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Network Working Group                                         Alan DeKok
INTERNET-DRAFT                                                FreeRADIUS
Category: Informational
<draft-ietf-radext-dtls-01.txt>
Expires: April 12, 2011
12 October 2010


                  DTLS as a Transport Layer for RADIUS
                       draft-ietf-radext-dtls-01


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 "hidden"
   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 April 12, 2011

Copyright Notice



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   Copyright (c) 2010 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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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.  Changes from RADIUS over TLS (RADIUS/TLS) ...........    6
      2.2.1.  Changes from RADIUS/TLS to RADIUS/DTLS .........    7
      2.2.2.  Reinforcement of RADIUS/TLS ....................    8
3.  Reception of Packets .....................................    8
   3.1.  Protocol Disambiguation .............................    9
4.  Connection Management ....................................   10
   4.1.  Server Connection Management ........................   10
      4.1.1.  Table Management ...............................   10
   4.2.  Client Connection Management ........................   11
5.  Processing Algorithm .....................................   12
6.  Diameter Considerations ..................................   14
7.  IANA Considerations ......................................   14
8.  Security Considerations ..................................   14
   8.1.  Legacy RADIUS Security ..............................   14
   8.2.  Network Address Translation .........................   15
9.  References ...............................................   16
   9.1.  Normative references ................................   16
   9.2.  Informative references ..............................   17


























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1.  Introduction

   The RADIUS protocol as described in [RFC2865], [RFC2866], and
   [RFC5176] 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, previous 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 [RFC4347] 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 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 continues to have all of
   the issues that RADIUS currently has with 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.

silently discard
     This means that the implementation discards the packet without
     further processing.  The implementation MAY provide the capability
     of logging the error, including the contents of the silently
     discarded packet, and SHOULD record the event in a statistics
     counter.

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 over 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 "encrypted" attributes such as Tunnel-Password.
      * UDP port numbering and usage

   The RADIUS packets are encapsulated in DTLS, which acts as a
   transport layer for it. The requirements above ensure the simplest
   possible implementation and widest interoperability of this
   specification.

   The only changes made to RADIUS in this specification 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
      length.

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

   All other portions of RADIUS are unchanged.

2.2.  Changes from RADIUS over TLS (RADIUS/TLS)

   While this specification is largely RADIUS/TLS over UDP instead of
   TCP, there are some differences between the two methods.



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   This section goes through the [RADIUS/TLS] document in detail,
   explaining the differences between RADIUS/TLS and RADIUS/DTLS.  As
   most of [RADIUS/TLS] also applies to RADIUS/DTLS, we highlight only
   the changes here, explaining how to interpret [RADIUS/TLS] for this
   specification:

      * 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 text below goes through some of
   the sections of [RADIUS/TLS], giving additional commentary only where
   necessary.

2.2.1.  Changes from RADIUS/TLS to RADIUS/DTLS

   Section 2.1 does not apply to RADIUS/DTLS.  The relationship between
   RADIUS packet codes and UDP ports in RADIUS/DTLS is unchanged from
   RADIUS/UDP.

   Section 2.2 applies also to RADIUS/DTLS, except for the
   recommendation that implementations "SHOULD" support
   TLS_RSA_WITH_RC4_128_SHA, which does not apply to RADIUS/DTLS.

   Section 2.3 applies also to RADIUS/TLS.

   Section 2.4 does not apply to RADIUS/DTLS.  See the comments above on
   Section 2.1.  The relationship between RADIUS packet codes and UDP
   ports in RADIUS/DTLS is unchanged from RADIUS/UDP.

   Section 3.3 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 3.3 item (3) does not apply to RTDLS.  The relationship
   between RADIUS packet codes and UDP ports in RADIUS/DTLS is unchanged
   from RADIUS.

   Section 3.3 item (4) does not apply to RADIUS/DTLS.  As RADIUS/DTLS
   still uses UDP for a transport, the use of negative ICMP responses is
   unchanged from RADIUS.



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2.2.2.  Reinforcement of RADIUS/TLS

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

3.  Reception of Packets

   As this specification permits implementations to to accept both
   traditional RADIUS and DTLS packets on the same port, we define a
   method to disambiguate between packets for the two protocols.  This
   method is applicable only to RADIUS servers.  RADIUS/DTLS clients
   SHOULD use connected sockets, as discussed in Section X.Y, below.

   RADIUS/DTLS servers MUST maintain a boolean flag for each RADIUS
   client that indicates whether or not it supports RADIUS/DTLS.  The
   interpretation of this flag is as follows. If the flag is "false",
   then the client may support RADIUS/DTLS.  Packets from the client
   need to be examined to see if they are RADIUS or RADIUS/DTLS.  If the
   flag is "true" then the client supports RADIUS/DTLS, and all packets
   from that client MUST be processed as RADIUS/DTLS.

   Note that this last requirement can impose significant changes for
   RADIUS clients.  Clients can no longer have multiple independent
   RADIUS implementations or processes that originate packets.  We
   RECOMMEND that RADIUS/DTLS clients implement a local RADIUS proxy
   that arbitrates all RADIUS traffic.

   This flag MUST be exposed to administrators of the RADIUS server.  As
   RADIUS clients are upgraded, administrators can then manually mark
   them as supporting RADIUS/DTLS.

   We recognize, however, the upgrade path from RADIUS to RADIUS/DTLS is
   important.  This path requires an RADIUS/DTLS server to accept
   packets from a RADIUS client without knowing beforehand if they are
   RADIUS or DTLS.  The method to distinguish between the two is defined
   in the next section.

   Once an RADIUS/DTLS server has established a DTLS session with a
   client that had the flag set to "false", it MUST set the flag to
   "true".  This change forces all subsequent traffic from that client
   to use DTLS, and prevents bidding-down attacks.  The server SHOULD
   also notify the administrator that it has successfully established
   the first DTLS session with that client.







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3.1.  Protocol Disambiguation

   When a RADIUS client is not marked as supporting RADIUS/DTLS, packets
   from that client may be, or may not be DTLS.  In order to provide a
   robust upgrade path, the RADIUS/DTLS server MUST examine the packet
   to see if it is RADIUS or DTLS.  In order to justify the examination
   methods, we first examine the packet formats for the two protocols.

   The DTLS record format ([RFC4347] Section 4.1) is shown below, in
   pseudo-code:

         struct {
                 uint8 type;
                 uint16 version;
                 uint16 epoch;
                 uint48 sequence_number;
                 uint16 length;
                 uint8 fragment[DTLSPlaintext.length];
         } DTLSPlaintext;

   The RADIUS record format ([RFC2865] Section 3) is shown below, in
   pseudo-code, with AuthVector.length=16.

         struct {
                 uint8 code;
                 uint8 id;
                 uint16 length;
                 uint8 vector[AuthVector.length];
                 uint8 data[RadiusPacket.length - 20];
         } RadiusPacket;

   We can see here that a number of fields overlap between the two
   protocols.  The low byte of the DTLS version and the high byte of the
   DTLS epoch overlap with the RADIUS length field.  The DTLS length
   field overlaps with the RADIUS authentication vector.  At first
   glance, it may be difficult for an application to accept both
   protocols on the same port.  However, this is not the case.

   For the initial packet of a DTLS connection, the type field has value
   22 (handshake), and the epoch and sequence number fields are
   initialized to zero.  The RADIUS code value of 22 has been assigned
   as Resource-Free-Response, but it is not in wide use.  In addition,
   that packet code is a response packet, and would not be sent by a
   RADIUS client to a server.

   As a result, protocol disambiguation is straightforward.  If the
   first byte of the packet has value 22, it is a DTLS packet, and is a
   DTLS connection initiation request.  Otherwise, it is a RADIUS



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   packet.

   Once a DTLS session has been established, a separate tracking table
   is used to disambiguate the protocols.  The definition of this
   tracking table is given in the next section.

   The full processing algorithm is given below, in Section X.Y.

4.  Connection Management

   Where [RADIUS/TLS] can rely on the TCP state machine to perform
   connection tracking, this specification cannot.  As a result,
   implementations of this specification will need to perform connection
   management of the DTLS session in the application layer.

4.1.  Server Connection Management

   An RADIUS/DTLS server MUST maintain a table that tracks ongoing DTLS
   sessions based on a key composed of the following 4-tuple:

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

   The contents of the tracking table are a implementation-specific
   value that describes an active DTLS session.  This connection
   tracking allows DTLS packets that have been received to be associated
   with an active DTLS session.

   RADIUS/DTLS servers SHOULD NOT use a "connect" API to manage DTLS
   connections, as a connected UDP socket will accept packets only from
   one source IP address and port.  This limitation would prevent the
   server from engaging in the normal RADIUS practice of accepting
   packets from multiple clients on the same port.

   Note that [RFC5080] Section 2.2.2 defines a duplicate detection cache
   which tracks packets by key similar to that defined above.

4.1.1.  Table Management

   This tracking table is subject to Denial of Service (DoS) attacks.
   RADIUS/DTLS servers SHOULD use the stateless cookie tracking
   technique described in [RFC4347] Section 4.2.1.  DTLS sessions SHOULD
   NOT be added to the tracking table until a ClientHello packet has
   been received with an appropriate Cookie value.

   Entries in the tracking table MUST deleted when a TLS Closure Alert



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   ([RFC5246] Section 7.2.1) or a TLS Error Alert ([RFC5246] Section
   7.2.2) is received.  Where the RADIUS specifications require that a
   RADIUS packet received via the DTLS session is to be "silently
   discarded", the entry in the tracking table corresponding to that
   DTLS session MUST also be deleted, the DTLS session MUST be closed,
   and any TLS session resumption parameters for that session MUST be
   discarded.

   As UDP does not offer guaranteed delivery of messages, RADIUS/DTLS
   servers MUST also maintain a timestamp per DTLS session.  The
   timestamp SHOULD be updated on reception of a valid DTLS packet.  The
   timestamp MUST NOT be updated in other situations.  When a session
   has not been used for a period of time, the server SHOULD pro-
   actively close it, and delete the DTLS session from the tracking
   table.  The server MAY cache the TLS session parameters, in order to
   provide for fast session resumption.

   This session lifetime SHOULD be exposed as 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 keep track of the total number of
   sessions in the tracking table, and refuse to create new sessions
   when a large number are already being tracked.  As system
   capabilities vary widely, we can only recommend that this number
   SHOULD be exposed as a configurable setting.

4.2.  Client Connection Management

   RADIUS/DTLS clients SHOULD use an operating system API to "connect" a
   UDP socket.  This "connected" socket will then rely on the operating
   system to perform connection tracking, and will be simpler than the
   method described above for servers.  RADIUS/DTLS clients SHOULD NOT
   use "unconnected" sockets, as it causes increased complexity in the
   client application.

   Once a DTLS session is established, an RADIUS/DTLS client SHOULD use
   the application-layer watchdog algorithm defined in [RFC3539] to
   determine server responsiveness.  The Status-Server packet defined in
   [RFC5997] MUST be used as the "watchdog packet" in the watchdog
   algorithm.

   RADIUS/DTLS clients SHOULD pro-actively close sessions when they have
   been idle for a period of time.  We RECOMMEND that a session be
   closed when no traffic over than watchdog packets and (possibly)
   responses have been sent for three watchdog timeouts.  This behavior



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   ensures that clients do not waste resources on the server by causing
   it to track idle sessions.

   RADIUS/DTLS clients SHOULD NOT send both normal RADIUS and
   RADIUS/DTLS packets 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 MUST NOT send both normal RADIUS and RADIUS/DTLS
   packets over the same key as defined in Section 4.1, abovre (source
   IP, source port, destination IP, destination port).  Doing so would
   require that servers perform RADIUS and RADIUS/DTLS determination for
   every packet that has been received.

   RADIUS/DTLS clients SHOULD use TLS session resumption, where
   possible.  This practice lowers the time and effort required to start
   a DTLS session with a server, and increases network responsiveness.

5.  Processing Algorithm

   The following algorithm MUST be used by an implementation of this
   protocol.  This algorithm is used to route packets to the appropriate
   destination.  We assume the following variables:

      D - implementation-specific handle to an existing DTLS session

      P - UDP packet received from the network.  This packet MUST
          also contain information about source IP/port, and
          destination IP/port.

      R - a RADIUS packet

      T - a tracking table used to manage ongoing DTLS sessions

   We also presume the following functions or functionality exists:

      receive_packet_from_network() - a function that reads a packet
      from the network, and returns P as above.  We presume also that
      this function performs the normal RADIUS client validation, and
      does not return P if the packet is from an unknown client.

      lookup_dtls_session() - a function that takes a packet P, a table
      T, and uses P to look up the corresponding DTLS session in T.  It
      returns either a session D, or a "null" indicator that no
      corresponding session exists.

      client_supports_rdtls() - a function that takes a packet P, and
      returns a boolean value as to whether or not the client



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      originating the packet was marked as supporting RADIUS/DTLS.

      process_dtls_packet() - a function that takes a DTLS packet P, and
      a DTLS session D.  It performs all necessary steps to use D to
      setup a DTLS session, and to decode P (where possible) into a
      RADIUS packet.  This function is also expected to perform checks
      for TLS errors.  On any fatal errors, it closes the session, and
      deletes D from the tracking table T.  If a RADIUS packet is
      decoded from P, it is returned by the function as R, otherwise a
      "null" indicator is returned.

      process_dtls_clienthello() - a function that takes a DTLS packet
      P, and initiates a DTLS session.  If P contains a valid DTLS
      Cookie, a DTLS session D is created, and stored in the tracking
      table T.  If P does not contain a DTLS Cookie, no session is
      created, and instead a HelloVerifyRequest containing a cookie is
      sent in response.  Packets containing invalid cookies are
      discarded.

      process_radius_packet() - a function that takes a RADIUS packet P,
      and processes it using the normal RADIUS methods.

   The algorithm is as follows:

         P = receive_packet_from_network()
         D = lookup_dtls_session(T, P)

         if (D || client_supports_rdtls(P)) {
            R = process_dtls_packet(D, P)
            if (R) {
               process_radius_packet(R)
            }

         } else if (first_octet_of_packet_is_22(P)) {
            process_dtls_clienthello(P)

         } else {
            process_radius_packet(P)
         }

   For simplicity, the timers necessary to perform expiry of "old"
   sessions are not included in the above algorithm.  This algorithm may
   also need to be modified if the RADIUS/DTLS server supports client
   validation by methods other than source IP address.







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6.  Diameter Considerations

   This specification is for a transport layer specific to RADIUS.  As a
   result, there are no Diameter considerations.

7.  IANA Considerations

   This specification does not create any new registries, nor does it
   require assignment of any protocol parameters.

8.  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, 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 [RADIUS/TLS] 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 only new 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 byte.
   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.

8.1.  Legacy RADIUS Security

   We reiterate here the poor security of the legacy RADIUS protocol.
   We RECOMMEND that all RADIUS clients and servers implement this
   specification as soon as possible.  New attacks on MD5 have appeared
   over the past few years, and there is a distinct possibility that MD5



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   may be completely broken in the near future.

   The existence of fast and cheap attacks on MD5 could result in a loss
   of all network security that depends on RADIUS.  Attackers could
   obtain user passwords, and possibly gain complete network access.  It
   is difficult to 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 MUST NOT be used for DTLS.  Re-using a shared
   secret between RADIUS and DTLS would negate all of the benefits found
   by using DTLS.

   When using PSK methods, RADIUS/DTLS clients MUST support keys (i.e.
   shared secrets) that are at least 32 characters in length.

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

   When using PSK methods, RADIUS/DTLS servers MUST support keys (i.e.
   shared secrets) that are at least 32 characters in length.
   RADIUS/DTLS server administrators MUST use strong shared secrets for
   those PSK methods.  We RECOMMEND using keys derived from a
   cryptographically secure pseudo-random number generator (CSPRNG).
   For example, a reasonable key may be 32 characters of a SHA-256 hash
   of at least 64 bytes of data taken from a CSPRNG.  If this method
   seems too complicated, a certificate-based TLS method SHOULD be used
   instead.

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

8.2.  Network Address Translation

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




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   The migration flag described above in Section 3 is also tracked per
   source IP address.  Using a NAT in front of many RADIUS clients
   negates the function of the flag, making it impossible to migrate
   clients in a secure fashion.

   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
   capability were 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.

9.  References

9.1.  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.

[RFC4347]
     Rescorla E., and Modadugu, N., "Datagram Transport Layer Security",
     RFC 4347, April 2006.

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

[RADIUS/TLS]
     Winter. S, et. al., "TLS encryption for RADIUS over TCP", draft-
     ietf-radext-radsec-06.txt, March 2010 (work in progress)

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






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9.2.  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.

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

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

[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.

   The author would like to thank Mike McCauley of Open Systems
   Consultants for making a Radiator server available for inter-
   operability testing.

Authors' Addresses

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

   Email: aland@freeradius.org




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