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RFC 7585

Experimental
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Dynamic Peer Discovery for RADIUS/TLS and RADIUS/DTLS Based on the Network Access Identifier (NAI)

 


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Internet Engineering Task Force (IETF)                         S. Winter
Request for Comments: 7585                                       RESTENA
Category: Experimental                                       M. McCauley
ISSN: 2070-1721                                                AirSpayce
                                                            October 2015


         Dynamic Peer Discovery for RADIUS/TLS and RADIUS/DTLS
              Based on the Network Access Identifier (NAI)

Abstract

   This document specifies a means to find authoritative RADIUS servers
   for a given realm.  It is used in conjunction with either RADIUS over
   Transport Layer Security (RADIUS/TLS) or RADIUS over Datagram
   Transport Layer Security (RADIUS/DTLS).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7585.

Page 2 
Copyright Notice

   Copyright (c) 2015 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
     1.3.  Document Status . . . . . . . . . . . . . . . . . . . . .   6
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  DNS Resource Record (RR) Definition . . . . . . . . . . .   7
       2.1.1.  S-NAPTR . . . . . . . . . . . . . . . . . . . . . . .   7
       2.1.2.  SRV . . . . . . . . . . . . . . . . . . . . . . . . .  12
       2.1.3.  Optional Name Mangling  . . . . . . . . . . . . . . .  12
     2.2.  Definition of the X.509 Certificate Property
           SubjectAltName:otherName:NAIRealm . . . . . . . . . . . .  14
   3.  DNS-Based NAPTR/SRV Peer Discovery  . . . . . . . . . . . . .  16
     3.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .  16
     3.2.  Configuration Variables . . . . . . . . . . . . . . . . .  16
     3.3.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     3.4.  Realm to RADIUS Server Resolution Algorithm . . . . . . .  17
       3.4.1.  Input . . . . . . . . . . . . . . . . . . . . . . . .  17
       3.4.2.  Output  . . . . . . . . . . . . . . . . . . . . . . .  18
       3.4.3.  Algorithm . . . . . . . . . . . . . . . . . . . . . .  18
       3.4.4.  Validity of Results . . . . . . . . . . . . . . . . .  20
       3.4.5.  Delay Considerations  . . . . . . . . . . . . . . . .  21
       3.4.6.  Example . . . . . . . . . . . . . . . . . . . . . . .  21
   4.  Operations and Manageability Considerations . . . . . . . . .  24
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  26
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  29
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  30
   Appendix A.  ASN.1 Syntax of NAIRealm . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

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1.  Introduction

   RADIUS in all its current transport variants (RADIUS/UDP, RADIUS/TCP,
   RADIUS/TLS, and RADIUS/DTLS) requires manual configuration of all
   peers (clients and servers).

   Where more than one administrative entity collaborates for RADIUS
   authentication of their respective customers (a "roaming
   consortium"), the Network Access Identifier (NAI) [RFC7542] is the
   suggested way of differentiating users between those entities; the
   part of a username to the right of the "@" delimiter in an NAI is
   called the user's "realm".  Where many realms and RADIUS forwarding
   servers are in use, the number of realms to be forwarded and the
   corresponding number of servers to configure may be significant.
   Where new realms with new servers are added or details of existing
   servers change on a regular basis, maintaining a single monolithic
   configuration file for all these details may prove too cumbersome to
   be useful.

   Furthermore, in cases where a roaming consortium consists of
   independently working branches (e.g., departments and national
   subsidiaries), each with their own forwarding servers, and who add or
   change their realm lists at their own discretion, there is additional
   complexity in synchronizing the changed data across all branches.

   Where realms can be partitioned (e.g., according to their top-level
   domain (TLD) ending), forwarding of requests can be realized with a
   hierarchy of RADIUS servers, all serving their partition of the realm
   space.  Figure 1 shows an example of this hierarchical routing.

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                                    +-------+
                                    |       |
                                    |   .   |
                                    |       |
                                    +---+---+
                                      / | \
                    +----------------/  |  \---------------------+
                    |                   |                        |
                    |                   |                        |
                    |                   |                        |
                 +--+---+            +--+--+                +----+---+
                 |      |            |     |                |        |
                 | .edu |    . . .   | .nl |      . . .     | .ac.uk |
                 |      |            |     |                |        |
                 +--+---+            +--+--+                +----+---+
                  / | \                 | \                      |
                 /  |  \                |  \                     |
                /   |   \               |   \                    |
         +-----+    |    +-----+        |    +------+            |
         |          |          |        |           |            |
         |          |          |        |           |            |
     +---+---+ +----+---+ +----+---+ +--+---+ +-----+----+ +-----+-----+
     |       | |        | |        | |      | |          | |           |
     |utk.edu| |utah.edu| |case.edu| |hva.nl| |surfnet.nl| |soton.ac.uk|
     |       | |        | |        | |      | |          | |           |
     +----+--+ +--------+ +--------+ +------+ +----+-----+ +-----------+
          |                                        |
          |                                        |
       +--+--+                                  +--+--+
       |     |                                  |     |
     +-+-----+-+                                |     |
     |         |                                +-----+
     +---------+
    user: paul@surfnet.nl             surfnet.nl Authentication server

     Figure 1: RADIUS Hierarchy Based on Top-Level Domain Partitioning

   However, such partitioning is not always possible.  As an example, in
   one real-life deployment, the administrative boundaries and RADIUS
   forwarding servers are organized along country borders, but generic
   top-level domains such as .edu do not map to this choice of
   boundaries (see [RFC7593] for details).  These situations can benefit
   significantly from a distributed mechanism for storing realm and
   server reachability information.  This document describes one such
   mechanism: storage of realm-to-server mappings in DNS; realm-based
   request forwarding can then be realized without a static hierarchy
   such as in the following figure:

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                                    ---------
                                   /         \
                          ---------           ------------
                         /                                \
                         |    DNS                          -
               ----------|                                  \
              /          \          surfnet.nl NAPTR?       |
        (1)  /            ----       -> radius.surfnet.nl   /
            /                 \                            /
           /                   --------           ---------
          /                            \---------/
         |
         |   ---------------------------------------
         |  /              (2) RADIUS               \
         |  |                                       |
     +---+---+ +----+---+ +----+---+ +--+---+ +-----+----+ +-----+-----+
     |       | |        | |        | |      | |          | |           |
     |utk.edu| |utah.edu| |case.edu| |hva.nl| |surfnet.nl| |soton.ac.uk|
     |       | |        | |        | |      | |          | |           |
     +----+--+ +--------+ +--------+ +------+ +----+-----+ +-----------+
          |                                        |
          |                                        |
       +--+--+                                  +--+--+
       |     |                                  |     |
     +-+-----+-+                                |     |
     |         |                                +-----+
     +---------+
     user: paul@surfnet.nl             surfnet.nl Authentication server

     Figure 2: RADIUS Hierarchy Based on Top-Level Domain Partitioning

   This document also specifies various approaches for verifying that
   server information that was retrieved from DNS was from an authorized
   party; for example, an organization that is not at all part of a
   given roaming consortium may alter its own DNS records to yield a
   result for its own realm.

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].

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1.2.  Terminology

   RADIUS/TLS Client: a RADIUS/TLS [RFC6614] instance that initiates a
   new connection.

   RADIUS/TLS Server: a RADIUS/TLS [RFC6614] instance that listens on a
   RADIUS/TLS port and accepts new connections.

   RADIUS/TLS Node: a RADIUS/TLS client or server.

   [RFC7542] defines the terms NAI, realm, and consortium.

1.3.  Document Status

   This document is an Experimental RFC.

   The communities expected to use this document are roaming consortia
   whose authentication services are based on the RADIUS protocol.

   The duration of the experiment is undetermined; as soon as enough
   experience is collected on the choice points mentioned below, it is
   expected to be obsoleted by a Standards Track version of the
   protocol, which trims down the choice points.

   If that removal of choice points obsoletes tags or service names as
   defined in this document and allocated by IANA, these items will be
   returned to IANA as per the provisions in [RFC6335].

   The document provides a discovery mechanism for RADIUS, which is very
   similar to the approach that is taken with the Diameter protocol
   [RFC6733].  As such, the basic approach (using Naming Authority
   Pointer (NAPTR) records in DNS domains that match NAI realms) is not
   of a very experimental nature.

   However, the document offers a few choice points and extensions that
   go beyond the provisions for Diameter.  The list of major additions/
   deviations is

   o  provisions for determining the authority of a server to act for
      users of a realm (declared out of scope for Diameter)

   o  much more in-depth guidance on DNS regarding timeouts, failure
      conditions, and alteration of Time-To-Live (TTL) information than
      the Diameter counterpart

   o  a partially correct routing error detection during DNS lookups

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2.  Definitions

2.1.  DNS Resource Record (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 3.4 below).

2.1.1.  S-NAPTR

2.1.1.1.  Registration of Application Service and Protocol Tags

   This specification defines three S-NAPTR service tags:

   +-----------------+-----------------------------------------+
   | Service Tag     | Use                                     |
   +-----------------+-----------------------------------------+
   | aaa+auth        | RADIUS Authentication, i.e., traffic as |
   |                 | defined in [RFC2865]                    |
   | - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
   | aaa+acct        | RADIUS Accounting, i.e., traffic as     |
   |                 | defined in [RFC2866]                    |
   | - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
   | aaa+dynauth     | RADIUS Dynamic Authorization, i.e.,     |
   |                 | traffic as defined in [RFC5176]         |
   +-----------------+-----------------------------------------+

                      Figure 3: List of Service Tags

   This specification defines two S-NAPTR protocol tags:

   +-----------------+-----------------------------------------+
   | Protocol Tag    | Use                                     |
   +-----------------+-----------------------------------------+
   | radius.tls.tcp  | RADIUS transported over TLS as defined  |
   |                 | in [RFC6614]                            |
   | - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
   | radius.dtls.udp | RADIUS transported over DTLS as defined |
   |                 | in [RFC7360]                            |
   +-----------------+-----------------------------------------+

                      Figure 4: List of Protocol Tags

   Note well:

      The S-NAPTR service and protocols are unrelated to the IANA
      "Service Name and Transport Protocol Port Number Registry".

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      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
      resolver.

      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.

2.1.1.2.  Definition of Conditions for Retry/Failure

   RADIUS is a time-critical protocol; RADIUS clients that do not
   receive an answer after a configurable, but short, amount of time
   will consider the request failed.  Due to this, there is little
   leeway for extensive retries.

   As a general rule, only error conditions that generate an immediate
   response from the other end are eligible for a retry of a discovered
   target.  Any error condition involving timeouts, or the absence of a
   reply for more than one second during the connection setup phase, is
   to be considered a failure; the next target in the set of discovered
   NAPTR targets is to be tried.

   Note that [RFC3958] already defines that a failure to identify the
   server as being authoritative for the realm is always considered a
   failure; so even if a discovered target returns a wrong credential
   instantly, it is not eligible for retry.

   Furthermore, the contacted RADIUS/TLS server verifies during
   connection setup whether or not it finds the connecting RADIUS/TLS
   client authorized.  If the connecting RADIUS/TLS client is not found
   acceptable, the server will close the TLS connection immediately with
   an appropriate alert.  Such TLS handshake failures are permanently
   fatal and not eligible for retry, unless the connecting client has
   more X.509 certificates to try; in this case, a retry with the
   remainder of its set of certificates SHOULD be attempted.  Not trying
   all available client certificates potentially creates a DoS for the
   end user whose authentication attempt triggered the discovery; one of
   the neglected certificates might have led to a successful RADIUS
   connection and subsequent end-user authentication.

   If the TLS session setup to a discovered target does not succeed,
   that target (as identified by the IP address and port number) SHOULD
   be ignored from the result set of any subsequent executions of the
   discovery algorithm at least until the target's Effective TTL (see
   Section 3.3) has expired or until the entity that executes the
   algorithm changes its TLS context to either send a new client
   certificate or expect a different server certificate.

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2.1.1.3.  Server Identification and Handshake

   After the algorithm in this document has been executed, a RADIUS/TLS
   session as per [RFC6614] is established.  Since the discovery
   algorithm does not have provisions to establish confidential keying
   material between the RADIUS/TLS client (i.e., the server that
   executes the discovery algorithm) and the RADIUS/TLS server that was
   discovered, Pre-Shared Key (PSK) ciphersuites for TLS cannot be used
   in the subsequent TLS handshake.  Only TLS ciphersuites using X.509
   certificates can be used with this algorithm.

   There are numerous ways to define which certificates are acceptable
   for use in this context.  This document defines one mandatory-to-
   implement mechanism that allows verification of whether the contacted
   host is authoritative for an NAI realm or not.  It also gives one
   example of another mechanism that is currently in widespread
   deployment and one possible approach based on DNSSEC, which is yet
   unimplemented.

   For the approaches that use trust roots (see the following two
   sections), a typical deployment will use a dedicated trust store for
   RADIUS/TLS certificate authorities, particularly a trust store that
   is independent from default "browser" trust stores.  Often, this will
   be one or a few Certification Authorities (CAs), and they only issue
   certificates for the specific purpose of establishing RADIUS server-
   to-server trust.  It is important not to trust a large set of CAs
   that operate outside the control of the roaming consortium, since
   their issuance of certificates with the properties important for
   authorization (such as NAIRealm and policyOID below) is difficult to
   verify.  Therefore, clients SHOULD NOT be preconfigured with a list
   of known public CAs by the vendor or manufacturer.  Instead, the
   clients SHOULD start off with an empty CA list.  The addition of a CA
   SHOULD be done only when manually configured by an administrator.

2.1.1.3.1.  Mandatory-to-Implement Mechanism: Trust Roots + NAIRealm

   Verification of authority to provide Authentication, Authorization,
   and Accounting (AAA) services over RADIUS/TLS is a two-step process.

   Step 1 is the verification of certificate well-formedness and
   validity as per [RFC5280] and whether it was issued from a root
   certificate that is deemed trustworthy by the RADIUS/TLS client.

   Step 2 is to compare the value of the algorithm's variable "R" after
   the execution of step 3 of the discovery algorithm in Section 3.4.3
   below (i.e., after a consortium name mangling but before conversion
   to a form usable by the name resolution library) to all values of the

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   contacted RADIUS/TLS server's X.509 certificate property
   "subjectAlternativeName:otherName:NAIRealm" as defined in
   Section 2.2.

2.1.1.3.2.  Other Mechanism: Trust Roots + policyOID

   Verification of authority to provide AAA services over RADIUS/TLS is
   a two-step process.

   Step 1 is the verification of certificate well-formedness and
   validity as per [RFC5280] and whether it was issued from a root
   certificate that is deemed trustworthy by the RADIUS/TLS client.

   Step 2 is to compare the values of the contacted RADIUS/TLS server's
   X.509 certificate's extensions of type "Policy OID" to a list of
   configured acceptable Policy OIDs for the roaming consortium.  If one
   of the configured OIDs is found in the certificate's Policy OID
   extensions, then the server is considered authorized; if there is no
   match, the server is considered unauthorized.

   This mechanism is inferior to the mandatory-to-implement mechanism in
   the previous section because all authorized servers are validated by
   the same OID value; the mechanism is not fine grained enough to
   express authority for one specific realm inside the consortium.  If
   the consortium contains members that are hostile against other
   members, this weakness can be exploited by one RADIUS/TLS server
   impersonating another if DNS responses can be spoofed by the hostile
   member.

   The shortcomings in server identification can be partially mitigated
   by using the RADIUS infrastructure only with authentication payloads
   that provide mutual authentication and credential protection (i.e.,
   Extensible Authentication Protocol (EAP) types passing the criteria
   of [RFC4017]): using mutual authentication prevents the hostile
   server from mimicking the real EAP server (it can't terminate the EAP
   authentication unnoticed because it does not have the server
   certificate from the real EAP server); protection of credentials
   prevents the impersonating server from learning usernames and
   passwords of the ongoing EAP conversation (other RADIUS attributes
   pertaining to the authentication, such as the EAP peer's Calling-
   Station-ID, can still be learned though).

2.1.1.3.3.  Other Mechanism: DNSSEC/DANE

   Where DNSSEC is used, the results of the algorithm can be trusted;
   that is, the entity that executes the algorithm can be certain that
   the realm that triggered the discovery is actually served by the
   server that was discovered via DNS.  However, this does not guarantee

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   that the server is also authorized (i.e., a recognized member of the
   roaming consortium).  The server still needs to present an X.509
   certificate proving its authority to serve a particular realm.

   The authorization can be sketched using DNSSEC and DNS-Based
   Authentication of Named Entities (DANE) as follows: DANE/TLSA records
   of all authorized servers are put into a DNSSEC zone that contains
   all known and authorized realms; the zone is rooted in a common,
   consortium-agreed branch of the DNS tree.  The entity executing the
   algorithm uses the realm information from the authentication attempt
   and then attempts to retrieve TLSA resource records (TLSA RRs) for
   the DNS label "realm.commonroot".  It then verifies that the
   presented server certificate during the RADIUS/TLS handshake matches
   the information in the TLSA record.

   Example:

      Realm = "example.com"

      Common Branch = "idp.roaming-consortium.example.

      label for TLSA query = "example.com.idp.roaming-
      consortium.example.

      result of discovery algorithm for realm "example.com" =
      192.0.2.1:2083

      ( TLS certificate of 192.0.2.1:2083 matches TLSA RR ? "PASS" :
      "FAIL" )

2.1.1.3.4.  Client Authentication and Authorization

   Note that RADIUS/TLS connections always mutually authenticate the
   RADIUS server and the RADIUS client.  This specification provides an
   algorithm for a RADIUS client to contact and verify authorization of
   a RADIUS server only.  During connection setup, the RADIUS server
   also needs to verify whether it considers the connecting RADIUS
   client authorized; this is outside the scope of this specification.

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2.1.2.  SRV

   This specification defines two SRV prefixes (i.e., two values for the
   "_service._proto" part of an SRV RR as per [RFC2782]):

   +-------------------+-----------------------------------------+
   | SRV Label         | Use                                     |
   +-------------------+-----------------------------------------+
   | _radiustls._tcp   | RADIUS transported over TLS as defined  |
   |                   | in [RFC6614]                            |
   | - - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
   | _radiusdtls._udp  | RADIUS transported over DTLS as defined |
   |                   | in [RFC7360]                            |
   +-------------------+-----------------------------------------+

                       Figure 5: List of SRV Labels

   Just like NAPTR records, the lookup and subsequent follow up of SRV
   records may yield more than one server to contact in a prioritized
   list.  [RFC2782] does not specify rules regarding "Definition of
   Conditions for Retry/Failure" nor "Server Identification and
   Handshake".  This specification states that the rules for these two
   topics as defined in Sections 2.1.1.2 and 2.1.1.3 SHALL be used both
   for targets retrieved via an initial NAPTR RR as well as for targets
   retrieved via an initial SRV RR (i.e., in the absence of NAPTR RRs).

2.1.3.  Optional Name Mangling

   It is expected that in most cases, the SRV and/or NAPTR label used
   for the records is the DNS A-label representation of the literal
   realm name for which the server is the authoritative RADIUS server
   (i.e., the realm name after conversion according to Section 5 of
   [RFC5891]).

   However, arbitrary other labels or service tags may be used if, for
   example, a roaming consortium uses realm names that 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 and/or which service tag to use.

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   Examples:

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

          example.com.  IN NAPTR 50 50 "s" "aaa+auth:radius.tls.tcp" ""
          _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 RADIUS servers use to resolve
       realm names.  "Company, Inc." is part of the consortium.  On the
       consortium's DNS server, realm company.example might have the
       following DNS entries:

          company.example.  IN NAPTR 50 50 "a"
          "aaa+auth:radius.dtls.udp" "" roamserv.company.example.

   c.  The eduroam consortium (see [RFC7593]) 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 RADIUS servers).  Due to that, the eduroam consortium uses
       the service tag "x-eduroam" for authentication purposes and
       eduroam RADIUS servers use this tag to look up other eduroam
       servers.  An eduroam participant example.org that 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.tcp" ""
          _radiustls._tcp.eduroam.example.org.

          example.org.  IN NAPTR 50 50 "s" "aaa+auth:radius.tls.tcp" ""
          _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.

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2.2.  Definition of the X.509 Certificate Property
      SubjectAltName:otherName:NAIRealm

   This specification retrieves IP addresses and port numbers from the
   Domain Name System that are subsequently used to authenticate users
   via the RADIUS/TLS protocol.  Regardless whether the results from DNS
   discovery are trustworthy or not (e.g., DNSSEC in use), it is always
   important to verify that the server that was contacted is authorized
   to service requests for the user that triggered the discovery
   process.

   The input to the algorithm is an NAI realm as specified in
   Section 3.4.1.  As a consequence, the X.509 certificate of the server
   that is ultimately contacted for user authentication needs to be able
   to express that it is authorized to handle requests for that realm.

   Current subjectAltName fields do not semantically allow an NAI realm
   to be expressed; the field subjectAltName:dNSName is syntactically a
   good match but would inappropriately conflate DNS names and NAI realm
   names.  Thus, this specification defines a new subjectAltName field
   to hold either a single NAI realm name or a wildcard name matching a
   set of NAI realms.

   The subjectAltName:otherName:sRVName field certifies that a
   certificate holder is authorized to provide a service; this can be
   compared to the target of a DNS label's SRV resource record.  If the
   Domain Name System is insecure, it is required that the label of the
   SRV record itself is known-correct.  In this specification, that
   label is not known-correct; it is potentially derived from a
   (potentially untrusted) NAPTR resource record of another label.  If
   DNS is not secured with DNSSEC, the NAPTR resource record may have
   been altered by an attacker with access to the Domain Name System
   resolution, and thus the label used to look up the SRV record may
   already be tainted.  This makes subjectAltName:otherName:sRVName not
   a trusted comparison item.

   Further to this, this specification's NAPTR entries may be of type
   "A", which does not involve resolution of any SRV records, which
   again makes subjectAltName:otherName:sRVName unsuited for this
   purpose.

   This section defines the NAIRealm name as a form of otherName from
   the GeneralName structure in subjectAltName defined in [RFC5280].

Top      ToC       Page 15 
      id-on-naiRealm OBJECT IDENTIFIER ::= { id-on 8 }

      ub-naiRealm-length INTEGER ::= 255

      NAIRealm ::= UTF8String (SIZE (1..ub-naiRealm-length))

   The NAIRealm, if present, MUST contain an NAI realm as defined in
   [RFC7542].  It MAY substitute the leftmost dot-separated label of the
   NAI with the single character "*" to indicate a wildcard match for
   "all labels in this part".  Further features of regular expressions,
   such as a number of characters followed by an "*" to indicate a
   common prefix inside the part, are not permitted.

   The comparison of an NAIRealm to the NAI realm as derived from user
   input with this algorithm is a byte-by-byte comparison, except for
   the optional leftmost dot-separated part of the value whose content
   is a single "*" character; such labels match all strings in the same
   dot-separated part of the NAI realm.  If at least one of the
   sAN:otherName:NAIRealm values match the NAI realm, the server is
   considered authorized; if none match, the server is considered
   unauthorized.

   Since multiple names and multiple name forms may occur in the
   subjectAltName extension, an arbitrary number of NAIRealms can be
   specified in a certificate.

   Examples:

   +---------------------+-------------------+-----------------------+
   | NAI realm (RADIUS)  | NAIRealm (cert)   | MATCH?                |
   +---------------------+-------------------+-----------------------+
   | foo.example         | foo.example       | YES                   |
   | foo.example         | *.example         | YES                   |
   | bar.foo.example     | *.example         | NO                    |
   | bar.foo.example     | *ar.foo.example   | NO (NAIRealm invalid) |
   | bar.foo.example     | bar.*.example     | NO (NAIRealm invalid) |
   | bar.foo.example     | *.*.example       | NO (NAIRealm invalid) |
   | sub.bar.foo.example | *.*.example       | NO (NAIRealm invalid) |
   | sub.bar.foo.example | *.bar.foo.example | YES                   |
   +-----------------+-----------------------------------------------+

         Figure 6: Examples for NAI Realm vs. Certificate Matching

   Appendix A contains the ASN.1 definition of the above objects.

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3.  DNS-Based NAPTR/SRV Peer Discovery

3.1.  Applicability

   Dynamic server discovery as defined in this document is only
   applicable for new AAA transactions and per service (i.e., distinct
   discovery is needed for Authentication, Accounting, and Dynamic
   Authorization) where a RADIUS entity that acts as a forwarding server
   for one or more realms receives a request with a realm for which it
   is not authoritative, and which no explicit next hop is configured.
   It is only applicable for

   a.  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.

   b.  the first accounting ticket for a user session.

   c.  the first RADIUS DynAuth packet for a user session.

3.2.  Configuration Variables

   The algorithm contains various variables for timeouts.  These
   variables are named here and reasonable default values are provided.
   Implementations wishing to deviate from these defaults should make
   sure they understand the implications of changes.

      DNS_TIMEOUT: maximum amount of time to wait for the complete set
      of all DNS queries to complete: Default = 3 seconds

      MIN_EFF_TTL: minimum DNS TTL of discovered targets: Default = 60
      seconds

      BACKOFF_TIME: if no conclusive DNS response was retrieved after
      DNS_TIMEOUT, do not attempt dynamic discovery before BACKOFF_TIME
      has elapsed: Default = 600 seconds

3.3.  Terms

   Positive DNS response: A response that contains the RR that was
   queried for.

   Negative DNS response: A response that does not contain the RR that
   was queried for but contains an SOA record along with a TTL
   indicating cache duration for this negative result.

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   DNS Error: Where the algorithm states "name resolution returns with
   an error", this shall mean that either the DNS request timed out or
   it is a DNS response, which is neither a positive nor a negative
   response (e.g., SERVFAIL).

   Effective TTL: The validity period for discovered RADIUS/TLS target
   hosts.  Calculated as: Effective TTL (set of DNS TTL values) = max {
   MIN_EFF_TTL, min { DNS TTL values } }

   SRV lookup: For the purpose of this specification, SRV lookup
   procedures are defined as per [RFC2782] but excluding that RFCs "A"
   fallback as defined in the "Usage Rules" section, final "else"
   clause.

   Greedy result evaluation: The NAPTR to SRV/A/AAAA resolution may lead
   to a tree of results, whose leafs are the IP addresses to contact.
   The branches of the tree are ordered according to their order/
   preference DNS properties.  An implementation is executing greedy
   result evaluation if it uses a depth-first search in the tree along
   the highest order results, attempts to connect to the corresponding
   resulting IP addresses, and only backtracks to other branches if the
   higher ordered results did not end in successful connection attempts.

3.4.  Realm to RADIUS Server Resolution Algorithm

3.4.1.  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 is 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.

   For RADIUS DynAuth server discovery, input I to the algorithm is the
   domain name of the operator of a RADIUS realm as was communicated
   during user authentication using the Operator-Name attribute
   ([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, preceded 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

Top      ToC       Page 18 
   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 "@".

      Encoding of User-Name in local encodings.

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

      UTF-8 realms that 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 RADIUS 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.

3.4.2.  Output

   Output O of the algorithm is a two-tuple consisting of: O-1) a set of
   tuples {hostname; port; protocol; order/preference; Effective TTL} --
   the set can be empty -- and O-2) an integer.  If the set in the first
   part of the tuple is empty, the integer contains the Effective TTL
   for backoff timeout; if the set is not empty, the integer is set to 0
   (and not used).

3.4.3.  Algorithm

   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.   Modify R according to agreed consortium procedures if
        applicable.

   4.   Convert R to a representation usable by the name resolution
        library if needed.

   5.   Initialize TIMER = 0; start TIMER.  If TIMER reaches
        DNS_TIMEOUT, continue at step 20.

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   6.   Using the host's name resolution library, perform a NAPTR query
        for R (see "Delay Considerations", Section 3.4.5, below).  If
        the result is a negative DNS response, O-2 = Effective TTL ( TTL
        value of the SOA record ) and continue at step 13.  If name
        resolution returns with error, O-1 = { empty set }, O-2 =
        BACKOFF_TIME, and terminate.

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

   8.   If no records are found, continue at step 13.

   9.   For the extracted NAPTRs, perform successive resolution as
        defined in [RFC3958], Section 2.2.  An implementation MAY use
        greedy result evaluation according to the NAPTR order/preference
        fields (i.e., can execute the subsequent steps of this algorithm
        for the highest-order entry in the set of results and only look
        up the remainder of the set if necessary).

   10.  If the set of hostnames is empty, O-1 = { empty set }, O-2 =
        BACKOFF_TIME, and terminate.

   11.  O' = (set of {hostname; port; protocol; order/preference;
        Effective TTL ( all DNS TTLs that led to this hostname ) } for
        all terminal lookup results).

   12.  Proceed with step 18.

   13.  Generate R' = (prefix R with "_radiustls._tcp." and/or
        "_radiustls._udp.").

   14.  Using the host's name resolution library, perform SRV lookup
        with R' as label (see "Delay Considerations", Section 3.4.5,
        below).

   15.  If name resolution returns with error, O-1 = { empty set }, O-2
        = BACKOFF_TIME, and terminate.

   16.  If the result is a negative DNS response, O-1 = { empty set },
        O-2 = min { O-2, Effective TTL ( TTL value of the SOA record )
        }, and terminate.

   17.  O' = (set of {hostname; port; protocol; order/preference;
        Effective TTL ( all DNS TTLs that led to this result ) } for all
        hostnames).

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   18.  Generate O-1 by resolving hostnames in O' into corresponding A
        and/or AAAA addresses: O-1 = (set of {IP address; port;
        protocol; order/preference; Effective TTL ( all DNS TTLs that
        led to this result ) } for all hostnames ), O-2 = 0.

   19.  For each element in O-1, test if the original request that
        triggered dynamic discovery was received on {IP address; port}.
        If yes, O-1 = { empty set }, O-2 = BACKOFF_TIME, log error, and
        terminate (see next section for a rationale).  If no, O is the
        result of dynamic discovery; terminate.

   20.  O-1 = { empty set }, O-2 = BACKOFF_TIME, log error, and
        terminate.

3.4.4.  Validity of Results

   The discovery algorithm is used by servers that do not have
   sufficient configuration information to process an incoming request
   on their own.  If the discovery algorithm result contains the
   server's own listening address (IP address and port), then there is a
   potential for an endless forwarding loop.  If the listening address
   is the DNS result with the highest priority, the server will enter a
   tight loop (the server would forward the request to itself,
   triggering dynamic discovery again in a perpetual loop).  If the
   address has a lower priority in the set of results, there is a
   potential loop with intermediate hops in between (the server could
   forward to another host with a higher priority, which might use DNS
   itself and forward the packet back to the first server).  The
   underlying reason that enables these loops is that the server
   executing the discovery algorithm is seriously misconfigured in that
   it does not recognize the request as one that is to be processed by
   itself.  RADIUS has no built-in loop detection, so any such loops
   would remain undetected.  So, if step 18 of the algorithm discovers
   such a possible-loop situation, the algorithm should be aborted and
   an error logged.  Note that this safeguard does not provide perfect
   protection against routing loops.  One reason that might introduce a
   loop includes the possibility that a subsequent hop has a statically
   configured next hop that leads to an earlier host in the loop.
   Another reason for occurring loops is if the algorithm was executed
   with greedy result evaluation, and the server's own address was in a
   lower-priority branch of the result set that was not retrieved from
   DNS at all, and thus can't be detected.

   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

Top      ToC       Page 21 
   records in the name resolution system are used to indicate how long
   it is safe to rely on the results of the name resolution.  If these
   TTLs are very low, thrashing of connections becomes possible; the
   Effective TTL mitigates that risk.  When a connection is open and the
   smallest of the Effective TTL value that was learned during
   discovering the server has not expired, subsequent new user sessions
   for the realm that corresponds to that open connection SHOULD reuse
   the existing connection and SHOULD NOT re-execute the discovery
   algorithm nor open a new connection.  To allow for a change of
   configuration, a RADIUS server SHOULD re-execute the discovery
   algorithm after the Effective TTL that is associated with this
   connection has expired.  The server SHOULD keep the session open
   during this reassessment to avoid closure and immediate reopening of
   the connection should the result not have changed.

   Should the algorithm above terminate with O-1 = { empty set }, the
   RADIUS server SHOULD NOT attempt another execution of this algorithm
   for the same target realm before the timeout O-2 has passed.

3.4.5.  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 timeout of its name resolution
   algorithms.  The algorithm therefore controls execution time with
   TIMER.  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 algorithm).

3.4.6.  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 forwarding server does
      not have the realm tu-m[U+00FC]nchen.example in its forwarding
      configuration but uses DNS for name resolution and has configured
      the use of dynamic discovery to discover RADIUS servers.

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      It is IPv6 enabled and prefers AAAA records over A records.

      It is listening for incoming RADIUS/TLS requests on 192.0.2.1,
      TCP/2083.

   May the configuration variables be

      DNS_TIMEOUT = 3 seconds

      MIN_EFF_TTL = 60 seconds

      BACKOFF_TIME = 3600 seconds

   If DNS contains the following records

      xn--tu-mnchen-t9a.example.  IN NAPTR 50 50 "s"
      "aaa+auth:radius.tls.tcp" "" _myradius._tcp.xn--tu-mnchen-
      t9a.example.

      xn--tu-mnchen-t9a.example.  IN NAPTR 50 50 "s"
      "fooservice:bar.dccp" "" _abc123._def.xn--tu-mnchen-t9a.example.

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

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

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

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

      backupserver.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

   5.   TIMER starts.

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   6.   Result:

           (TTL = 47) 50 50 "s" "aaa+auth:radius.tls.tcp" ""
           _myradius._tcp.xn--tu-mnchen-t9a.example.

           (TTL = 522) 50 50 "s" "fooservice:bar.dccp" ""
           _abc123._def.xn--tu-mnchen-t9a.example.

   7.   Result:

           (TTL = 47) 50 50 "s" "aaa+auth:radius.tls.tcp" ""
           _myradius._tcp.xn--tu-mnchen-t9a.example.

   8.   NOOP

   9.   Successive resolution performs SRV query for label
        _myradius._tcp.xn--tu-mnchen-t9a.example, which results in

           (TTL 499) 0 10 2083 radsec.xn--tu-mnchen-t9a.example.

           (TTL 2200) 0 20 2083 backup.xn--tu-mnchen-t9a.example.

   10.  NOOP

   11.  O' = {

           (radsec.xn--tu-mnchen-t9a.example.; 2083; RADIUS/TLS; 10;
           60),

           (backup.xn--tu-mnchen-t9a.example.; 2083; RADIUS/TLS; 20; 60)

        } // minimum TTL is 47, upped to MIN_EFF_TTL

   12.  Continuing at 18.

   13.  (not executed)

   14.  (not executed)

   15.  (not executed)

   16.  (not executed)

   17.  (not executed)

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   18.  O-1 = {

           (2001:0DB8::202:44ff:fe0a:f704; 2083; RADIUS/TLS; 10; 60),

           (192.0.2.7; 2083; RADIUS/TLS; 20; 60)

        }; O-2 = 0

   19.  No match with own listening address; terminate with tuple (O-1,
        O-2) from previous step.

   The implementation will then attempt to connect to two servers, with
   preference to [2001:0DB8::202:44ff:fe0a:f704]:2083 using the RADIUS/
   TLS protocol.

4.  Operations and Manageability Considerations

   The discovery algorithm as defined in this document contains several
   options: the major ones are use of NAPTR vs. SRV; how to determine
   the authorization status of a contacted server for a given realm; and
   which trust anchors to consider trustworthy for the RADIUS
   conversation setup.

   Random parties that do not agree on the same set of options may not
   be able to interoperate.  However, such a global interoperability is
   not intended by this document.

   Discovery as per this document becomes important inside a roaming
   consortium, which has set up roaming agreements with the other
   partners.  Such roaming agreements require much more than a technical
   means of server discovery; there are administrative and contractual
   considerations at play (service contracts, back-office compensations,
   procedures, etc.).

   A roaming consortium's roaming agreement must include a profile of
   which choice points in this document to use.  So as long as the
   roaming consortium can settle on one deployment profile, they will be
   able to interoperate based on that choice; this per-consortium
   interoperability is the intended scope of this document.

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5.  Security Considerations

   When using DNS without DNSSEC security extensions and validation for
   all of the replies to NAPTR, SRV, and A/AAAA requests as described in
   Section 3, the result of the discovery process can not be trusted.
   Even if it can be trusted (i.e., DNSSEC is in use), actual
   authorization of the discovered server to provide service for the
   given realm needs to be verified.  A mechanism from Section 2.1.1.3
   or equivalent MUST be used to verify authorization.

   The algorithm has a configurable completion timeout DNS_TIMEOUT
   defaulting to three seconds for RADIUS' operational reasons.  The
   lookup of DNS resource records based on unverified user input is an
   attack vector for DoS attacks: an attacker might intentionally craft
   bogus DNS zones that take a very long time to reply (e.g., due to a
   particularly byzantine tree structure or artificial delays in
   responses).

   To mitigate this DoS vector, implementations SHOULD consider rate
   limiting either the amount of new executions of the discovery
   algorithm as a whole or the amount of intermediate responses to
   track, or at least the number of pending DNS queries.
   Implementations MAY choose lower values than the default for
   DNS_TIMEOUT to limit the impact of DoS attacks via that vector.  They
   MAY also continue their attempt to resolve DNS records even after
   DNS_TIMEOUT has passed; a subsequent request for the same realm might
   benefit from retrieving the results anyway.  The amount of time spent
   waiting for a result will influence the impact of a possible DoS
   attack; the waiting time value is implementation dependent and
   outside the scope of this specification.

   With dynamic discovery being enabled for a RADIUS server, and
   depending on the deployment scenario, the server may need to open up
   its target IP address and port for the entire Internet because
   arbitrary clients may discover it as a target for their
   authentication requests.  If such clients are not part of the roaming
   consortium, the RADIUS/TLS connection setup phase will fail (which is
   intended), but the computational cost for the connection attempt is
   significant.  When the port for a TLS-based service is open, the
   RADIUS server shares all the typical attack vectors for services
   based on TLS (such as HTTPS and SMTPS).  Deployments of RADIUS/TLS
   with dynamic discovery should consider these attack vectors and take
   appropriate countermeasures (e.g., blacklisting known bad IPs on a
   firewall, rate limiting new connection attempts, etc.).

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6.  Privacy Considerations

   The classic RADIUS operational model (known, preconfigured peers,
   shared secret security, and mostly plaintext communication) and this
   new RADIUS dynamic discovery model (peer discovery with DNS, PKI
   security, and packet confidentiality) differ significantly in their
   impact on the privacy of end users trying to authenticate to a RADIUS
   server.

   With classic RADIUS, traffic in large environments gets aggregated by
   statically configured clearinghouses.  The packets sent to those
   clearinghouses and their responses are mostly unprotected.  As a
   consequence,

   o  All intermediate IP hops can inspect most of the packet payload in
      clear text, including the User-Name and Calling-Station-Id
      attributes, and can observe which client sent the packet to which
      clearinghouse.  This allows the creation of mobility profiles for
      any passive observer on the IP path.

   o  The existence of a central clearinghouse creates an opportunity
      for the clearinghouse to trivially create the same mobility
      profiles.  The clearinghouse may or may not be trusted not to do
      this, e.g., by sufficiently threatening contractual obligations.

   o  In addition to that, with the clearinghouse being a RADIUS
      intermediate in possession of a valid shared secret, the
      clearinghouse can observe and record even the security-critical
      RADIUS attributes such as User-Password.  This risk may be
      mitigated by choosing authentication payloads that are
      cryptographically secured and do not use the attribute User-
      Password -- such as certain EAP types.

   o  There is no additional information disclosure to parties outside
      the IP path between the RADIUS client and server (in particular,
      no DNS servers learn about realms of current ongoing
      authentications).

   With RADIUS and dynamic discovery,

   o  This protocol allows for RADIUS clients to identify and directly
      connect to the RADIUS home server.  This can eliminate the use of
      clearinghouses to do forwarding of requests, and it also
      eliminates the ability of the clearinghouse to then aggregate the
      user information that flows through it.  However, there are
      reasons why clearinghouses might still be used.  One reason to
      keep a clearinghouse is to act as a gateway for multiple backends

Top      ToC       Page 27 
      in a company; another reason may be a requirement to sanitize
      RADIUS datagrams (filter attributes, tag requests with new
      attributes, etc.).

   o  Even where intermediate proxies continue to be used for reasons
      unrelated to dynamic discovery, the number of such intermediates
      may be reduced by removing those proxies that are only deployed
      for pure request routing reasons.  This reduces the number of
      entities that can inspect the RADIUS traffic.

   o  RADIUS clients that make use of dynamic discovery will need to
      query the Domain Name System and use a user's realm name as the
      query label.  A passive observer on the IP path between the RADIUS
      client and the DNS server(s) being queried can learn that a user
      of that specific realm was trying to authenticate at that RADIUS
      client at a certain point in time.  This may or may not be
      sufficient for the passive observer to create a mobility profile.
      During the recursive DNS resolution, a fair number of DNS servers
      and the IP hops in between those get to learn that information.
      Not every single authentication triggers DNS lookups, so there is
      no one-to-one relation of leaked realm information and the number
      of authentications for that realm.

   o  Since dynamic discovery operates on a RADIUS hop-by-hop basis,
      there is no guarantee that the RADIUS payload is not transmitted
      between RADIUS systems that do not make use of this algorithm, and
      they possibly use other transports such as RADIUS/UDP.  On such
      hops, the enhanced privacy is jeopardized.

   In summary, with classic RADIUS, few intermediate entities learn very
   detailed data about every ongoing authentication, while with dynamic
   discovery, many entities learn only very little about recently
   authenticated realms.

7.  IANA Considerations

   Per this document, IANA has added the following entries in existing
   registries:

   o  S-NAPTR Application Service Tags registry

      *  aaa+auth

      *  aaa+acct

      *  aaa+dynauth

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   o  S-NAPTR Application Protocol Tags registry

      *  radius.tls.tcp

      *  radius.dtls.udp

   This document reserves the use of the "radiustls" and "radiusdtls"
   service names.  Registration information as per Section 8.1.1 of
   [RFC6335] is as follows:

      Service Name: radiustls; radiusdtls

      Transport Protocols: TCP (for radiustls), UDP (for radiusdtls)

      Assignee: IESG <iesg@ietf.org>

      Contact: IETF Chair <chair@ietf.org>

      Description: Authentication, Accounting, and Dynamic Authorization
      via the RADIUS protocol.  These service names are used to
      construct the SRV service labels "_radiustls" and "_radiusdtls"
      for discovery of RADIUS/TLS and RADIUS/DTLS servers, respectively.

      Reference: RFC 7585

   This specification makes use of the SRV protocol identifiers "_tcp"
   and "_udp", which are mentioned as early as [RFC2782] but do not
   appear to be assigned in an actual registry.  Since they are in
   widespread use in other protocols, this specification refrains from
   requesting a new registry "RADIUS/TLS SRV Protocol Registry" and
   continues to make use of these tags implicitly.

   Per this document, a number of Object Identifiers have been assigned.
   They are now under the control of IANA following [RFC7299].

   IANA has assigned the following identifiers:

      85 has been assigned from the "SMI Security for PKIX Module
      Identifier" registry.  The description is id-mod-nai-realm-08.

      8 has been assigned from the "SMI Security for PKIX Other Name
      Forms" registry.  The description is id-on-naiRealm.

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8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              DOI 10.17487/RFC2782, February 2000,
              <http://www.rfc-editor.org/info/rfc2782>.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, DOI 10.17487/RFC2865, June 2000,
              <http://www.rfc-editor.org/info/rfc2865>.

   [RFC2866]  Rigney, C., "RADIUS Accounting", RFC 2866,
              DOI 10.17487/RFC2866, June 2000,
              <http://www.rfc-editor.org/info/rfc2866>.

   [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, DOI 10.17487/RFC3958,
              January 2005, <http://www.rfc-editor.org/info/rfc3958>.

   [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,
              DOI 10.17487/RFC5176, January 2008,
              <http://www.rfc-editor.org/info/rfc5176>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5580]  Tschofenig, H., Ed., Adrangi, F., Jones, M., Lior, A., and
              B. Aboba, "Carrying Location Objects in RADIUS and
              Diameter", RFC 5580, DOI 10.17487/RFC5580, August 2009,
              <http://www.rfc-editor.org/info/rfc5580>.

Top      ToC       Page 30 
   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <http://www.rfc-editor.org/info/rfc5891>.

   [RFC6614]  Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
              "Transport Layer Security (TLS) Encryption for RADIUS",
              RFC 6614, DOI 10.17487/RFC6614, May 2012,
              <http://www.rfc-editor.org/info/rfc6614>.

   [RFC7360]  DeKok, A., "Datagram Transport Layer Security (DTLS) as a
              Transport Layer for RADIUS", RFC 7360,
              DOI 10.17487/RFC7360, September 2014,
              <http://www.rfc-editor.org/info/rfc7360>.

   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
              DOI 10.17487/RFC7542, May 2015,
              <http://www.rfc-editor.org/info/rfc7542>.

8.2.  Informative References

   [RFC4017]  Stanley, D., Walker, J., and B. Aboba, "Extensible
              Authentication Protocol (EAP) Method Requirements for
              Wireless LANs", RFC 4017, DOI 10.17487/RFC4017, March
              2005, <http://www.rfc-editor.org/info/rfc4017>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <http://www.rfc-editor.org/info/rfc6335>.

   [RFC6733]  Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
              Ed., "Diameter Base Protocol", RFC 6733,
              DOI 10.17487/RFC6733, October 2012,
              <http://www.rfc-editor.org/info/rfc6733>.

   [RFC7299]  Housley, R., "Object Identifier Registry for the PKIX
              Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
              <http://www.rfc-editor.org/info/rfc7299>.

   [RFC7593]  Wierenga, K., Winter, S., and T. Wolniewicz, "The eduroam
              Architecture for Network Roaming", RFC 7593,
              DOI 10.17487/RFC7593, September 2015,
              <http://www.rfc-editor.org/info/rfc7593>.

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Appendix A.  ASN.1 Syntax of NAIRealm

PKIXNaiRealm08 {iso(1) identified-organization(3) dod(6)
     internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
     id-mod-nai-realm-08(85) }

 DEFINITIONS EXPLICIT TAGS ::=

 BEGIN

 -- EXPORTS ALL --

 IMPORTS

    id-pkix
    FROM PKIX1Explicit-2009
        {iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-explicit-02(51)}
           -- from RFCs 5280 and 5912

    OTHER-NAME
    FROM PKIX1Implicit-2009
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59)}
             -- from RFCs 5280 and 5912
 ;


 -- Service Name Object Identifier

 id-on   OBJECT IDENTIFIER ::= { id-pkix 8 }

 id-on-naiRealm OBJECT IDENTIFIER ::= { id-on 8 }

 -- Service Name

 naiRealm OTHER-NAME ::= { NAIRealm IDENTIFIED BY { id-on-naiRealm }}

 ub-naiRealm-length INTEGER ::= 255

 NAIRealm ::= UTF8String (SIZE (1..ub-naiRealm-length))

 END

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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
   AirSpayce Pty Ltd
   9 Bulbul Place
   Currumbin Waters  QLD 4223
   Australia

   Phone: +61 7 5598 7474
   Email: mikem@airspayce.com
   URI:   http://www.airspayce.com