NAI-based Dynamic Peer Discovery for RADIUS/TLS and RADIUS/DTLS
draft-ietf-radext-dynamic-discovery-03

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

    NAI-based Dynamic Peer Discovery for RADIUS/TLS and RADIUS/DTLS
                 draft-ietf-radext-dynamic-discovery-03

Abstract

   This document specifies a means to find authoritative RADIUS servers
   for a given realm.  It can be used in conjunction with RADIUS/TLS and
   RADIUS/DTLS.

Status of This Memo

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   This Internet-Draft will expire on January 10, 2012.

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   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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 . . . . . . . . . . . . . . . . . . . . 8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
   5.  Normative References  . . . . . . . . . . . . . . . . . . . . . 8

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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",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   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 RADIUS server receives a
   request with a realm for which no home 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 and Access-Accepts use the
   previously-established communication channel between client and
   server.

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
   below).

   This specification defines three S-NAPTR service tags: "aaa+auth",
   "aaa+acct" and "aaa+dynauth".  This specification defines two S-NAPTR
   protocol tags: "radius.tls" for RADIUS/TLS [I-D.ietf-radext-radsec]
   and "radius.dtls" for RADIUS/DTLS [I-D.dekok-radext-dtls].

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   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
   server.

   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.

   Examples:

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

          example.com.  IN NAPTR 50 50 "s" "aaa+auth:radius.tls" ""
          _radiustls._tcp.foobar.example.com.

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

   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" "aaa+auth:radius.dtls" ""
          "very.bad.example"

   c.  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 "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 AAA on a different server uses the general "aaa+auth"
       tag:

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

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          example.org.  IN NAPTR 50 50 "s" "aaa+auth:radius.tls" ""
          _radiustls._tcp.aaa.example.org

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

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

2.3.  Realm to AAA server resolution algorithm

   Input I to the algorithm is the RADIUS User-Name attribute with
   content of the form "user@realm"; the literal @ sign being the
   separator between a local user identifier within a realm and its
   realm.  The use of multiple literal @ signs in a User-Name 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; the set can be empty.

   Note well: The attribute User-Name is defined to contain UTF-8 text.
   In practice, the content may or may not 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 realms mapping to DNS domain
   names in UTF-8 encoding.  The result of all other possible contents
   of 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 RADIUS server to contact is as
   follows:

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

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

   3.   Optional: modify R according to agreed consortium procedures

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   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 "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
        "_radiustls._udp")

   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" "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
      radsec.xn--tu-mnchen-t9a.example.

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

      radsec.xn--tu-mnchen-t9a.example.  IN AAAA 2001:0DB8::202:44ff:
      fe0a:f704

      radsec.xn--tu-mnchen-t9a.example.  IN A 192.0.2.3

      backup.xn--tu-mnchen-t9a.example.  IN A 192.0.2.7

   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" "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" "aaa+auth:radius.tls" "" _radiustls._tcp.xn--
        tu-mnchen-t9a.example.

   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

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   IP stack's capabilities.

3.  Security Considerations

   When using DNS without 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:
   certificate verification or TLS-PSK keys.

4.  IANA Considerations

   This document requests IANA registration of the following S-NAPTR
   parameter:

   o  Application Service Tags

      *  aaa+auth

      *  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.

   [RFC5891]                 Klensin, J., "Internationalized Domain
                             Names in Applications (IDNA): Protocol",
                             RFC 5891, August 2010.

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

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

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.

   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.

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