Network Working Group                                         Alan DeKok
INTERNET-DRAFT                                                FreeRADIUS
Category: Informational
Expires: May 16, 2013
16 July 2012

                  DTLS as a Transport Layer for RADIUS


   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

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
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   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

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on May 16, 2013

Copyright Notice

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   Copyright (c) 2012 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
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   ( in effect on the date of
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   carefully, as they describe your rights and restrictions with respect
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   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.  Reception of Packets .....................................    8
4.  Connection Management ....................................    9
   4.1.  Server Connection Management ........................    9
      4.1.1.  Table Management ...............................   10
      4.1.2.  Protocol Disambiguation ........................   11
      4.1.3.  Processing Algorithm ...........................   12
   4.2.  Client Connection Management ........................   13
5.  Diameter Considerations ..................................   14
6.  IANA Considerations ......................................   14
7.  Security Considerations ..................................   14
   7.1.  Legacy RADIUS Security ..............................   14
   7.2.  Resource Exhaustion .................................   15
   7.3.  Network Address Translation .........................   16
   7.4.  Wildcard Clients ....................................   16
8.  References ...............................................   16
   8.1.  Normative references ................................   16
   8.2.  Informative references ..............................   17

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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 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

   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:

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

     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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     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 over UDP, as defined in [RFC2865].

     RADIUS over TLS, as defined in [RFC6614].

silently discard
     This means that the implementation discards the packet without
     further processing.  See Section X.Y for additional requirements on
     packets being silently discarded.

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",
   and "OPTIONAL" in this document are to be interpreted as described in

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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.
      * UDP port numbering and relationship between code and port

   In short, the application creates a RADIUS packet as usual, 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

   We note that the DTLS encapsulation of RADIUS means that the minimum
   and maximum UDP packet sizes increase by the DTLS overhead.
   Implementations should be aware of this, and take it into account
   when allocating buffers to read and write RADIUS/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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      (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 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

      * 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

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

   Section 2.2 applies also to RADIUS/DTLS, 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.

   Section 2.3 does not apply to RADIUS/DTLS.

   Section 2.4 does not apply to RADIUS/DTLS.  The relationship between
   RADIUS packet codes and UDP ports in RADIUS/DTLS is unchanged from

   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,

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   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 RADIUS/TDLS.  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.

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
   in their entirety to RADIUS/DTLS.

3.  Reception of Packets

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

   RADIUS/DTLS servers MUST maintain a boolean "DTLS Required" flag for
   each client that indicates if it requires a client to use
   RADIUS/DTLS.  The interpretation of this flag is as follows. If the
   flag is "true" then the client supports RADIUS/DTLS, and all packets
   from that client MUST be processed as RADIUS/DTLS.  If the flag is
   "false", then the client supports RADIUS/UDP, but may still support
   RADIUS/DTLS.  Packets from the client need to be examined to see if
   they are RADIUS/UDP or RADIUS/DTLS.

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

   Once a RADIUS/DTLS server has established a DTLS session with a
   client that previously had the flag set to "false", the server MUST
   set the "DTLS Required" flag to "true".  This change requires 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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   Note that this last requirement on servers can impose significant
   changes for clients.  Clients can no longer have multiple independent
   RADIUS implementations or processes that originate RADIUS/UDP and
   RADIUS/DTLS packets.  Instead, they need to use only one transport
   layer, either UDP or DTLS.

   It is therefore RECOMMENDED that RADIUS/DTLS clients use a local
   proxy which arbitrates all traffic between the client and any
   servers.  The proxy SHOULD accept traffic only from the authorized
   subsystems on the client machine, and SHOULD proxy that traffic to
   one or more known servers.

4.  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 will need to perform connection
   management of the DTLS session in the application layer.

4.1.  Server Connection Management

   A RADIUS/DTLS server MUST maintain a table that tracks ongoing 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 table is independent of IP address version (IPv4 or

   Each table entry contains the following information:

Protocol Type
     A flag which is either "RADIUS/UDP" for old-style RADIUS traffic,
     or "RADIUS/DTLS" for RADIUS/DTLS connections.

     An implementation-specific variable containing information about
     the active DTLS connection.  For non-DTLS connections, this
     variable MUST be empty.

Last Packet
     A variable containing a timestamp which indicates when the last
     valid packet was received for this connection.  Packets which are
     "silently discarded" MUST NOT update this variable.

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

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

4.1.1.  Table Management

   This tracking table 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 added to the
   tracking table 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.

   Entries in the tracking table MUST deleted when a TLS Closure Alert
   ([RFC5246] Section 7.2.1) or a TLS Error Alert ([RFC5246] Section
   7.2.2) is received.  Where the specifications require that a packet
   received via a DTLS session 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.  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.

   As UDP does not guarantee delivery of messages, RADIUS/DTLS servers
   MUST also maintain a "Last Packet" 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 received a packet for a period of time, it is labelled
   "idle".  The server SHOULD delete idle DTLS session from the tracking
   table 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

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   RADIUS/DTLS servers SHOULD also keep track of the total number of
   sessions in the tracking table.  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.

4.1.2.  Protocol Disambiguation

   When the "DTLS Required" flag for a client is set to "false", the
   client may, or may not be sending DTLS packets.  For existing
   connections, protocol disambiguation is simple, the "Protocol Type"
   field in the tracking table entry is examined.  New connections must
   still be disambiguated.

   In order to provide a robust upgrade path, the RADIUS/DTLS server
   MUST examine the packet to see if it is RADIUS/UDP or RADIUS/DTLS.
   This examination method is defined here.

   We justify the examination methods by analysing the packet formats
   for the two protocols.  We assume that the server has a buffer in
   which it has received a UDP packet matching no entry on the
   conneciton tracking table.  It must then analyse this buffer to
   determine which protocol is used to process the packet.

   The DTLS record format ([RFC6347] Section 4.1) is shown below, in

         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

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   protocols.  At first glance, it seems difficult for an application to
   accept both protocols on the same port.  However, this is not the

   The initial DTLS packet of a connection requires that the type field
   (first octet) has value 22 (handshake).  The first octet of a RADIUS
   packet is the code field.  The code value of 22 has been assigned as
   Resource-Free-Response.  That code is intended to be a response from
   a server to a client, and will therefore never be sent by a client to
   a server.

   As a result, protocol disambiguation for new connections to a server
   is straightforward.  Only the first octet of the packet needs to be
   examined to disambiguate RADIUS/DTLS from RADIUS/UDP.  If that octet
   has value 22, then the packet is likely to be RADIUS/DTLS.
   Otherwise, the packet is likely to be RADIUS/UDP.

4.1.3.  Processing Algorithm

   When a RADIUS/DTLS server recieves a packet, it uses the following
   algorithm to process that packet.  As with RADIUS/UDP, packets from
   unknown clients MUST be silently discarded.

   The "DTLS Required" flag for that client is examined.  If it is set
   to "true", then the packet MUST be processed as RADIUS/DTLS.

   If the "DTLS Required" flag is set to "false", the connection
   tracking table is examined.  Packets matching an existing entry MUST
   be processed as defined by the "Protocol Type" field of that entry.

   If the "DTLS Required" flag is set to "false" and no entry exists in
   the connection tracking table, then the first octet of the packet is
   examined.  If it has value 22, then the packet MUST be processed as
   RADIUS/DTLS.  Otherwise, the packet MUST be processed as RADIUS/UDP.

   In all cases, the packet MUST be checked for correctness.  For
   RADIUS/UDP, any packets which are silently discarded MUST NOT affect
   the state of any variable in the session tracking table.  For
   RADIUS/DTLS, any packets which are discarded by the DTLS layer MUST
   NOT affect the state of any variable in the session tracking table.
   For RADIUS/DTLS, any RADIUS packets which are subsequently silently
   discarded MUST result in the removal of the associated entry from the
   connection tracking table.

   When the packet matches an existing entry in the connection table,
   and is accepted for processing by the server, the "Last Packet"
   timestamp is updated.  Where the packet does not match any entry in
   the connection table, a new connection is created using the 4-tuple

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   key defined above.  The "Protocol Type" flag for that connection is
   set to "RADIUS/DTLS", or "RADIUS/UDP", as determined by examining the
   first octet of the packet.

   When a server has the clients "DTLS Required" flag set to "false", it
   MUST set the flag to "true" after establishing a DTLS session with
   that client.  It MUST NOT set the flag to "true" until a DTLS session
   has been fully established.  Doing so would mean that attackers could
   perform a DoS attack by sending forged DTLS ClientHello packets to a

4.2.  Client Connection Management

   Clients SHOULD use "connected" UDP sockets for RADIUS/DTLS traffic.
   A connected socket will then rely on the operating system to perform
   connection tracking.  Clients SHOULD NOT use "unconnected" sockets
   for RADIUS/DTLS traffic.  Using unconnected sockets would require the
   client to implement a connection tracking table, which is complex and

   Once a DTLS session is established, a 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

   RADIUS/DTLS clients SHOULD pro-actively close sessions when they have
   been idle for a period of time.  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.

   packets over the same key of (source IP, source port, destination IP,
   destination port) as defined in Section 4.1, above .  Doing so would
   make it impossible to correctly process either kind of packet.

   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

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

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5.  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.

6.  IANA Considerations

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

7.  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

   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 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 ignored,
   and will not change the security profile of the server.

7.1.  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

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   specification.  New attacks on MD5 have appeared over the past few
   years, and there is a distinct possibility that MD5 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 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.

   When using PSK methods, RADIUS/DTLS servers MUST support keys (i.e.
   shared secrets) that are at least 32 characters in length.  These
   keys SHOULD be able to contain arbitrary binary data.  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
   octetss 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.

7.2.  Resource Exhaustion

   The use of DTLS allows DoS attacks, and resource exhaustion attacks
   which were not possible in RADIUS/UDP.  These attacks are the same as
   described in [RFC6614] Section X.Y.

   Use of the connection tracking table defined in Section X.Y can
   result in resource exhaustion.  Servers MUST therefore limit the

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

7.3.  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.

   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
   multiple 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
   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.

7.4.  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", then RADIUS/DTLS MUST be used.
   Clients MUST be uniquely identified, and any certificate or PSK used
   MUST be unique to each client.

8.  References

8.1.  Normative references

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

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

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

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

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

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

8.2.  Informative references

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

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

     Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

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

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

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

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   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


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