RADIUS Extensions Working Group                                S. Winter
Internet-Draft                                                   RESTENA
Intended status: Experimental                                M. McCauley
Expires: January 3, 14, 2010                                            OSC
                                                           July 02, 13, 2009

     NAI-based Dynamic Peer Discovery for RADIUS over TLS and DTLS

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   This document specifies a means to find authoritative AAA servers for
   a given NAI realm as defined in [RFC4282].  It can be used in
   conjunction with RADIUS over TLS and RADIUS over DTLS.

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  . . . . . . . . . 4 5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6 7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   5.  Normative References  . . . . . . . . . . . . . . . . . . . . . 7 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

   RadSec node: a RadSec client or server

   RadSec Client: a RadSec instance which initiates a new connection.

   RadSec Server: a RadSec instance which listens on a RadSec port and
   accepts new connections

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 server receives a request
   with a NAI realm for which no home AAA 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 use the previously-
   established communication channel between client and server.

2.2.  DNS RR definition

   DNS definitions of RadSec servers can be either NAPTR records or SRV
   records.  When both are defined, the resolution algorithm prefers
   NAPTR results (see section Section 2.2 2.3 below).  The NAPTR service
   field used is "AAAS+RADSECT".  The SRV prefix used is "_radsec._tcp".
   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.


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

          example.com.  IN NAPTR 50 50 "s" "AAAS+RADSECT" ""

          _radsec._tcp.example.com.  IN SRV 0 10 2083

   b.  Consortium "foo" provides roaming services for banks.  The realms
       used are of the form enterprise-name.foobankroam.  The consortium
       operates a special purpose DNS server for the (private) TLD
       "foobankroam" which all AAA servers use to resolve realm names.
       "Rupt, Inc." is part of the consortium.  On the consortium's DNS
       server, realm bank-rupt.foobankroam might have the following DNS

          bank-rupt.foobankroam IN NAPTR 50 50 "a" "AAAS+RADSECT" ""

          _radsec._tcp.bank-rupt.foobankroam IN SRV 0 10 2083 triple-a-

   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 labels
       prefixed with "eduroam." and eduroam AAA servers use these labels
       to look up servers.  An eduroam participant which also provides
       general-purpose AAA on a different server might have the
       following DNS entries:

          eduroam.restena.lu.  IN NAPTR 50 50 "a" "AAAS+RADSECT" "" aaa-

          restena.lu.  IN NAPTR 50 50 "a" "AAAS+RADSECT" "" aaa-

          _radsec._tcp.eduroam.restena.lu.  IN SRV 0 10 2083 aaa-

          _radsec._tcp.restena.lu.  IN SRV 0 10 2083 aaa-


2.3.  Realm to AAA server resolution algorithm

   Input I to the algorithm is a User-Name in the form of a NAI as
   defined in [RFC4282] as extracted from the User-Name attribute in an
   Access-Request.  Output O of the algorithm is a set of hostname:port
   and an assoiciated order/preference; the set can be empty.

   Note well: The attribute User-Name does not necessarily contain well-
   formed NAIs and may not even contain well-formed UTF-8 strings.  This
   document describes server discovery only for well-formed NAIs 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

   The algorithm to determine the AAA server to contact is as follows:

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

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

   3.   Optional: modify R according to agreed consortium procedures

   4.   Generate R' = ( DNS library transformation of R to an FQDN in

   5.   If generation of R' failed, O = {}; terminate.

   6.   Perform   Using the host's name resolution library, perform a NAPTR query
        for service "AAAS+RADSECT" with for R as label


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

   6.   If no result, continue at step 10.

   8. 9.

   7.   Evaluate NAPTR result, 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'' R' = (prefix R' R with "_radsec._tcp.")

   11.  Perform

   10.  Using the host's name resolution library, perform SRV lookup
        with R'' 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".  If DNS contains the following

      xn--tu-mnchen-t9a.example.  IN NAPTR 50 50 "s" "AAAS+RADSECT" ""

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

      _radsec._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.   R' = "xn--tu-mnchen-t9a.example."

   5.   NOOP

   6.   Query result: ( 0 10 2083 radsec.xn--tu-mnchen-t9a.example. ; 0
        20 2083 backup.xn--tu-mnchen-t9a.example. )


   5.   NOOP


   6.   NOOP

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

   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, 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
   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.

4.  IANA Considerations

   This document contains no actions for IANA.  Maybe.  Not sure about
   the labels "AAAS+RADSECT" and "_radsec._tcp.".

5.  Normative References

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

   [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
              Network Access Identifier", RFC 4282, December 2005.

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.