Network Working Group G. Zorn Internet-Draft Cisco Systems Updates: 2865 (if approved) July 7, 2004 Expires: January 5, 2005 User Session Tracking in RADIUS draft-zorn-radius-logoff-03.txt Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 January 5, 2005. 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 January 5, 2005 [Page 1]
Internet-Draft User Session Tracking in RADIUS July 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 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1 Normative References . . . . . . . . . . . . . . . . . . . . 10 8.2 Informative References . . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 11 Intellectual Property and Copyright Statements . . . . . . . . 12 Zorn Expires January 5, 2005 [Page 2]
Internet-Draft User Session Tracking in RADIUS July 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 January 5, 2005 [Page 3]
Internet-Draft User Session Tracking in RADIUS July 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 January 5, 2005 [Page 4]
Internet-Draft User Session Tracking in RADIUS July 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+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 Zorn Expires January 5, 2005 [Page 5]
Internet-Draft User Session Tracking in RADIUS July 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 January 5, 2005 [Page 6]
Internet-Draft User Session Tracking in RADIUS July 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 January 5, 2005 [Page 7]
Internet-Draft User Session Tracking in RADIUS July 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 January 5, 2005 [Page 8]
Internet-Draft User Session Tracking in RADIUS July 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 January 5, 2005 [Page 9]
Internet-Draft User Session Tracking in RADIUS July 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. 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. [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. Zorn Expires January 5, 2005 [Page 10]
Internet-Draft User Session Tracking in RADIUS July 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 January 5, 2005 [Page 11]
Internet-Draft User Session Tracking in RADIUS July 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Zorn Expires January 5, 2005 [Page 12]