RADIUS Extensions Working Group                                S. Winter
Internet-Draft                                                   RESTENA
Intended status: Experimental                                M. McCauley
Expires: September 6, 2010 January 10, 2012                                            OSC
                                                          March 05, 2010
                                                           July 09, 2011

    NAI-based Dynamic Peer Discovery for RADIUS over TLS RADIUS/TLS and DTLS
                 draft-ietf-radext-dynamic-discovery-02 RADIUS/DTLS


   This document specifies a means to find authoritative AAA RADIUS servers
   for a given NAI realm.  It can be used in conjunction with RADIUS over

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  DNS-based NAPTR/SRV Peer Discovery  . . . . . . . . . . . . . . 3
     2.1.  Applicability . . . . . . . . . . . . . . . . . . . . . . . 3
     2.2.  DNS RR definition . . . . . . . . . . . . . . . . . . . . . 3
     2.3.  Realm to AAA server resolution algorithm  . . . . . . . . . 5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7 8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   5.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8

1.  Introduction

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",
   and "OPTIONAL" in this document are to be interpreted as described in
   RFC 2119.  [RFC2119]

1.2.  Terminology

   RADIUS/TLS Client: a RADIUS/TLS [I-D.ietf-radext-radsec] instance
   which initiates a new connection.

   RADIUS/TLS Server: a RADIUS/TLS [I-D.ietf-radext-radsec] instance
   which listens on a RADIUS/TLS port and accepts new connections

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

2.  DNS-based NAPTR/SRV Peer Discovery

2.1.  Applicability

   Dynamic server discovery as defined in this document is only
   applicable for AAA transactions where a AAA RADIUS server receives a
   request with a NAI realm for which no home AAA RADIUS server is known.  I.e.
   where static server configuration does not contain a known home
   authentication server, or where the server configuration explicitly
   states that the realm destination is to be looked up dynamically.
   Furthermore, it is only applicable for new user sessions, i.e. for
   the initial Access-Request.  Subsequent messages concerning this
   session, for example Access-Challenges, Access-Accepts, Accounting
   Messages or Change-of-Authorisation messages Access-Challenges and Access-Accepts use the previously-
   previously-established communication channel between client and

2.2.  DNS RR definition

   DNS definitions of RADIUS/TLS servers can be either S-NAPTR records
   (see [RFC3958]) or SRV records.  When both are defined, the
   resolution algorithm prefers S-NAPTR results (see section Section 2.3

   This specification defines two three S-NAPTR service tag: a general-purpose
   tag "nai-roaming" tags: "aaa+auth",
   "aaa+acct" and a special-purpose tag "eduroam" for the eduroam
   roaming consortium. "aaa+dynauth".  This specification defines two S-NAPTR
   protocol tags: "radius.tls" for RADIUS over TLS RADIUS/TLS [I-D.ietf-radext-radsec]
   and "radius.dtls" for RADIUS over DTLS RADIUS/DTLS [I-D.dekok-radext-dtls].

   This specification defines the SRV prefix "_radiustls._tcp" for
   RADIUS over TLS [I-D.ietf-radext-radsec] and "_radiustls._udp" for
   RADIUS over DTLS [I-D.dekok-radext-dtls].  It is expected that in
   most cases, the label used for the records is the DNS representation
   (punycode) of the literal realm name for which the server is the AAA

   However, arbitrary other labels may be used if, for example, a
   roaming consortium uses realm names which are not associated to DNS
   names or special-purpose consortia where a globally valid discovery
   is not a use case.  Such other labels require a consortium-wide
   agreement about the transformation from realm name to lookup label.


   a.  A general-purpose AAA server for realm example.com might have DNS
       entries as follows:

          example.com.  IN NAPTR 50 50 "s" "nai-roaming:radius.tls" "aaa+auth:radius.tls" ""


          _radiustls._tcp.foobar.example.com.  IN SRV 0 10 2083

   b.  The consortium "foo" provides roaming services for its members
       only.  The realms used are of the form enterprise-name.example.
       The consortium operates a special purpose DNS server for the
       (private) TLD "example" which all AAA servers use to resolve
       realm names.  "Bad, Inc." is part of the consortium.  On the
       consortium's DNS server, realm bad.example might have the
       following DNS entries:

          bad.example IN NAPTR 50 50 "a" "nai-roaming:radius.dtls" "aaa+auth:radius.dtls" ""

   c.  the  The eduroam consortium uses realms based on DNS, but provides its
       services to a closed community only.  However, a AAA domain
       participating in eduroam may also want to expose AAA services to
       other, general-purpose, applications (on the same or other AAA
       servers).  Due to that, the eduroam consortium uses the service
       tag "eduroam" "x-eduroam" for authentication purposes and eduroam AAA
       servers use this tag to look up other eduroam servers.  An
       eduroam participant example.org which also provides general-purpose general-
       purpose AAA on a different server uses the general "nai-roaming" "aaa+auth"

          example.org.  IN NAPTR 50 50 "s" "eduroam:radius.tls" "x-eduroam:radius.tls" ""

          example.org.  IN NAPTR 50 50 "s" "nai-roaming:radius.tls" "aaa+auth:radius.tls" ""

          _radiustls._tcp.eduroam.example.org.  IN SRV 0 10 2083 aaa-

          _radiustls._tcp.aaa.example.org.  IN SRV 0 10 2083 aaa-

2.3.  Realm to AAA server resolution algorithm

   Input I to the algorithm is a the RADIUS User-Name in attribute with
   content of the form of "user@realm"; the literal @ sign being the
   separator between a NAI as
   defined local user identifier within a realm and its
   realm.  The use of multiple literal @ signs in [RFC4282] as extracted from the a User-Name attribute in an
   Access-Request. is
   strongly discouraged; but if present, the last @ sign is to be
   considered the separator.  All previous instances of the @ sign are
   to be considered part of the local user identifier.  Output O of the
   algorithm is a set of hostname:port and an associated order/preference; order/
   preference; the set can be empty.

   Note well: The attribute User-Name does not necessarily is defined to contain well-
   formed NAIs and UTF-8 text.
   In practice, the content may or may not even contain well-formed UTF-8 strings. be UTF-8.  Even if UTF-8, it
   may or may not map to a domain name in the realm part.  Implementors
   MUST take possible conversion error paths into consideration when
   parsing incoming User-Name attributes.  This document describes
   server discovery only for well-formed NAIs realms mapping to DNS domain
   names in UTF-8 encoding.  The result of all other possible contents
   of User-
   Name User-Name is unspecified; this includes, but is not limited to:

      Usage of separators other than @

      Usage of multiple @ separators

      Encoding of User-Name in local encodings

      UTF-8 realms which fail the conversion rules as per [RFC5891]

   The algorithm to determine the AAA RADIUS server to contact is as

   1.   Determine P = (position of first last "@" character) in I.

   2.   generate R = (substring from P+1 to end of I)

   3.   Optional: modify R according to agreed consortium procedures
   4.   Using the host's name resolution library, perform a NAPTR query
        for R. The name resolution library may need to convert R to a
        different respresentation, depending on the resolution backend
        used.  If no result, continue at step 9.  If name resolution
        returns with error, O = { }.  Terminate.

   5.   Extract NAPTR records with service tag "nai-roaming" (replace
        with other service tags where applicable). "aaa+auth", "aaa+acct",
        "aaa+dynauth" as appropriate.

   6.   If no result, continue at step 9.

   7.   Evaluate NAPTR result(s) for desired protocol tag, perform
        subsequent lookup steps until lookup yields one or more
        hostnames.  O = (set of {Order/Preference, hostname:port} for
        all lookup results).

   8.   Terminate.

   9.   Generate R' = (prefix R with "_radiustls._tcp." or

   10.  Using the host's name resolution library, perform SRV lookup
        with R' as label.

   11.  If name resolution returns with error, O = { }.  Terminate.

   12.  If no result, O = {}; terminate.

   13.  Perform subsequent lookup steps until lookup yields one or more
        hostnames.  O = (set of {Order/Preference, hostname} for all
        hostnames).  Terminate.

   Example: Assume a user from the Technical University of Munich,
   Germany, has a RADIUS User-Name of
   "foobar@tu-m[U+00FC]nchen.example".  The name resolution library on
   the RADIUS client uses DNS for name resolution.  If DNS contains the
   following records:

      xn--tu-mnchen-t9a.example.  IN NAPTR 50 50 "s" "nai-
      roaming:radius.tls" "aaa+
      auth:radius.tls" "" _radiustls._tcp.xn--tu-mnchen-t9a.example.

      xn--tu-mnchen-t9a.example.  IN NAPTR 50 50 "s" "fooservice:
      bar.dccp" "" _abc._def.xn--tu-mnchen-t9a.example.

      _radiustls._tcp.xn--tu-mnchen-t9a.example.  IN SRV 0 10 2083

      _radiustls._tcp.xn--tu-mnchen-t9a.example.  IN SRV 0 20 2083

      radsec.xn--tu-mnchen-t9a.example.  IN AAAA 2001:0DB8::202:44ff:

      radsec.xn--tu-mnchen-t9a.example.  IN A

      backup.xn--tu-mnchen-t9a.example.  IN A

   Then the algorithm executes as follows, with I =
   "foobar@tu-m[U+00FC]nchen.example", and no consortium name mangling
   in use:

   1.   P = 7

   2.   R = "tu-m[U+00FC]nchen.example"

   3.   NOOP

   4.   [name resolution library converts R to xn--tu-mnchen-
        t9a.example] Query result: ( 50 50 "s" "nai-roaming:radius.tls" "aaa+auth:radius.tls" ""
        _radiustls._tcp.xn--tu-mnchen-t9a.example. ; 50 50 "s"
        "fooservice:bar.dccp" "" _abc._def.xn--tu-mnchen-t9a.example. )

   5.   Result: 50 50 "s" "nai-roaming:radius.tls" "aaa+auth:radius.tls" ""
        _radiustls._tcp.xn--tu-mnchen-t9a.example. _radiustls._tcp.xn--

   6.   NOOP

   7.   O = {(10,radsec.xn--tu-mnchen-t9a.example.:2083),(20,backup.xn--
        tu-mnchen-t9a. example.:2083)}

   8.   Terminate.

   9.   (not executed)

   10.  (not executed)

   11.  (not executed)

   12.  (not executed)

   13.  (not executed)

   The implementation will then attempt to connect to two servers, with
   preference to radsec.xn--tu-mnchen-t9a.example.:2083, using either
   the AAAA or A addresses depending on the host configuration and its
   IP stack's capabilities.

3.  Security Considerations

   When using DNS without security, DNSSEC security extensions, the replies to
   NAPTR, SRV and A/AAAA requests as described in section Section 2 can
   not be trusted.  RADIUS transports have an out-of-DNS-band means to
   verify that the discovery attempt led to the intended target (TLS/DTLS: ceritifcate target:
   certificate verification or TLS shared secret ciphers; UDP/TCP: the RADIUS shared
   secret) and are safe from DNS-based redirection attacks.  [Note:
   assuming here that a hypothetical RADIUS/UDP SRV discovery will NOT
   deliver the shared secret in the DNS response!]

   The discovery process is always susceptible to bidding down attacks
   if a realm has SRV records for RADIUS/UDP and/or RADIUS/TCP as well
   as for RADIUS/TLS and/or RADIUS/DTLS.  While the SRV query will
   expose both transports, an attacker in the routing path might
   suppress the subsequent A/AAAA results for the TLS or DTLS peer and
   trick the initiating peer into using the weakly protected UDP or TCP
   transports.  The use of DNSSEC can not fully mitigate this attack,
   since it does not provide a means to detect packet suppression.  The
   only way to disable such bidding down attacks is by intiating
   connections only to the peer(s) which match or exceed a configured
   minimum security level.  All implementations SHOULD provide a means
   to configure the administratively desired minimum security level. TLS-PSK keys.

4.  IANA Considerations

   This document requests IANA registration of the following S-NAPTR

   o  Application Service Tags

      *  nai-roaming  aaa+auth

      *  eduroam  aaa+acct

      *  aaa+dynauth

   o  Application Protocol Tags

      *  radius.tls

      *  radius.dtls

5.  Normative References

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

   [RFC3958]                 Daigle, L. and A. Newton, "Domain-Based
                             Application Service Location Using SRV RRs
                             and the Dynamic Delegation Discovery
                             Service (DDDS)", RFC 3958, January 2005.

   [RFC4282]                 Aboba, B., Beadles, M., Arkko,

   [RFC5891]                 Klensin, J., and P.
                             Eronen, "The Network Access Identifier", "Internationalized Domain
                             Names in Applications (IDNA): Protocol",
                             RFC 4282, December 2005. 5891, August 2010.

   [I-D.dekok-radext-dtls]   DeKok, A., "DTLS as a Transport Layer for
                             RADIUS", draft-dekok-radext-dtls-01 draft-dekok-radext-dtls-03 (work
                             in progress), June 2009. July 2010.

   [I-D.ietf-radext-radsec]  Winter, S., McCauley, M., Venaas, S., and
                             K. Wierenga, "TLS encryption for RADIUS
                             over TCP", draft-ietf-radext-radsec-06 RADIUS",
                             draft-ietf-radext-radsec-09 (work in
                             progress), March 2010. July 2011.

Authors' Addresses

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

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

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

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