One document matched: draft-zorn-radius-logoff-08.txt
Differences from draft-zorn-radius-logoff-07.txt
Network Working Group G. Zorn
Internet-Draft Cisco Systems
Updates: 2865 3576 (if approved) A. Lior
Expires: April 20, 2007 Bridgewater Systems
October 17, 2006
User Session Tracking in RADIUS
draft-zorn-radius-logoff-08.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines a set of new messages and attributes 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-Session-Notification . . . . . . . . . . . . . . . . 6
4.2 User-Session-Acknowledgement . . . . . . . . . . . . . . . 8
5. Attributes . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1 Session-Id . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2 Session-Notification-Type . . . . . . . . . . . . . . . . 10
5.3 Table of Attributes . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1 Normative References . . . . . . . . . . . . . . . . . . . 11
8.2 Informative References . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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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. 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]; the RADIUS server has no way of knowing if user
sessions (as defined above) have actually begun or ended. 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 implement RADIUS Accounting even if they would not otherwise
do so.
This document defines a pair of message exchanges and associated
Attributes that can be used to notify a RADIUS server that a user
session has begun or ended.
Discussion of this draft may be directed to the authors.
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-Session-Notification
<MSG2> User-Session-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-Session-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-Session-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-Session-
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-Session-
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.
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
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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-Session-Notification
Description
User-Session-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-
Session-Notification).
Upon receipt of an User-Session-Notification packet from a valid
client, the server MUST reply using either a User-Session-
Acknowledgement message or an Error-Notification message [ERRMSG].
A User-Session-Notification message MUST contain either a NAS-IP-
Address Attribute [RFC2865] or a NAS-Identifier Attribute
[RFC2865] or both.
A User-Session-Notification message MUST contain a Session-Id
Attribute (see below) if one was returned from the server in the
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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.).
A User-Session-Notification message MUST include the Session-
Notification-Type Attribute with the Value field set appropriately
(see below).
To help avoid spoofing attacks, a User-Session-Notification
message SHOULD contain a Message-Authenticator Attribute
[RFC2869].
A summary of the User-Session-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-Session-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.
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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-Session-Acknowledgement
Description
User-Session-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-Session-Acknowledgement).
No Attributes are required in a User-Session-Acknowledgement
packet.
A summary of the User-Session-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-Session-Acknowledgement
Identifier
The Identifier field is a copy of the Identifier field of the
User-Session-Notification packet which caused this User-Session-
Acknowledgement packet to be created. .
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Acknowledgement Authenticator
The Acknowledgement Authenticator value is calculated from the
User-Session-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 assigned by the 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 [RFC2865] the associated User-Session-
Notification packet. The value of the Attribute in both messages
MUST be the same. Because the Session-Id Attribute completely
identifies the session in question, it MAY also be used as a
replacement for the session identication attributes discussed in
[RFC3576].
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
<ATR1> for Session-Id.
Length
>= 3
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Identifier
The Identifier field SHOULD be a string of UTF-8 encoded 10646
characters [RFC3629].
5.2 Session-Notification-Type
Description
This attribute contains an integer that identifies the type of
notification contained in the User-Session-Notification packet.
This Arribute MUST be present in any User-Session-Notification
packet.
A summary of the Session-Notification-Type attribute 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Notification-Type
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Notification-Type (cont'd.) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
<ATR2> for Session-Notification-Type.
Length
6
Notification-Type
The Notification-Type field in a 32-bit unsigned integer. The
following values are defined for this field:
0 Start
1 End
5.3 Table of Attributes
The following table provides a guide to which attributes may be found
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in which kinds of packets, and in what quantity; note that since none
of the Attributes described here may be present in the either the
Access-Reject or Access-Challenge packets, those columns have been
omitted from the table.
Accept Notify-Req Notify-Ack CoA-Req # Attribute
0-1 0-1 0-1 0-1 ATR1 Session-Id
0 1 0-1 0 ATR2 Notification-Type
The following table defines the meaning of the above table entries.
0 This Attribute MUST NOT be present in packet.
0-1 Zero or one instances of this Attribute MAY be present in
packet.
6. IANA Considerations
This document requires the allocation of four new numbers: two RADIUS
Packet Type Codes (denoted herein as <MSG1> and <MSG2>) and two
RADIUS Attribute Types (denoted as <ATR1> and <ATR2>). The criteria
to be used by the Internet Assigned Numbers Authority (IANA) for
assignment of these are identical to those given in [RFC3575].
7. Security Considerations
If the User-Session-Notification packet is unauthenticated or if the
shared secret is compromised, an attacker might be able to convince
the server that user sessions had been initiated (if the
Notification-Type is "Start") or completed (if the Notification-Type
is "Stop" 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-06.txt (work in progress),
October 2006.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
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[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.
[RFC3576] Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
Aboba, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 3576,
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.
Authors' Addresses
Glen Zorn
Cisco Systems
2901 Third Avenue, Suite 600
Seattle, WA 98121
US
Phone: +1 (425) 344-8113
Email: gwz@cisco.com
Avi Lior
Bridgewater Systems
303 Terry Fox Drive
Ottawa, ON K2K 3J1
Canada
Phone: +1 (613) 591-6655
Email: avi@bridgewatersystems.com
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