RADIUS Extensions Working Group                                S. Winter
Internet-Draft                                                   RESTENA
Intended status: Experimental                                M. McCauley
Expires: January 10, December 30, 2012                                           OSC
                                                           July 09, 2011
                                                           June 28, 2012

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


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

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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
       2.3.1.  Input  . . . . . . . . . . . . . . . . . . . . . . . .  5
       2.3.2.  Output . . . . . . . . . . . . . . . . . . . . . . . .  6
       2.3.3.  Algorithm  . . . . . . . . . . . . . . . . . . . . . .  6
       2.3.4.  Validity of results  . . . . . . . . . . . . . . . . .  7
       2.3.5.  Delay considerations . . . . . . . . . . . . . . . . .  8
       2.3.6.  Example  . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . . . 8 10
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 8 10
   5.  Normative References . . . . . . . . . . . . . . . . . . . . . 8 10

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] [RFC6614] instance which initiates a
   new connection.

   RADIUS/TLS Server: a RADIUS/TLS [I-D.ietf-radext-radsec] [RFC6614] 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

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 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] [RFC6614] and
   "radius.dtls" for RADIUS/DTLS [I-D.dekok-radext-dtls].

   Note well:

      The S-NAPTR service and protocols are unrelated to the IANA
      Service Name and Transport Protocol Number registry

      The delimiter '.' in the protocol tags is only a separator for
      human reading convenience - not for structure or namespacing; it
      MUST NOT be parsed in any way by the querying application or

      The use of the separator '.' is common also in other protocols'
      protocol tags.  This is coincidence and does not imply a shared
      semantics with such protocols.

   This specification defines the SRV prefix "_radiustls._tcp" for
   RADIUS over TLS [I-D.ietf-radext-radsec] [RFC6614] 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.


   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.  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" "aaa+auth:radius.dtls" ""

   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"

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

          example.org.  IN NAPTR 50 50 "s" "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

   This algorithm can be used to discover RADIUS servers (for RADIUS
   Authentication and RADIUS Accounting) or to discover RADIUS DynAuth

2.3.1.  Input

   For RADIUS Authentication and RADIUS Accounting server discovery,
   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

   For RADIUS DynAuth Server discovery, input I to the algorithm is a set the
   domain name of hostname:port and an associated order/
   preference; the set can be empty.

   Note well: The operator of a RADIUS realm as was communicated
   during user authentication using the Operator-Name attribute User-Name is defined to
   ([RFC5580], section 4.1).  Only Operator-Name values with the
   namespace "1" are supported by this algorithm - the input to the
   algorithm is the actual domain name, preceeded with an "@" (but
   without the "1" namespace identifier byte of that attribute).

   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]

      UTF-8 realms which end with a . ("dot") character.

   For the last bullet point, "trailing dot", special precautions should
   be taken to avoid problems when resolving servers with the algorithm
   below: they may resolve to a AAA server even if the peer RADIUS
   server only is configured to handle the realm without the trailing
   dot.  If that RADIUS server again uses NAI discovery to determine the
   authoritative server, the server will forward the request to
   localhost, resulting in a tight endless loop.

2.3.2.  Output

   Output O of the algorithm is a set of the tuple {hostname; port;
   order/preference; TTL} - the set can be empty.

2.3.3.  Algorithm

   The algorithm to determine the RADIUS server to contact is as

   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

   4.   Using the host's name resolution library, perform a NAPTR query
        for R. R (see "Delay considerations" below).  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. } and terminate.

   5.   Extract NAPTR records with service tag "aaa+auth", "aaa+acct",
        "aaa+dynauth" as appropriate.  Keep note of the remaining TTL of
        each of the discovered NAPTR records.

   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} {hostname; port; order/preference;
        min{all TTLs that led to this result} } for all lookup results).
        Keep note of the remaining TTL of each of the discovered records
        (e.g.  SRV and AAAA).

   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. label (see "Delay considerations" below).  Keep note
        of the TTL of each of the discovered SRV records.

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

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

   13.  Perform subsequent lookup steps until lookup yields one or more
        hostnames (see "Delay considerations" below).  Keep note of the
        TTL of each of the discovered records.

   14.  O = (set of {Order/Preference, hostname} {hostname; port; order/preference; min{all TTLs that
        led to this result} } for all hostnames).  Terminate.

2.3.4.  Validity of results

   After executing the above algorithm, the RADIUS server establishes a
   connection to a home server from the result set.  This connection can
   potentially remain open for an indefinite amount of time.  This
   conflicts with the possibility of changing device and network
   configurations on the receiving end.  Typically, TTL values for
   records in the name resolution system are used to indicate how long
   it is safe to rely on the results of the name resolution.  To allow
   for a change of configuration, a RADIUS server SHOULD re-execute the
   algorithm above after the lowest of the TTL values that are
   associated with this connection have expired.  The server MAY keep
   the session open during this re-assessment to avoid closure and
   immediate re-opening of the connection should the result not have

   Should the algorithm above terminate with an empty set (but no
   error), the RADIUS server SHOULD NOT attempt another execution of
   this algorithm for the same target realm before the negative TTL has

   Should the algorithm above terminate due to an error with no TTL
   value known (e.g.  DNS SERVFAIL), the RADIUS server SHOULD NOT
   attempt another execution of this algorithm for the same target realm
   before a configurable timeout interval has passed.

2.3.5.  Delay considerations

   The host's name resolution library may need to contact outside
   entities to perform the name resolution (e.g. authoritative name
   servers for a domain), and since the NAI discovery algorithm is based
   on uncontrollable user input, the destination of the lookups is out
   of control of the server that performs NAI discovery.  If such
   outside entities are misconfigured or unreachable, the algorithm
   above may need an unacceptably long time to terminate.  Many RADIUS
   implementations time out after five seconds of delay between Request
   and Response.  It is not useful to wait until the host name
   resolution library signals a time-out of its name resolution
   algorithms; instead, implementations of NAI discovery SHOULD
   terminate the algorithm after the fixed upper bound of time of three
   seconds.  If no final output of the algorithm is available after this
   timeout, the RADIUS server MUST assume the empty set as a result and
   treat the pending request according to its static configuration
   (e.g., fallback to a default route to a home server).  Execution of
   the NAI discovery algorithm SHOULD be non-blocking (i.e. allow other
   requests to be processed in parallel to the execution of the

2.3.6.  Example

   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

      _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" "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--

   6.   NOOP

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

   8.   Terminate.

   9.   (not executed)

   10.  (not executed)

   11.  (not executed)

   12.  (not executed)

   13.  (not executed)
   14.  (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 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

   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.

   [RFC5580]                Tschofenig, H., Adrangi, F., Jones, M.,
                            Lior, A., and B. Aboba, "Carrying Location
                            Objects in RADIUS and Diameter", RFC 5580,
                            August 2009.

   [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.


   [RFC6614]                Winter, S., McCauley, M., Venaas, S., and K.
                            Wierenga, "TLS encryption "Transport Layer Security (TLS)
                            Encryption for RADIUS",
                             draft-ietf-radext-radsec-09 (work in
                             progress), July 2011. RFC 6614, May 2012.

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.