . . . . . . . . . . . . . . . . . 10
Informative References . . . . . . . . . . . . . . . . . . . . 10
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 to
authenticate the reply from the RADIUS server.
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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.
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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 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
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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
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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
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.
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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. for a given
session MUST have the same Acct-Session-Id.
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].
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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.
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.
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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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