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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 6614

RADIUS Extensions Working Group                                S. Winter
Internet-Draft                                                   RESTENA
Intended status: Experimental                                M. McCauley
Expires: August 17, 2012                                             OSC
                                                               S. Venaas
                                                             K. Wierenga
                                                                   Cisco
                                                       February 14, 2012


          Transport Layer Security (TLS) encryption for RADIUS
                      draft-ietf-radext-radsec-12

Abstract

   This document specifies a transport profile for RADIUS using
   Transport Layer Security (TLS) over TCP as the transport protocol.
   This enables dynamic trust relationships between RADIUS servers.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on August 17, 2012.

Copyright Notice

   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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   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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Document Status  . . . . . . . . . . . . . . . . . . . . .  4
   2.  Normative: Transport Layer Security for RADIUS/TCP . . . . . .  5
     2.1.  TCP port and packet types  . . . . . . . . . . . . . . . .  5
     2.2.  TLS negotiation  . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  Connection Setup . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Connecting Client Identity . . . . . . . . . . . . . . . .  7
     2.5.  RADIUS Datagrams . . . . . . . . . . . . . . . . . . . . .  8
   3.  Informative: Design Decisions  . . . . . . . . . . . . . . . . 10
     3.1.  Implications of Dynamic Peer Discovery . . . . . . . . . . 10
     3.2.  X.509 Certificate Considerations . . . . . . . . . . . . . 10
     3.3.  Ciphersuites and Compression Negotiation Considerations  . 11
     3.4.  RADIUS Datagram Considerations . . . . . . . . . . . . . . 11
   4.  Compatibility with other RADIUS transports . . . . . . . . . . 12
   5.  Diameter Compatibility . . . . . . . . . . . . . . . . . . . . 13
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   8.  Notes to the RFC Editor  . . . . . . . . . . . . . . . . . . . 15
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     10.2. Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Implementation Overview: Radiator . . . . . . . . . . 18
   Appendix B.  Implementation Overview: radsecproxy  . . . . . . . . 19
   Appendix C.  Assessment of Crypto-Agility Requirements . . . . . . 20







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1.  Introduction

   The RADIUS protocol [RFC2865] is a widely deployed authentication and
   authorisation protocol.  The supplementary RADIUS Accounting
   specification [RFC2866] also provides accounting mechanisms, thus
   delivering a full Authentication, Authorization, and Accounting (AAA)
   solution.  However, RADIUS is experiencing several shortcomings, such
   as its dependency on the unreliable transport protocol UDP and the
   lack of security for large parts of its packet payload.  RADIUS
   security is based on the MD5 algorithm, which has been proven to be
   insecure.

   The main focus of RADIUS over TLS is to provide a means to secure the
   communication between RADIUS/TCP peers using TLS.  The most important
   use of this specification lies in roaming environments where RADIUS
   packets need to be transferred through different administrative
   domains and untrusted, potentially hostile networks.  An example for
   a world-wide roaming environment that uses RADIUS over TLS to secure
   communication is "eduroam", see [eduroam].

   There are multiple known attacks on the MD5 algorithm which is used
   in RADIUS to provide integrity protection and a limited
   confidentiality protection (see [MD5-attacks]).  RADIUS over TLS
   wraps the entire RADIUS packet payload into a TLS stream and thus
   mitigates the risk of attacks on MD5.

   Because of the static trust establishment between RADIUS peers (IP
   address and shared secret) the only scalable way of creating a
   massive deployment of RADIUS-servers under control by different
   administrative entities is to introduce some form of a proxy chain to
   route the access requests to their home server.  This creates a lot
   of overhead in terms of possible points of failure, longer
   transmission times as well as middleboxes through which
   authentication traffic flows.  These middleboxes may learn privacy-
   relevant data while forwarding requests.  The new features in RADIUS
   over TLS obsolete the use of IP addresses and shared MD5 secrets to
   identify other peers and thus allow the use of more contemporary
   trust models, e.g. checking a certificate by inspecting the issuer
   and other certificate properties.

1.1.  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", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
   interpreted as described in RFC 2119.  [RFC2119]




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1.2.  Terminology

   RADIUS/TLS node: a RADIUS over TLS client or server

   RADIUS/TLS Client: a RADIUS over TLS instance which initiates a new
   connection.

   RADIUS/TLS Server: a RADIUS over TLS instance which listens on a
   RADIUS over TLS port and accepts new connections

   RADIUS/UDP: classic RADIUS transport over UDP as defined in [RFC2865]

1.3.  Document Status

   This document is an Experimental RFC.

   It is one out of several approaches to address known cryptographic
   weaknesses of the RADIUS protocol (see also Section 4).  The
   specification does not fulfill all recommendations on a AAA transport
   profile as per [RFC3539]; in particular, by being based on TCP as a
   transport layer, it does not prevent head-of-line blocking issues.

   If this specification is indeed selected for advancement to standards
   track, certificate verification options (section 2.3.2) need to be
   refined.

   Another experimental characteristic of this specification is the
   question of key management between RADIUS/TLS peers.  RADIUS/UDP only
   allowed for manual key management, i.e. distribution of a shared
   secret between a client and a server.  RADIUS/TLS allows manual
   distribution of long-term proofs of peer identity as well (by using
   TLS-PSK cipher suites, or identifying clients by a certificate
   fingerprint), but as a new feature enables use of X.509 certificates
   in a PKIX infrastructure.  It remains to be seen if one of these
   methods prevail, or if both will find their place in real-life
   deployments.  The authors can imagine pre-shared keys to be popular
   in small-scale deployments (SOHO or isolated enterprise deployments)
   where scalability is not an issue and the deployment of a CA is
   considered too much a hassle; but can also imagine large roaming
   consortia to make use of PKIX.  Readers of this specification are
   encouraged to read the discussion of key management issus within
   [RFC6421] as well as [RFC4107].

   It has yet to be decided whether this approach is to be chosen for
   standards track.  One key aspect to judge whether the approach is
   usable at large scale is by observing the uptake, usability and
   operational behaviour of the protocol in large-scale, real-life
   deployments.



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   An example for a world-wide roaming environment that uses RADIUS over
   TLS to secure communication is "eduroam", see [eduroam].

2.  Normative: Transport Layer Security for RADIUS/TCP

2.1.  TCP port and packet types

   The default destination port number for RADIUS over TLS is TCP/2083.
   There are no separate ports for authentication, accounting and
   dynamic authorisation changes.  The source port is arbitrary.  See
   section Section 3.4 for considerations regarding separation of
   authentication, accounting and dynamic authorization traffic.

2.2.  TLS negotiation

   RADIUS/TLS has no notion of negotiating TLS in an established
   connection.  Servers and clients need to be preconfigured to use
   RADIUS/TLS for a given endpoint.

2.3.  Connection Setup

   RADIUS/TLS nodes

   1.  establish TCP connections as per [I-D.ietf-radext-tcp-transport].
       Failure to connect leads to continuous retries, with
       exponentially growing intervals between every try.  If multiple
       servers are defined, the node MAY attempt to establish a
       connection to these other servers in parallel, in order to
       implement quick failover.

   2.  after completing the TCP handshake, immediately negotiate TLS
       sessions according to [RFC5246] or its predecessor TLS 1.1.  The
       following restrictions apply:

       *  Support for TLS v1.1 [RFC4346] or later (e.g.  TLS 1.2
          [RFC5246] ]) is REQUIRED.  To prevent known attacks on TLS
          versions prior to 1.1, implementations MUST NOT negotiate TLS
          versions prior to 1.1.

       *  Support for certificate-based mutual authentication is
          REQUIRED.

       *  Negotiation of mutual authentication is REQUIRED.

       *  Negotiation of a ciphersuite providing for confidentiality as
          well as integrity protection is REQUIRED.  Failure to comply
          with this requirement can lead to severe security problmes,
          like user passwords being recoverable by third parties.  See



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          Section 6 for details.

       *  Support for and negotiation of compression is OPTIONAL.

       *  Support for TLS-PSK mutual authentication [RFC4279] is
          OPTIONAL.

       *  RADIUS/TLS implementations MUST at a minimum support
          negotiation of the TLS_RSA_WITH_3DES_EDE_CBC_SHA), and SHOULD
          support TLS_RSA_WITH_RC4_128_SHA and
          TLS_RSA_WITH_AES_128_CBC_SHA as well (see Section 3.3 ).

       *  In addition, RADIUS/TLS implementations MUST support
          negotiation of the mandatory-to-implement ciphersuites
          required by the versions of TLS that they support.

   3.  Peer authentication can be performed in any of the following
       three operation models:

       *  TLS with X.509 certificates using PKIX trust models (this
          model is mandatory to implement):

          +  Implementations MUST allow to configure a list of trusted
             Certification Authorities for incoming connections.

          +  Certificate validation MUST include the verification rules
             as per [RFC5280].

          +  Implementations SHOULD indicate their trusted Certification
             Authorities (CAs).  For TLS 1.2, this is done using
             [RFC5246] section 7.4.4 "certificate authorities" (server
             side) and [RFC6066] Section 6 "Trusted CA Indication"
             (client side).  See also Section 3.2.

          +  Peer validation always includes a check on whether the
             locally configured expected DNS name or IP address of the
             server that is contacted matches its presented certificate.
             DNS names and IP addresses can be contained in the Common
             Name (CN) or subjectAltName entries.  For verification,
             only one of these entries is to be considered.  The
             following precedence applies: for DNS name validation,
             subjectAltName:DNS has precedence over CN; for IP address
             validation, subjectAltName:iPAddr has precedence over CN.
             Implementors of this specification are advised to read
             [RFC6125] Section 6 for more details on DNS name
             validation.





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          +  Implementations MAY allow to configure a set of additional
             properties of the certificate to check for a peer's
             authorisation to communicate (e.g. a set of allowed values
             in subjectAltName:URI or a set of allowed X509v3
             Certificate Policies)

          +  When the configured trust base changes (e.g. removal of a
             CA from the list of trusted CAs; issuance of a new CRL for
             a given CA) implementations MAY re-negotiate the TLS
             session to re-assess the connecting peer's continued
             authorisation.

       *  TLS with X.509 certificates using certificate fingerprints
          (this model is optional to implement): Implementations SHOULD
          allow to configure a list of trusted certificates, identified
          via fingerprint of the DER encoded certificate octets.
          Implementations MUST support SHA-1 as the hash algorithm for
          the fingerprint.  To prevent attacks based on hash collisions,
          support for a more contemporary hash function such as SHA-256
          is RECOMMENDED.

       *  TLS using TLS-PSK (this model is optional to implement)

   4.  start exchanging RADIUS datagrams (note Section 3.4 (1) ).  The
       shared secret to compute the (obsolete) MD5 integrity checks and
       attribute encryption MUST be "radsec" (see Section 3.4 (2) ).

2.4.  Connecting Client Identity

   In RADIUS/UDP, clients are uniquely identified by their IP address.
   Since the shared secret is associated with the origin IP address, if
   more than one RADIUS client is associated with the same IP address,
   then those clients also must utilize the same shared secret, a
   practice which is inherently insecure as noted in [RFC5247].

   RADIUS/TLS supports multiple operation modes.

   In TLS-PSK operation, a client is uniquely identified by its TLS
   identifier.

   In TLS-X.509 mode using fingerprints, a client is uniquely identified
   by the fingerprint of the presented client certificate.

   In TLS-X.509 mode using PKIX trust models, a client is uniquely
   identified by the tuple (serial number of presented client
   certificate;Issuer).

   Note well: having identified a connecting entity does not mean the



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   server necessarily wants to communicate with that client.  E.g. if
   the Issuer is not in a trusted set of Issuers, the server may decline
   to perform RADIUS transactions with this client.

   There are numerous trust models in PKIX environments, and it is
   beyond the scope of this document to define how a particular
   deployment determines whether a client is trustworthy.
   Implementations which want to support a wide variety of trust models
   should expose as many details of the presented certificate to the
   administrator as possible so that the trust model can be implemented
   by the administrator.  As a suggestion, at least the following
   parameters of the X.509 client certificate should be exposed:

   o  Originating IP address

   o  Certificate Fingerprint

   o  Issuer

   o  Subject

   o  all X509v3 Extended Key Usage

   o  all X509v3 Subject Alternative Name

   o  all X509v3 Certificate Policies

   In TLS-PSK operation, at least the following parameters of the TLS
   connection should be exposed:

   o  Originating IP address

   o  TLS Identifier

2.5.  RADIUS Datagrams

   Authentication, Accounting and Authorization packets are sent
   according to the following rules:

   RADIUS/TLS clients transmit the same packet types on the connection
   they initiated as a RADIUS/UDP client would (see Section 3.4 (3) and
   (4) ).  E.g. they send

   o  Access-Request

   o  Accounting-Request





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   o  Status-Server

   o  Disconnect-ACK

   o  Disconnect-NAK

   o  ...

   and they receive

   o  Access-Accept

   o  Accounting-Response

   o  Disconnect-Request

   o  ...

   RADIUS/TLS servers transmit the same packet types on connections they
   have accepted as a RADIUS/UDP server would.  E.g. they send

   o  Access-Challenge

   o  Access-Accept

   o  Access-Reject

   o  Accounting-Response

   o  Disconnect-Request

   o  ...

   and they receive

   o  Access-Request

   o  Accounting-Request

   o  Status-Server

   o  Disconnect-ACK

   o  ...

   Due to the use of one single TCP port for all packet types, it is
   required for a RADIUS/TLS server to signal to a connecting peer which
   types of packets are supported on a server.  See also section



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   Section 3.4 for a discussion of signaling.

   o  When receiving an unwanted packet of type 'CoA-Request' or
      'Disconnect-Request', it needs to be replied to with a 'CoA-NAK'
      or 'Disconnect-NAK' respectively.  The NAK SHOULD contain an
      attribute Error-Cause with the value 406 ("Unsupported
      Extension"); see [RFC5176] for details.

   o  When receiving an unwanted packet of type 'Accounting-Request',
      the RADIUS/TLS server SHOULD reply with an Accounting-Response
      containing an Error-Cause attribute with value 406 "Unsupported
      Extension" as defined in [RFC5176].  A RADIUS/TLS accounting
      client receiving such an Accounting-Response SHOULD log the error
      and stop sending Accounting-Request packets.

3.  Informative: Design Decisions

   This section explains the design decisions that led to the rules
   defined in the previous section.

3.1.  Implications of Dynamic Peer Discovery

   One mechanism to discover RADIUS over TLS peers dynamically via DNS
   is specified in [I-D.ietf-radext-dynamic-discovery].  While this
   mechanism is still under development and therefore is not a normative
   dependency of RADIUS/TLS, the use of dynamic discovery has potential
   future implications that are important to understand.

   Readers of this document who are considering the deployment of DNS-
   based dynamic discovery are thus encouraged to read
   [I-D.ietf-radext-dynamic-discovery] and follow its future
   development.

3.2.  X.509 Certificate Considerations

   (1) If a RADIUS/TLS client is in possession of multiple certificates
   from different CAs (i.e. is part of multiple roaming consortia) and
   dynamic discovery is used, the discovery mechanism possibly does not
   yield sufficient information to identify the consortium uniquely
   (e.g.  DNS discovery).  Subsequently, the client may not know by
   itself which client certificate to use for the TLS handshake.  Then
   it is necessary for the server to signal which consortium it belongs
   to, and which certificates it expects.  If there is no risk of
   confusing multiple roaming consortia, providing this information in
   the handshake is not crucial.

   (2) If a RADIUS/TLS server is in possession of multiple certificates
   from different CAs (i.e. is part of multiple roaming consortia), it



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   will need to select one of its certificates to present to the RADIUS/
   TLS client.  If the client sends the Trusted CA Indication, this hint
   can make the server select the appropriate certificate and prevent a
   handshake failure.  Omitting this indication makes it impossible to
   deterministically select the right certificate in this case.  If
   there is no risk of confusing multiple roaming consortia, providing
   this indication in the handshake is not crucial.

3.3.  Ciphersuites and Compression Negotiation Considerations

   Not all TLS ciphersuites in [RFC5246] are supported by available TLS
   tool kits, and licenses may be required in some cases.  The existing
   implementations of RADIUS/TLS use OpenSSL as cryptographic backend,
   which supports all of the ciphersuites listed in the rules in the
   normative section.

   The TLS ciphersuite TLS_RSA_WITH_3DES_EDE_CBC_SHA is mandatory-to-
   implement according to [RFC4346] and thus has to be supported by
   RADIUS/TLS nodes.

   The two other ciphersuites in the normative section are widely
   implemented in TLS toolkits and are considered good practice to
   implement.

3.4.  RADIUS Datagram Considerations

   (1) After the TLS session is established, RADIUS packet payloads are
   exchanged over the encrypted TLS tunnel.  In RADIUS/UDP, the packet
   size can be determined by evaluating the size of the datagram that
   arrived.  Due to the stream nature of TCP and TLS, this does not hold
   true for RADIUS/TLS packet exchange.  Instead, packet boundaries of
   RADIUS packets that arrive in the stream are calculated by evaluating
   the packet's Length field.  Special care needs to be taken on the
   packet sender side that the value of the Length field is indeed
   correct before sending it over the TLS tunnel, because incorrect
   packet lengths can no longer be detected by a differing datagram
   boundary.  See section 2.6.4 of [I-D.ietf-radext-tcp-transport] for
   more details.

   (2) Within RADIUS/UDP [RFC2865], a shared secret is used for hiding
   of attributes such as User-Password, as well as in computation of
   the Response Authenticator.  In RADIUS accounting [RFC2866], the
   shared secret is used in computation of both the Request
   Authenticator and the Response Authenticator.  Since TLS provides
   integrity protection and encryption sufficient to substitute for
   RADIUS application-layer security, it is not necessary to configure a
   RADIUS shared secret.  The use of a fixed string for the obsolete
   shared secret eliminates possible node misconfigurations.



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   (3) RADIUS/UDP [RFC2865] uses different UDP ports for authentication,
   accounting and dynamic authorisation changes.  RADIUS/TLS allocates a
   single port for all RADIUS packet types.  Nevertheless, in RADIUS/TLS
   the notion of a client which sends authentication requests and
   processes replies associated with it's users' sessions and the notion
   of a server which receives requests, processes them and sends the
   appropriate replies is to be preserved.  The normative rules about
   acceptable packet types for clients and servers mirror the packet
   flow behaviour from RADIUS/UDP.

   (4) RADIUS/UDP [RFC2865] uses negative ICMP responses to a newly
   allocated UDP port to signal that a peer RADIUS server does not
   support reception and processing of the packet types in [RFC5176].
   These packet types are listed as to be received in RADIUS/TLS
   implementations.  Note well: it is not required for an implementation
   to actually process these packet types; it is only required to send
   the NAK as defined above.

   (5) RADIUS/UDP [RFC2865] uses negative ICMP responses to a newly
   allocated UDP port to signal that a peer RADIUS server does not
   support reception and processing of RADIUS Accounting packets.  There
   is no RADIUS datagram to signal an Accounting NAK.  Clients may be
   misconfigured to send Accounting packets to a RADIUS/TLS server which
   does not wish to process their Accounting packet.  To prevent a
   regression of detectability of this situation, the Accounting-
   Response + Error-Cause sgnaling was introduced.

4.  Compatibility with other RADIUS transports

   Ongoing work in the IETF defines multiple alternative transports to
   the classic UDP transport model as defined in [RFC2865], namely
   RADIUS over TCP [I-D.ietf-radext-tcp-transport], RADIUS over Datagram
   Transport Layer Security (DTLS) [I-D.ietf-radext-dtls] and this
   present document on RADIUS over TLS.

   RADIUS/TLS does not specify any inherent backwards compatibility to
   RADIUS/UDP or cross compatibility to the other transports, i.e. an
   implementation which implements RADIUS/TLS only will not be able to
   receive or send RADIUS packet payloads over other transports.  An
   implementation wishing to be backward or cross compatible (i.e.
   wishes to serve clients using other transports than RADIUS/TLS) will
   need to implement these other transports along with the RADIUS/TLS
   transport and be prepared to send and receive on all implemented
   transports, which is called a multi-stack implementation.

   If a given IP device is able to receive RADIUS payloads on multiple
   transports, this may or may not be the same instance of software, and
   it may or may not serve the same purposes.  It is not safe to assume



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   that both ports are interchangeable.  In particular, it can not be
   assumed that state is maintained for the packet payloads between the
   transports.  Two such instances MUST be considered separate RADIUS
   server entities.

5.  Diameter Compatibility

   Since RADIUS/TLS is only a new transport profile for RADIUS,
   compatibility of RADIUS/TLS - Diameter [RFC3588] vs. RADIUS/UDP
   [RFC2865] - Diameter [RFC3588] is identical.  The considerations
   regarding payload size in [I-D.ietf-radext-tcp-transport] apply.

6.  Security Considerations

   The computational resources to establish a TLS tunnel are
   significantly higher than simply sending mostly unencrypted UDP
   datagrams.  Therefore, clients connecting to a RADIUS/TLS node will
   more easily create high load conditions and a malicious client might
   create a Denial-of-Service attack more easily.

   Some TLS ciphersuites only provide integrity validation of their
   payload, and provide no encryption.  This specification forbids the
   use of such ciphersuites.  Since the RADIUS payload's shared secret
   is fixed to the well-known term "radsec" (see Section 2.3 (4) ) ,
   failure to comply with this requirement will expose the entire
   datagram payload in plain text, including User-Password, to
   intermediate IP nodes.

   By virtue of being based on TCP, there are several generic attack
   vectors to slow down or prevent the TCP connection from being
   established; see [RFC4953] for details.  If a TCP connection is not
   up when a packet is to be processed, it gets re-established, so such
   attacks in general lead only to a minor performance degradation (the
   time it takes to re-establish the connection).  There is one notable
   exception where an attacker might create a bidding-down attack
   though: If peer communication between two devices is configured for
   both RADIUS/TLS (i.e TLS security over TCP as a transport, shared
   secret fixed to "radsec") and RADIUS/UDP (i.e. shared secret security
   with a secret manually configured by the administrator), and where
   the RADIUS/UDP transport is the failover option if the TLS session
   cannot be established, a bidding-down attack can occur if an
   adversary can maliciously close the TCP connection, or prevent it
   from being established.  Situations where clients are configured in
   such a way are likely to occur during a migration phase from RADIUS/
   UDP to RADIUS/TLS.  By preventing the TLS session setup, the attacker
   can reduce the security of the packet payload from the selected TLS
   cipher suite packet encryption to the classic MD5 per-attribute
   encryption.  The situation should be avoided by disabling the weaker



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   RADIUS/UDP transport as soon as the new RADIUS/TLS connection is
   established and tested.  Disabling can happen at either the RADIUS
   client or server side:

   o  Client side: de-configure the failover setup, leaving RADIUS/TLS
      as the only communication option

   o  Server side: de-configure the RADIUS/UDP client from the list of
      valid RADIUS clients

   RADIUS/TLS provides authentication and encryption between RADIUS
   peers.  In the presence of proxies, the intermediate proxies can
   still inspect the individual RADIUS packets, i.e. "end-to-end"
   encryption is not provided.  Where intermediate proxies are
   untrusted, it is desirable to use other RADIUS mechanisms to prevent
   RADIUS packet payload from inspection by such proxies.  One common
   method to protect passwords is the use of the Extensible
   Authentication Protocol (EAP) and EAP methods which utilize TLS.

   When using certificate fingerprints to identify RADIUS/TLS peers, any
   two certificates which produce the same hash value (i.e. which have a
   hash collision) will be considered the same client.  It is therefore
   important to make sure that the hash function used is
   cryptographically uncompromised so that an attacker is very unlikely
   to be able to produce a hash collision with a certificate of his
   choice.  While this specification mandates support for SHA-1, a later
   revision will likely demand support for more contemporary hash
   functions because as of issuance of this document there are already
   attacks on SHA-1.

7.  IANA Considerations

   No new RADIUS attributes or packet codes are defined.  IANA is
   requested to update the already-assigned TCP 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 TCP port 2083 was already
      previously assigned by IANA for "RadSec", an early implementation
      of RADIUS/TLS, prior to issuance of this RFC.  This early
      implementation can be configured to be compatible to RADIUS/TLS as
      specified by the IETF.  See RFC (RFC number of this document),
      Appendix A for details."







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8.  Notes to the RFC Editor

   [I-D.ietf-radext-tcp-transport] is currently in the publication queue
   because it has a normative reference on this draft; it has no other
   blocking dependencies.  The two drafts should be published as an RFC
   simultaneously, ideally with consequtive numbers.  The references in
   this draft to [I-D.ietf-radext-tcp-transport] should be changed to
   references to the corresponding RFC prior to publication.

   This section, "Notes to the RFC Editor" should be deleted from the
   draft prior to publication.

9.  Acknowledgements

   RADIUS/TLS was first implemented as "RADSec" by Open Systems
   Consultants, Currumbin Waters, Australia, for their "Radiator" RADIUS
   server product (see [radsec-whitepaper]).

   Funding and input for the development of this Internet Draft was
   provided by the European Commission co-funded project "GEANT2"
   [geant2] and further feedback was provided by the TERENA Task Force
   Mobility [terena].

10.  References

10.1.  Normative References

   [RFC2119]                            Bradner, S., "Key words for use
                                        in RFCs to Indicate Requirement
                                        Levels", BCP 14, RFC 2119,
                                        March 1997.

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

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

   [RFC4279]                            Eronen, P. and H. Tschofenig,
                                        "Pre-Shared Key Ciphersuites for
                                        Transport Layer Security (TLS)",
                                        RFC 4279, December 2005.

   [RFC5280]                            Cooper, D., Santesson, S.,
                                        Farrell, S., Boeyen, S.,



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                                        Housley, R., and W. Polk,
                                        "Internet X.509 Public Key
                                        Infrastructure Certificate and
                                        Certificate Revocation List
                                        (CRL) Profile", RFC 5280,
                                        May 2008.

   [RFC5176]                            Chiba, M., Dommety, G., Eklund,
                                        M., Mitton, D., and B. Aboba,
                                        "Dynamic Authorization
                                        Extensions to Remote
                                        Authentication Dial In User
                                        Service (RADIUS)", RFC 5176,
                                        January 2008.

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

   [RFC5247]                            Aboba, B., Simon, D., and P.
                                        Eronen, "Extensible
                                        Authentication Protocol (EAP)
                                        Key Management Framework",
                                        RFC 5247, August 2008.

   [RFC6066]                            Eastlake, D., "Transport Layer
                                        Security (TLS) Extensions:
                                        Extension Definitions",
                                        RFC 6066, January 2011.

   [I-D.ietf-radext-tcp-transport]      DeKok, A., "RADIUS Over TCP", dr
                                        aft-ietf-radext-tcp-transport-09
                                        (work in progress),
                                        October 2010.

10.2.  Informative References

   [I-D.ietf-radext-dtls]               DeKok, A., "DTLS as a Transport
                                        Layer for RADIUS",
                                        draft-ietf-radext-dtls-01 (work
                                        in progress), October 2010.

   [I-D.ietf-radext-dynamic-discovery]  Winter, S. and M. McCauley,
                                        "NAI-based Dynamic Peer
                                        Discovery for RADIUS/TLS and
                                        RADIUS/DTLS", draft-ietf-radext-
                                        dynamic-discovery-03 (work in



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                                        progress), July 2011.

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

   [RFC3588]                            Calhoun, P., Loughney, J.,
                                        Guttman, E., Zorn, G., and J.
                                        Arkko, "Diameter Base Protocol",
                                        RFC 3588, September 2003.

   [RFC4107]                            Bellovin, S. and R. Housley,
                                        "Guidelines for Cryptographic
                                        Key Management", BCP 107,
                                        RFC 4107, June 2005.

   [RFC4346]                            Dierks, T. and E. Rescorla, "The
                                        Transport Layer Security (TLS)
                                        Protocol Version 1.1", RFC 4346,
                                        April 2006.

   [RFC4953]                            Touch, J., "Defending TCP
                                        Against Spoofing Attacks",
                                        RFC 4953, July 2007.

   [RFC6125]                            Saint-Andre, P. and J. Hodges,
                                        "Representation and Verification
                                        of Domain-Based Application
                                        Service Identity within Internet
                                        Public Key Infrastructure Using
                                        X.509 (PKIX) Certificates in the
                                        Context of Transport Layer
                                        Security (TLS)", RFC 6125,
                                        March 2011.

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

   [radsec-whitepaper]                  Open System Consultants, "RadSec
                                        - a secure, reliable RADIUS
                                        Protocol", May 2005, <http://
                                        www.open.com.au/radiator/
                                        radsec-whitepaper.pdf>.




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   [MD5-attacks]                        Black, J., Cochran, M., and T.
                                        Highland, "A Study of the MD5
                                        Attacks: Insights and
                                        Improvements", October 2006, <ht
                                        tp://www.springerlink.com/
                                        content/40867l85727r7084/>.

   [radsecproxy-impl]                   Venaas, S., "radsecproxy Project
                                        Homepage", 2007, <http://
                                        software.uninett.no/
                                        radsecproxy/>.

   [eduroam]                            Trans-European Research and
                                        Education Networking
                                        Association, "eduroam Homepage",
                                        2007, <http://www.eduroam.org/>.

   [geant2]                             Delivery of Advanced Network
                                        Technology to Europe, "European
                                        Commission Information Society
                                        and Media: GEANT2", 2008,
                                        <http://www.geant2.net/>.

   [terena]                             TERENA, "Trans-European Research
                                        and Education Networking
                                        Association", 2008,
                                        <http://www.terena.org/>.

Appendix A.  Implementation Overview: Radiator

   Radiator implements the RadSec protocol for proxying requests with
   the <Authby RADSEC> and <ServerRADSEC> clauses in the Radiator
   configuration file.

   The <AuthBy RADSEC> clause defines a RadSec client, and causes
   Radiator to send RADIUS requests to the configured RadSec server
   using the RadSec protocol.

   The <ServerRADSEC> clause defines a RadSec server, and causes
   Radiator to listen on the configured port and address(es) for
   connections from <Authby RADSEC> clients.  When an <Authby RADSEC>
   client connects to a <ServerRADSEC> server, the client sends RADIUS
   requests through the stream to the server.  The server then handles
   the request in the same way as if the request had been received from
   a conventional UDP RADIUS client.

   Radiator is compliant to RADIUS/TLS if the following options are
   used:



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      <AuthBy RADSEC>

      *  Protocol tcp

      *  UseTLS

      *  TLS_CertificateFile

      *  Secret radsec

      <ServerRADSEC>

      *  Protocol tcp

      *  UseTLS

      *  TLS_RequireClientCert

      *  Secret radsec

   As of Radiator 3.15, the default shared secret for RadSec connections
   is configurable and defaults to "mysecret" (without quotes).  For
   compliance with this document, this setting needs to be configured
   for the shared secret "radsec".  The implementation uses TCP
   keepalive socket options, but does not send Status-Server packets.
   Once established, TLS connections are kept open throughout the server
   instance lifetime.

Appendix B.  Implementation Overview: radsecproxy

   The RADIUS proxy named radsecproxy was written in order to allow use
   of RadSec in current RADIUS deployments.  This is a generic proxy
   that supports any number and combination of clients and servers,
   supporting RADIUS over UDP and RadSec.  The main idea is that it can
   be used on the same host as a non-RadSec client or server to ensure
   RadSec is used on the wire, however as a generic proxy it can be used
   in other circumstances as well.

   The configuration file consists of client and server clauses, where
   there is one such clause for each client or server.  In such a clause
   one specifies either "type tls" or "type udp" for RadSec or UDP
   transport.  For RadSec the default shared secret "mysecret" (without
   quotes), the same as Radiator, is used.  For compliance with this
   document, this setting needs to be configured for the shared secret
   "radsec".  A secret may be specified by putting say "secret
   somesharedsecret" inside a client or server clause.

   In order to use TLS for clients and/or servers, one must also specify



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   where to locate CA certificates, as well as certificate and key for
   the client or server.  This is done in a TLS clause.  There may be
   one or several TLS clauses.  A client or server clause may reference
   a particular TLS clause, or just use a default one.  One use for
   multiple TLS clauses may be to present one certificate to clients and
   another to servers.

   If any RadSec (TLS) clients are configured, the proxy will at startup
   listen on port 2083, as assigned by IANA for the OSC RadSec
   implementation.  An alternative port may be specified.  When a client
   connects, the client certificate will be verified, including checking
   that the configured FQDN or IP address matches what is in the
   certificate.  Requests coming from a RadSec client are treated
   exactly like requests from UDP clients.

   The proxy will at startup try to establish a TLS connection to each
   (if any) of the configured RadSec (TLS) servers.  If it fails to
   connect to a server, it will retry regularly.  There is some back-off
   where it will retry quickly at first, and with longer intervals
   later.  If a connection to a server goes down it will also start
   retrying regularly.  When setting up the TLS connection, the server
   certificate will be verified, including checking that the configured
   FQDN or IP address matches what is in the certificate.  Requests are
   sent to a RadSec server just like they would to a UDP server.

   The proxy supports Status-Server messages.  They are only sent to a
   server if enabled for that particular server.  Status-Server requests
   are always responded to.

   This RadSec implementation has been successfully tested together with
   Radiator.  It is a freely available open-source implementation.  For
   source code and documentation, see [radsecproxy-impl].

Appendix C.  Assessment of Crypto-Agility Requirements

   The RADIUS Crypto-Agility Requirements [RFC6421] defines numerous
   classification criteria for protocols that strive to enhance the
   security of RADIUS.  It contains mandatory (M) and recommended (R)
   criteria which crypto-agile protocols have to fulfill.  The authors
   believe that the following assessment about the crypto-agility
   properties of RADIUS/TLS are true.

   By virtue of being a transport profile using TLS over TCP as a
   transport protocol, the cryptographically agile properties of TLS are
   inherited, and RADIUS/TLS subsequently meets the following points:

      (M) negotiation of cryptographic algorithms for integrity and auth




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      (M) negotiation of cryptographic algorithms for encryption

      (M) replay protection

      (M) define mandatory-to-implement cryptographic algorithms

      (M) generate fresh session keys for use between client and server

      (R) support for Perfect Forward Secrecy in session keys

      (R) support X.509 certificate based operation

      (R) support Pre-Shared keys

      (R) support for confidentiality of the entire packet

      (M/R) support Automated Key Management

   The remainder of the requirements is discussed individually below in
   more detail:

      (M) "avoid security compromise, even in situations where the
      existing cryptographic alogrithms used by RADIUS implementations
      are shown to be weak enough to provide little or no security" -
      The existing algorithm, based on MD5, is not of any significance
      in RADIUS/TLS; its compromise does not compromise the outer
      transport security.

      (R) mandatory-to-implement alogrithms are to be NIST-Acceptable
      with no deprecation date - The mandatory-to-implement algorithm is
      TLS_RSA_WITH_3DES_EDE_CBC_SHA.  This ciphersuite supports three-
      key 3DES operation, which is classified as Acceptable with no
      known deprecation date by NIST.

      (M) demonstrate backward compatibility with RADIUS - There are
      multiple implementations supporting both RADIUS and RADIUS/TLS,
      and the translation between them.

      (M) After legacy mechanisms have been compromised, secure
      algorithms MUST be used, so that backward compatibility is no
      longer possible - In RADIUS, communication between client and
      server is always a manual configuration; after a compromise, the
      legacy client in question can be de-configured by the same manual
      configuration.

      (M) indicate a willingness to cede change control to the IETF -
      Change control of this protocol is with the IETF.




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      (M) be interoperable between implementations based purely on the
      information in the specification - At least one implementation was
      created exclusively based on this specification and is
      interoperable with other RADIUS/TLS implementations.

      (M) apply to all packet types - RADIUS/TLS operates on the
      transport layer, and can carry all packet types.

      (R) message data exchanged with Diameter SHOULD NOT be affected -
      The solution is Diameter-agnostic.

      (M) discuss any inherent assumptions - The authors are not aware
      of any implicit assumptions which would be yet-unarticulated in
      the draft

      (R) provide recommendations for transition - The Security
      Considerations section contains a transition path.

      (R) discuss legacy interoperability and potential for bidding-down
      attacks - The Security Considerations section contains an
      corresponding discussion.

   Summarizing, it is believed that this specification fulfills all the
   mandatory and all the recommended requirements for a crypto-agile
   solution and should thus be considered UNCONDITIONALLY COMPLIANT.

Authors' Addresses

   Stefan Winter
   Fondation RESTENA
   6, rue Richard Coudenhove-Kalergi
   Luxembourg  1359
   LUXEMBOURG

   Phone: +352 424409 1
   Fax:   +352 422473
   EMail: stefan.winter@restena.lu
   URI:   http://www.restena.lu.













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   Mike McCauley
   Open Systems Consultants
   9 Bulbul Place
   Currumbin Waters  QLD 4223
   AUSTRALIA

   Phone: +61 7 5598 7474
   Fax:   +61 7 5598 7070
   EMail: mikem@open.com.au
   URI:   http://www.open.com.au.


   Stig Venaas
   cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: stig@cisco.com


   Klaas Wierenga
   Cisco Systems International BV
   Haarlerbergweg 13-19
   Amsterdam  1101 CH
   The Netherlands

   Phone: +31 (0)20 3571752
   Fax:
   EMail: kwiereng@cisco.com
   URI:   http://www.cisco.com.




















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