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Versions: (draft-dekok-radext-dtls) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 7360

Network Working Group                                         Alan DeKok
INTERNET-DRAFT                                                FreeRADIUS
Category: Experimental
<draft-ietf-radext-dtls-06.txt>
Expires: January 12, 2014
12 July 2013


                  DTLS as a Transport Layer for RADIUS
                       draft-ietf-radext-dtls-06


Abstract

   The RADIUS protocol [RFC2865] has limited support for authentication
   and encryption of RADIUS packets.  The protocol transports data "in
   the clear", although some parts of the packets can have "obfuscated"
   content.  Packets may be replayed verbatim by an attacker, and
   client-server authentication is based on fixed shared secrets.  This
   document specifies how the Datagram Transport Layer Security (DTLS)
   protocol may be used as a fix for these problems.  It also describes
   how implementations of this proposal can co-exist with current RADIUS
   systems.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   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 12, 2014

Copyright Notice



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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info/) in effect on the date of
   publication of this document.  Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.







































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Table of Contents

1.  Introduction .............................................    4
   1.1.  Terminology .........................................    4
   1.2.  Requirements Language ...............................    5
2.  Building on Existing Foundations .........................    6
   2.1.  Changes to RADIUS ...................................    6
   2.2.  Similarities with RADIUS/TLS ........................    7
      2.2.1.  Changes from RADIUS/TLS to RADIUS/DTLS .........    7
      2.2.2.  Reinforcement of RADIUS/TLS ....................    8
3.  Interaction with RADIUS/UDP ..............................    8
   3.1.  DTLS Port and Packet Types ..........................    9
   3.2.  Server Behavior .....................................    9
4.  Client Behavior ..........................................   10
5.  Connection Management ....................................   10
   5.1.  Server Connection Management ........................   11
      5.1.1.  Session Management .............................   11
   5.2.  Client Connection Management ........................   13
6.  Implementation Guidelines ................................   14
   6.1.  Client Implementations ..............................   15
   6.2.  Server Implementations ..............................   15
7.  Implementation Experience ................................   16
8.  Diameter Considerations ..................................   16
9.  IANA Considerations ......................................   16
10.  Security Considerations .................................   17
   10.1.  Legacy RADIUS Security .............................   17
   10.2.  Resource Exhaustion ................................   18
   10.3.  Network Address Translation ........................   19
   10.4.  Wildcard Clients ...................................   19
   10.5.  Session Closing ....................................   19
   10.6.  Clients Subsystems .................................   20
11.  References ..............................................   20
   11.1.  Normative references ...............................   20
   11.2.  Informative references .............................   21

















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1.  Introduction

   The RADIUS protocol as described in [RFC2865], [RFC2866], [RFC5176],
   and others has traditionally used methods based on MD5 [RFC1321] for
   per-packet authentication and integrity checks.  However, the MD5
   algorithm has known weaknesses such as [MD5Attack] and [MD5Break].
   As a result, some specifications such as [RFC5176] have recommended
   using IPSec to secure RADIUS traffic.

   While RADIUS over IPSec has been widely deployed, there are
   difficulties with this approach.  The simplest point against IPSec is
   that there is no straightforward way for a RADIUS application to
   control or monitor the network security policies.  That is, the
   requirement that the RADIUS traffic be encrypted and/or authenticated
   is implicit in the network configuration, and is not enforced by the
   RADIUS application.

   This specification takes a different approach.  We define a method
   for using DTLS [RFC6347] as a RADIUS transport protocol.  This
   approach has the benefit that the RADIUS application can directly
   monitor and control the security policies associated with the traffic
   that it processes.

   Another benefit is that RADIUS over DTLS continues to be a User
   Datagram Protocol (UDP) based protocol.  This continuity ensures that
   existing network-layer infrastructure (firewall rules, etc.) does not
   need to be changed when RADIUS clients and servers are upgraded to
   support RADIUS over DTLS.

   This specification does not, however, solve all of the problems
   associated with RADIUS.  The DTLS protocol does not add reliable or
   in-order transport to RADIUS.  DTLS also does not support
   fragmentation of application-layer messages, or of the DTLS messages
   themselves.  This specification therefore shares with traditional
   RADIUS the issues of order, reliability, and fragmentation.

1.1.  Terminology

   This document uses the following terms:

RADIUS/DTLS
     This term is a short-hand for "RADIUS over DTLS".

RADIUS/DTLS client
     This term refers both to RADIUS clients as defined in [RFC2865],
     and to Dynamic Authorization clients as defined in [RFC5176], that
     implement RADIUS/DTLS.




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RADIUS/DTLS server
     This term refers both to RADIUS servers as defined in [RFC2865],
     and to Dynamic Authorization servers as defined in [RFC5176], that
     implement RADIUS/DTLS.

RADIUS/UDP
     RADIUS over UDP, as defined in [RFC2865].

RADIUS/TLS
     RADIUS over TLS, as defined in [RFC6614].

silently discard
     This means that the implementation discards the packet without
     further processing.

1.2.  Requirements Language

   In this document, several words are used to signify the requirements
   of the specification.  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].





























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2.  Building on Existing Foundations

   Adding DTLS as a RADIUS transport protocol requires a number of
   changes to systems implementing standard RADIUS. This section
   outlines those changes, and defines new behaviors necessary to
   implement DTLS.

2.1.  Changes to RADIUS

   The RADIUS packet format is unchanged from [RFC2865], [RFC2866], and
   [RFC5176].  Specifically, all of the following portions of RADIUS
   MUST be unchanged when using RADIUS/DTLS:

      * Packet format
      * Permitted codes
      * Request Authenticator calculation
      * Response Authenticator calculation
      * Minimum packet length
      * Maximum packet length
      * Attribute format
      * Vendor-Specific Attribute (VSA) format
      * Permitted data types
      * Calculations of dynamic attributes such as CHAP-Challenge,
        or Message-Authenticator.
      * Calculation of "obfuscated" attributes such as User-Password
        and Tunnel-Password.

   In short, the application creates a RADIUS packet via the usual
   methods, and then instead of sending it over a UDP socket, sends the
   packet to a DTLS layer for encapsulation.  DTLS then acts as a
   transport layer for RADIUS, hence the names "RADIUS/UDP" and
   "RADIUS/DTLS".

   The requirement that RADIUS remain largely unchanged ensures the
   simplest possible implementation and widest interoperability of this
   specification.

   We note that the DTLS encapsulation of RADIUS means that RADIUS
   packets have an additional overhead due to DTLS.  Implementations
   MUST support encapsulated RADIUS packets of 4096 in length, with a
   corresponding increase in the maximum size of the encapsulated DTLS
   packets.

   The only changes made from RADIUS/UDP to RADIUS/DTLS are the
   following two items:

      (1) The Length checks defined in [RFC2865] Section 3 MUST use the
      length of the decrypted DTLS data instead of the UDP packet



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      length.

      (2) The shared secret secret used to compute the MD5 integrity
      checks and the attribute encryption MUST be "radius/dtls".

   All other aspects of RADIUS are unchanged.

2.2.  Similarities with RADIUS/TLS

   While this specification can be thought of as RADIUS/TLS over UDP
   instead of the Transmission Control Protocol (TCP), there are some
   differences between the two methods.  The bulk of [RFC6614] applies
   to this specification, so we do not repeat it here.

   This section explains the differences between RADIUS/TLS and
   RADIUS/DTLS, as semantic "patches" to [RFC6614].  The changes are as
   follows:

      * We replace references to "TCP" with "UDP"

      * We replace references to "RADIUS/TLS" with "RADIUS/DTLS"

      * We replace references to "TLS" with "DTLS"

   Those changes are sufficient to cover the majority of the differences
   between the two specifications.  The next section reviews some more
   detailed changes from [RFC6614], giving additional commentary only
   where necessary.

2.2.1.  Changes from RADIUS/TLS to RADIUS/DTLS

   This section describes where this specification is similar to
   [RFC6614], and where it differs.

   Section 2.1 applies to RADIUS/DTLS, with the exception that the
   RADIUS/DTLS port is UDP/2083.

   Section 2.2 applies to RADIUS/DTLS.  Servers and clients need to be
   preconfigured to use RADIUS/DTLS for a given endpoint.

   Most of Section 2.3 applies also to RADIUS/DTLS.  Item (1) should be
   interpreted as applying to DTLS session initiation, instead of TCP
   connection establishment.  Item (2) applies, except for the
   recommendation that implementations "SHOULD" support
   TLS_RSA_WITH_RC4_128_SHA.  This recommendation is a historical
   artifact of RADIUS/TLS, and does not apply to RADIUS/DTLS.  Item (3)
   applies to RADIUS/DTLS.  Item (4) applies, except that the fixed
   shared secret is "radius/dtls", as described above.



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   Section 2.4 applies to RADIUS/DTLS.  Client identies can be
   determined from TLS parameters, instead of relying solely on the
   source IP address of the packet.

   Section 2.5 does not apply to RADIUS/DTLS.  The relationship between
   RADIUS packet codes and UDP ports in RADIUS/DTLS is unchanged from
   RADIUS/UDP.

   Sections 3.1, 3.2, and 3.3 apply to RADIUS/DTLS.

   Section 3.4 item (1) does not apply to RADIUS/DTLS.  Each RADIUS
   packet is encapsulated in one DTLS packet, and there is no "stream"
   of RADIUS packets inside of a TLS session.  Implementors MUST enforce
   the requirements of [RFC2865] Section 3 for the RADIUS Length field,
   using the length of the decrypted DTLS data for the checks.  This
   check replaces the RADIUS method of using the length field from the
   UDP packet.

   Section 3.4 item (3) applies to RADIUS/DTLS when the new port is
   used.  When DTLS is used over the existing RADIUS/UDP ports, the
   relationship between RADIUS packet codes and UDP ports in RADIUS/DTLS
   is unchanged from RADIUS.

   Section 3.4 item (4) applies to RADIUS/DTLS when the new port is
   used.  When DTLS is used over the existing RADIUS/UDP ports, the use
   of negative ICMP responses is unchanged from RADIUS.

   Section 3.4 item (5) applies to RADIUS/DTLS when the new port is
   used.  When DTLS is used over the existing RADIUS/UDP ports, the use
   of negative ICMP responses is unchanged from RADIUS.

   Section 4 does not apply to RADIUS/DTLS.  Protocol compatibility
   considerations are defined in this document.

2.2.2.  Reinforcement of RADIUS/TLS

   We re-iterate that much of [RFC6614] applies to this document.
   Specifically, Section 4 and Section 6 of that document are applicable
   to RADIUS/DTLS.

3.  Interaction with RADIUS/UDP

   Transitioning to DTLS is a process which needs to be done carefully.
   A poorly handled transition is complex for administrators, and
   potentially subject to security downgrade attacks.  It is not
   sufficient to just disable RADIUS/UDP and enable RADIUS/DTLS.  That
   approach would result in timeouts, lost traffic, and network
   instabilities.



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   The end result of this specification is that nearly all RADIUS/UDP
   implementations should transition to using a secure alternative.  In
   some cases, RADIUS/UDP may remain where IPSec is used as a transport,
   or where implementation and/or business reasons preclude a change.
   However, long-term use of RADIUS/UDP is NOT RECOMMENDED.

   This section describes how clients and servers should use
   RADIUS/DTLS, and how it interacts with RADIUS/UDP.

3.1.  DTLS Port and Packet Types

   The default destination port number for RADIUS/DTLS is UDP/2083.
   There are no separate ports for authentication, accounting, and
   dynamic authorization changes.  The source port is arbitrary.  The
   text above in Section 2.2.1 describes issues surrounding the use of
   one port for multiple packet types, by referencing [RFC6614] Section
   3.4.

3.2.  Server Behavior

   When a server receives packets on UDP/2083, all packets MUST be
   treated as being DTLS.  RADIUS/UDP packets MUST NOT be accepted on
   this port.

   Servers MUST NOT accept DTLS packets on the old RADIUS/UDP ports.
   Early drafts of this specification permitted this behavior.  It is
   forbidden here, as it depended on behavior in DTLS which may change
   without notice.

   As RADIUS has no provisions for capability signalling, there is no
   way for a RADIUS server to indicate to a client that it should
   transition to using DTLS.  This action has to be taken by the
   administrators of the two systems, using a method other than RADIUS.

   Some servers maintain a list of allowed clients per destination port.
   Others maintain a global list of clients, which are permitted to send
   packets to any port.  Where a client can send packets to multiple
   ports, the server MUST maintain a "DTLS Required" flag per client.

   This flag indicates whether or not the client is required to use
   DTLS.  When set, the flag indicates that the only traffic accepted
   from the client is over UDP/2083.  When packets are received from a
   client on non-DTLS ports, for which DTLS is required, the server MUST
   silently discard these packets, as there is no RADIUS/UDP shared
   secret available.

   This flag will often be set by an administrator.  However, if a
   server receives DTLS traffic from a client, it SHOULD notify the



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   administrator that DTLS is available for that client.  It MAY mark
   the client as "DTLS Required".

   Allowing RADIUS/UDP and RADIUS/DTLS from the same client exposes the
   traffic to downbidding attacks, and is NOT RECOMMENDED.

4.  Client Behavior

   When a client sends packets to the assigned RADIUS/DTLS port, all
   packets MUST be DTLS.  RADIUS/UDP packets MUST NOT be sent to this
   port.

   RADIUS/DTLS clients SHOULD NOT probe servers to see if they support
   DTLS transport.  Doing so could cause problems for servers which do
   not implement DTLS.  Instead, clients SHOULD use DTLS as a transport
   layer only when administratively configured.

   RADIUS clients often had multiple independent RADIUS implementations,
   or processes that originate packets.  This practice was simple to
   implement, but means that each independent subsystem must
   independently discover network issues or server failures.  It is
   therefore RECOMMENDED that clients use a local proxy as described in
   Section 6.1, below.

   Clients may implement "pools" of servers for fail-over or load-
   balancing.  These pools SHOULD NOT mix RADIUS/UDP and RADIUS/DTLS
   servers.

5.  Connection Management

   Where [RFC6614] can rely on the TCP state machine to perform
   connection tracking, this specification cannot.  As a result,
   implementations of this specification may need to perform connection
   management of the DTLS session in the application layer.  This
   section describes logically how this tracking is done.
   Implementations may choose to use the method described here, or
   another, equivalent method.

   We note that [RFC5080] Section 2.2.2 already mandates a duplicate
   detection cache.  The connection tracking described below can be seen
   as an extension of that cache, where entries contain DTLS sessions
   instead of RADIUS/UDP packets.

   [RFC5080] section 2.2.2 describes how duplicate RADIUS/UDP requests
   result in the retransmission of a previously cached RADIUS/UDP
   response.  Due to DTLS sequence window requirements, a server MUST
   NOT retransmit a previously sent DTLS packet.  Instead, it should
   cache the RADIUS response packet, and re-process it through DTLS to



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   create a new RADIUS/DTLS packet, every time it is necessary to
   retransmit a RADIUS response.

5.1.  Server Connection Management

   A RADIUS/DTLS server MUST track ongoing DTLS client connections based
   on a key composed of the following 4-tuple:

      * source IP address
      * source port
      * destination IP address
      * destination port

   Note that this key is independent of IP address version (IPv4 or
   IPv6).

   Each entry associated with a key contains the following information:

DTLS Data
     An implementation-specific variable containing information about
     the active DTLS connection.

Last Taffic
     A variable containing a timestamp which indicates when this
     connection last received valid traffic.

     Each entry may contain other information, such as idle timeouts,
     connection lifetimes, and other implementation-specific data.

5.1.1.  Session Management

   Session tracking is subject to Denial of Service (DoS) attacks due to
   the ability of an attacker to forge UDP traffic.  RADIUS/DTLS servers
   SHOULD use the stateless cookie tracking technique described in
   [RFC6347] Section 4.2.1.  DTLS sessions SHOULD NOT be tracked until a
   ClientHello packet has been received with an appropriate Cookie
   value.  The requirement to accept RADIUS/UDP and RADIUS/DTLS on the
   same port makes this recommendation difficult to implement in
   practice.  Server implementation SHOULD therefore have a way of
   tracking partially setup DTLS connections.  Servers SHOULD limit both
   the number and impact on resources of partial connections.

   Sessions (both key and entry) MUST deleted when a TLS Closure Alert
   ([RFC5246] Section 7.2.1) or a fatal TLS Error Alert ([RFC5246]
   Section 7.2.2) is received.  When a session is deleted due to failed
   security, the DTLS session MUST be closed, and any TLS session
   resumption parameters for that session MUST be discarded, and all
   tracking information MUST be deleted.



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   Sessions MUST also be deleted when a RADIUS packet fails validation
   due to a packet being malformed, or when it has an invalid Message-
   Authenticator, or invalid Request Authenticator.  There are other
   cases when the specifications require that a packet received via a
   DTLS session be "silently discarded".  In those cases,
   implementations MAY delete the underlying session as described above.
   There are few reasons to communicate with a NAS which is not
   implementing RADIUS.

   The above paragraph can be rephrased more generically.  A session
   MUST be deleted when non-RADIUS traffic is received over it.  This
   specification is for RADIUS, and there is no reason to allow non-
   RADIUS traffic over a RADIUS/DTLS connection.  A session MUST be
   deleted when RADIUS traffic fails to pass security checks.  There is
   no reason to permit insecure networks.  A session SHOULD NOT be
   deleted when a well-formed, but "unexpected" RADIUS packet is
   received over it.  Future specifications may extend RADIUS/DTLS, and
   we do not want to forbid those specifications.

   Once a DTLS session is established, a RADIUS/DTLS server SHOULD use
   DTLS Heartbeats [RFC6520] to determine connectivity between the two
   servers.  A server SHOULD also use watchdog packets from the client
   to determine that the connection is still active.

   As UDP does not guarantee delivery of messages, RADIUS/DTLS servers
   which do not implement an application-layer watchdog MUST also
   maintain a "Last Traffic" timestamp per DTLS session.  The timestamp
   SHOULD be updated on reception of a valid RADIUS/DTLS packet, or a
   DTLS heartbeat.  The timestamp MUST NOT be updated in other
   situations.  When a session has not received a packet for a period of
   time, it is labelled "idle".  The server SHOULD delete idle DTLS
   sessions after an "idle timeout".  The server MAY cache the TLS
   session parameters, in order to provide for fast session resumption.

   This session "idle timeout" SHOULD be exposed to the administrator as
   a configurable setting.  It SHOULD NOT be set to less than 60
   seconds, and SHOULD NOT be set to more than 600 seconds (10 minutes).
   The minimum value useful value for this timer is determined by the
   application-layer watchdog mechanism defined in the following
   section.

   RADIUS/DTLS servers SHOULD also monitor the total number of sessions
   they are tracking.  They SHOULD stop the creating of new sessions
   when a large number are already being tracked.  This "maximum
   sessions" number SHOULD be exposed to administrators as a
   configurable setting.

   RADIUS/DTLS servers SHOULD implement session resumption, preferably



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   stateless session resumption as given in [RFC5077].  This practice
   lowers the time and effort required to start a DTLS session with a
   client, and increases network responsiveness.

   Since UDP is stateless, the potential exists for the client to
   initiate a new DTLS session using a particular 4-tuple, before the
   server has closed the old session.  For security reasons, the server
   must keep the old session active until it has received secure
   notification from the client that the session is closed.  Or, when
   the server has decided for itself that the session is closed.  Taking
   any other action would permit unauthenticated clients to perform a
   DoS attack, by closing active DTLS session.

   As a result, servers MUST ignore any attempts to re-use an existing
   4-tuple from an active session.  This requirement can likely be
   reached by simply processing the packet through the existing session,
   as with any other packet received via that 4-tuple.  Non-compliant,
   or unexpected packets will be ignored by the DTLS layer.

   The above requirement is mitigated by the suggestion in Section 6.1,
   below, that the client use a local proxy for all RADIUS traffic.
   That proxy can then track the ports which it uses, and ensure that
   re-use of 4-tuples is avoided.  The exact process by which this
   tracking is done is outside of the scope of this document.

5.2.  Client Connection Management

   Clients SHOULD use Path MTU (PMTU) discovery [RFC6520] to determine
   the PMTU between the client and server, prior to sending any RADIUS
   traffic.  Once a DTLS session is established, a RADIUS/DTLS client
   SHOULD use DTLS Heartbeats [RFC6520] to determine connectivity
   between the two systems.  Alternatively, RADIUS/DTLS clients may use
   the application-layer watchdog algorithm defined in [RFC3539] to
   determine server responsiveness.  The Status-Server packet defined in
   [RFC5997] SHOULD be used as the "watchdog packet" in any application-
   layer watchdog algorithm.

   RADIUS/DTLS clients SHOULD pro-actively close sessions when they have
   been idle for a period of time.  Clients SHOULD close a session when
   the DTLS Heartbeat algorithm indicates that the session is no longer
   active.  Clients SHOULD close a session when no traffic other than
   watchdog packets and (possibly) watchdog responses have been sent for
   three watchdog timeouts.  This behavior ensures that clients do not
   waste resources on the server by causing it to track idle sessions.

   A client may choose to avoid DTLS heartbeats and watchdog packets
   entirely.  However, DTLS provides no signal that a session has been
   closed.  There is therefore the possibility that the server closes



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   the session without the client knowing.  When that happens, the
   client may later transmit packets in a session, and those packets
   will be ignored by the server.  The client is then forced to time out
   those packets and then the session, leading to delays and network
   instabilities.

   For these reasons, it is RECOMMENDED that RADIUS/DTLS clients
   implement DTLS heartbeats and/or watchdog packets for all DTLS
   sessions.

   DTLS sessions MUST also be deleted when a RADIUS packet fails
   validation due to a packet being malformed, or when it has an invalid
   Message-Authenticator, or invalid Response Authenticator.  There are
   other cases when the specifications require that a packet received
   via a DTLS session be "silently discarded".  In those cases,
   implementations MAY delete the underlying DTLS session.

   RADIUS/DTLS clients SHOULD NOT send both RADIUS/UDP and RADIUS/DTLS
   packets to different servers from the same source socket.  This
   practice causes increased complexity in the client application, and
   increases the potential for security breaches due to implementation
   issues.

   RADIUS/DTLS clients SHOULD implement session resumption, preferably
   stateless session resumption as given in [RFC5077].  This practice
   lowers the time and effort required to start a DTLS session with a
   server, and increases network responsiveness.

6.  Implementation Guidelines

   The text above describes the protocol.  In this section, we give
   additional implementation guidelines.  These guidelines are not part
   of the protocol, but may help implementors create simple, secure, and
   inter-operable implementations.

   Where a TLS pre-shared key (PSK) method is used, implementations MUST
   support keys of at least 16 octets in length.  Implementations SHOULD
   support key lengths of 32 octets, and SHOULD allow for longer keys.
   The key data MUST be capable of being any value (0 through 255,
   inclusive).  Implementations MUST NOT limit themselves to using
   textual keys.  It is RECOMMENDED that the administration interface
   allows for the keys to be entered as hex strings.

   It is RECOMMENDED that keys be derived from a cryptographically
   secure pseudo-random number generator (CSPRNG).  If managing keys is
   too complicated, a certificate-based TLS method SHOULD be used
   instead.




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6.1.  Client Implementations

   RADIUS/DTLS clients SHOULD use connected sockets where possible.  Use
   of connected sockets means that the underlying kernel tracks the
   sessions, so that the client subsystem does not need to.  It is a
   good idea to leverage existing functionality.

   RADIUS/DTLS clients SHOULD use one source when sending packets to a
   particular RADIUS/DTLS server.  Doing so minimizes the number of DTLS
   session setups.  It also ensures that information about the home
   server state is discovered only once.

   In practice, this means that RADIUS/DTLS clients SHOULD use a local
   proxy which arbitrates all RADIUS traffic between the client and all
   servers.  The proxy SHOULD accept traffic only from the authorized
   subsystems on the client machine, and SHOULD proxy that traffic to
   known servers.  Each authorized subsystem SHOULD include an attribute
   which uniquely identifies that subsystem to the proxy, so that the
   proxy can apply origin-specific proxy rules and security policies.
   We suggest using NAS-Identifier for this purpose.

   The local proxy SHOULD be able to interact with multiple servers at
   the same time.  There is no requirement that each server have its own
   unique proxy on the client, as that would be inefficient.

   Each client subsystem can include a subsystem-specific NAS-Identifier
   in each request.  The format of this attribute is implementation-
   specific.  The proxy SHOULD verify that the request originated from
   the local system, ideally via a loopback address.  The proxy MUST
   then re-write any subsystem-specific NAS-Identifier to a NAS-
   Identifier which identifies the client as a whole.  Or, remove NAS-
   Identifier entirely and replace it with NAS-IP-Address or NAS-
   IPv6-Address.

   In traditional RADIUS, the cost to set up a new "session" between a
   client and server was minimal.  The client subsystem could simply
   open a port, send a packet, wait for the response, and the close the
   port.  With RADIUS/DTLS, the connection setup is significantly more
   expensive.  In addition, there may be a requirement to use DTLS in
   order to communicate with a server, as RADIUS/UDP may not be
   supported by that server.  The knowledge of what protocol to use is
   best managed by a dedicated RADIUS subsystem, rather than by each
   individual subsystem on the client.

6.2.  Server Implementations

   RADIUS/DTLS servers SHOULD NOT use connected sockets to read DTLS
   packets from a client.  This recommendation is because a connected



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   UDP socket will accept packets only from one source IP address and
   port.  This limitation would prevent the server from accepting
   packets from multiple clients on the same port.

7.  Implementation Experience

   Two implementations of RADIUS/DTLS exist, Radsecproxy, and jradius
   (http://www.coova.org/JRadius).  Some experimental tests have been
   performed, but there are at this time no production implementations
   using RADIUS/DTLS.

   Section 4.2 of [RFC6421] makes a number of recommendations about
   security properties of new RADIUS proposals.  All of those
   recommendations are satisfied by using DTLS as the transport layer.

   Section 4.3 of [RFC6421] makes a number of recommendations about
   backwards compatibility with RADIUS.  Section 3, above, addresses
   these concerns in detail.

   Section 4.4 of [RFC6421] recommends that change control be ceded to
   the IETF, and that interoperability is possible.  Both requirements
   are satisfied.

   Section 4.5 of [RFC6421] requires that the new security methods apply
   to all packet types.  This requirement is satisfied by allowing DTLS
   to be used for all RADIUS traffic.  In addition, Section 3, above,
   addresses concerns about documenting the transition from legacy
   RADIUS to crypto-agile RADIUS.

   Section 4.6 of [RFC6421] requires automated key management.  This
   requirement is satisfied by leveraging DTLS.

8.  Diameter Considerations

   This specification defines a transport layer for RADIUS.  It makes no
   other changes to the RADIUS protocol.  As a result, there are no
   Diameter considerations.

9.  IANA Considerations

   No new RADIUS attributes or packet codes are defined.  IANA is
   requested to update the already-assigned UDP port number 2083 in the
   following ways:

   o  Reference: list the RFC number of this document as the reference

   o  Assignment Notes: add the text "The UDP port 2083 was already
      previously assigned by IANA for "RadSec", an early implementation



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      of RADIUS/TLS, prior to issuance of this RFC."

10.  Security Considerations

   This entire specification is devoted to discussing security
   considerations related to RADIUS.  However, we discuss a few
   additional issues here.

   This specification relies on the existing DTLS, RADIUS/UDP, and
   RADIUS/TLS specifications.  As a result, all security considerations
   for DTLS apply to the DTLS portion of RADIUS/DTLS.  Similarly, the
   TLS and RADIUS security issues discussed in [RFC6614] also apply to
   this specification.  All of the security considerations for RADIUS
   apply to the RADIUS portion of the specification.

   However, many security considerations raised in the RADIUS documents
   are related to RADIUS encryption and authorization.  Those issues are
   largely mitigated when DTLS is used as a transport method.  The
   issues that are not mitigated by this specification are related to
   the RADIUS packet format and handling, which is unchanged in this
   specification.

   The main portion of the specification that could have security
   implications is a servers ability to accept both RADIUS and DTLS
   packets on the same port.  The filter that disambiguates the two
   protocols is simple, and is just a check for the value of one octet.
   We do not expect this check to have any security issues.

   We also note that nothing prevents malicious clients from sending
   DTLS packets to existing RADIUS implementations, or RADIUS packets to
   existing DTLS implementations.  There should therefore be no issue
   with clients sending RADIUS/DTLS packets to legacy servers that do
   not support the protocol.  These packets will be silently discarded,
   and will not change the security profile of the server.

   This specification also suggests that implementations use a
   connection tracking table.  This table is an extension of the
   duplicate detection cache mandated in [RFC5080] Section 2.2.2.  The
   changes given here are that DTLS-specific information is tracked for
   each table entry.  Section 5.1.1, above, describes steps to mitigate
   any DoS issues which result from tracking additional information.

10.1.  Legacy RADIUS Security

   We reiterate here the poor security of the legacy RADIUS protocol.
   It is RECOMMENDED that all RADIUS clients and servers implement this
   specification.  New attacks on MD5 have appeared over the past few
   years, and there is a distinct possibility that MD5 may be completely



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   broken in the near future.

   The existence of fast and cheap attacks on MD5 could result in a loss
   of all network security which depends on RADIUS.  Attackers could
   obtain user passwords, and possibly gain complete network access.  We
   cannot overstate the disastrous consequences of a successful attack
   on RADIUS.

   We also caution implementors (especially client implementors) about
   using RADIUS/DTLS.  It may be tempting to use the shared secret as
   the basis for a TLS pre-shared key (PSK) method, and to leave the
   user interface otherwise unchanged.  This practice MUST NOT be used.
   The administrator MUST be given the option to use DTLS.  Any shared
   secret used for RADIUS/UDP MUST NOT be used for DTLS.  Re-using a
   shared secret between RADIUS/UDP and RADIUS/DTLS would negate all of
   the benefits found by using DTLS.

   RADIUS/DTLS client implementors MUST expose a configuration that
   allows the administrator to choose the cipher suite.  Where
   certificates are used, RADIUS/DTLS client implementors MUST expose a
   configuration which allows an administrator to configure all
   certificates necessary for certificate-based authentication.  These
   certificates include client, server, and root certificates.

   TLS-PSK methods are susceptible to dictionary attacks.  Section 6,
   above, recommends deriving TLS-PSK keys from a CSPRNG, which makes
   dictionary attacks significantly more difficult.  Servers SHOULD
   track failed client connections by TLS-PSK ID, and block TLS-PSK IDs
   which seem to be attempting brute-force searchs of the keyspace.

   The historic RADIUS practice of using shared secrets that are minor
   variations of words is NOT RECOMMENDED, as it would negate all of the
   security of DTLS.

10.2.  Resource Exhaustion

   The use of DTLS allows DoS attacks, and resource exhaustion attacks
   which were not possible in RADIUS/UDP.  These attacks are the similar
   to those described in [RFC6614] Section 6, for TCP.

   Session tracking as described in Section 5.1 can result in resource
   exhaustion.  Servers MUST therefore limit the absolute number of
   sessions that they track.  When the total number of sessions tracked
   is going to exceed the configured limit, servers MAY free up
   resources by closing the session which has been idle for the longest
   time.  Doing so may free up idle resources which then allow the
   server to accept a new session.




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   Servers MUST limit the number of partially open DTLS sessions.  These
   limits SHOULD be exposed to the administrator as configurable
   settings.

10.3.  Network Address Translation

   Network Address Translation (NAT) is fundamentally incompatible with
   RADIUS/UDP.  RADIUS/UDP uses the source IP address to determine the
   shared secret for the client, and NAT hides many clients behind one
   source IP address.

   In addition, port re-use on a NAT gateway means that packets from
   different clients may appear to come from the same source port on the
   NAT.  That is, a RADIUS server may receive a RADIUS/DTLS packet from
   a client IP/port combination, followed by the reception of a
   RADIUS/UDP packet from that same client IP/port combination.  If this
   behavior is allowed, it would permit a downgrade attack to occur, and
   would negate all of the security added by RADIUS/DTLS.

   As a result, RADIUS clients SHOULD NOT be located behind a NAT
   gateway.  If clients are located behind a NAT gateway, then a secure
   transport such as DTLS MUST be used.  As discussed below, a method
   for uniquely identifying each client MUST be used.

10.4.  Wildcard Clients

   Some RADIUS server implementations allow for "wildcard" clients.
   That is, clients with an IPv4 netmask of other than 32, or an IPv6
   netmask of other than 128.  That practice is NOT RECOMMENDED for
   RADIUS/UDP, as it means multiple clients use the same shared secret.

   When a client is a "wildcard", RADIUS/DTLS MUST be used.  Clients
   MUST be uniquely identified, and any certificate or PSK used MUST be
   unique to each client.

10.5.  Session Closing

   Section 5.1.1 above requires that DTLS sessions be closed when the
   transported RADIUS packets are malformed, or fail various
   authenticator checks.  This requirement is due to security
   considerations.

   When an implementation has a DTLS connection, it is expected that the
   connection be used to transport RADIUS.  Any non-RADIUS traffic on
   that connection means the other party is misbehaving, and a potential
   security risk.  Similarly, any RADIUS traffic failing validation
   means that two parties do not share the same security parameters, and
   the session is therefore a security risk.



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   We wish to avoid the situation where a third party can send well-
   formed RADIUS packets which cause a DTLS connection to close.
   Therefore, in other situations, the session may remain open in the
   face of non-conformant packets.

10.6.  Clients Subsystems

   Many traditional clients treat RADIUS as subsystem-specific.  That
   is, each subsystem on the client has its own RADIUS implementation
   and configuration.  These independent implementations work for simple
   systems, but break down for RADIUS when multiple servers, fail-over,
   and load-balancing are required.  They have even worse issues when
   DTLS is enabled.

   As noted in Section 6.1, above, clients SHOULD use a local proxy
   which arbitrates all RADIUS traffic between the client and all
   servers.  This proxy will encapsulate all knowledge about servers,
   including security policies, fail-over, and load-balancing.  All
   client subsystems SHOULD communicate with this local proxy, ideally
   over a loopback address.  The requirements on using strong shared
   secrets still apply.

   The benefit of this configuration is that there is one place in the
   client which arbitrates all RADIUS traffic.  Subsystems which do not
   implement DTLS can remain unaware of DTLS.  DTLS connections opened
   by the proxy can remain open for long periods of time, even when
   client subsystems are restarted.  The proxy can do RADIUS/UDP to some
   servers, and RADIUS/DTLS to others.

   Delegation of responsibilities and separation of tasks are important
   security principles.  By moving all RADIUS/DTLS knowledge to a DTLS-
   aware proxy, security analysis becomes simpler, and enforcement of
   correct security becomes easier.

11.  References

11.1.  Normative references

[RFC2865]
     Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
     Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.

[RFC3539]
     Aboba, B. et al., "Authentication, Authorization and Accounting
     (AAA) Transport Profile", RFC 3539, June 2003.

[RFC5077]
     Salowey, J, et al., "Transport Layer Security (TLS) Session



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     Resumption without Server-Side State", RFC 5077, January 2008

[RFC5080]
     Nelson, D. and DeKok, A, "Common Remote Authentication Dial In User
     Service (RADIUS) Implementation Issues and Suggested Fixes", RFC
     5080, December 2007.

[RFC5246]
     Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
     Protocol Version 1.2", RFC 5246, August 2008.

[RFC5997]
     DeKok, A., "Use of Status-Server Packets in the Remote
     Authentication Dial In User Service (RADIUS) Protocol", RFC 5997,
     August 2010.

[RFC6347]
     Rescorla E., and Modadugu, N., "Datagram Transport Layer Security",
     RFC 6347, April 2006.

[RFC6520]
     Seggelmann, R., et al.,"Transport Layer Security (TLS) and Datagram
     Transport Layer Security (DTLS) Heartbeat Extension", RFC 6520,
     February 2012.

[RFC6614]
     Winter. S, et. al., "TLS encryption for RADIUS over TCP", RFFC
     6614, May 2012

11.2.  Informative references

[RFC1321]
     Rivest, R. and S. Dusse, "The MD5 Message-Digest Algorithm", RFC
     1321, April 1992.

[RFC2119]
     Bradner, S., "Key words for use in RFCs to Indicate Requirement
     Levels", RFC 2119, March, 1997.

[RFC2866]
     Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

[RFC5176]
     Chiba, M. et al., "Dynamic Authorization Extensions to Remote
     Authentication Dial In User Service (RADIUS)", RFC 5176, January
     2008.





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[RFC6421]
     Nelson, D. (Ed), "Crypto-Agility Requirements for Remote
     Authentication Dial-In User Service (RADIUS)", RFC 6421, November
     2011.

[MD5Attack]
     Dobbertin, H., "The Status of MD5 After a Recent Attack",
     CryptoBytes Vol.2 No.2, Summer 1996.

[MD5Break]
     Wang, Xiaoyun and Yu, Hongbo, "How to Break MD5 and Other Hash
     Functions", EUROCRYPT. ISBN 3-540-25910-4, 2005.

Acknowledgments

   Parts of the text in Section 3 defining the Request and Response
   Authenticators were taken with minor edits from [RFC2865] Section 3.

Authors' Addresses

   Alan DeKok
   The FreeRADIUS Server Project
   http://freeradius.org

   Email: aland@freeradius.org


























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