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rfc:rfc7585

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.

Winter & McCauley Experimental [Page 1] RFC 7585 RADIUS Peer Discovery October 2015

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

Winter & McCauley Experimental [Page 2] RFC 7585 RADIUS Peer Discovery October 2015

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.

Winter & McCauley Experimental [Page 3] RFC 7585 RADIUS Peer Discovery October 2015

                                  +-------+
                                  |       |
                                  |   .   |
                                  |       |
                                  +---+---+
                                    / | \
                  +----------------/  |  \---------------------+
                  |                   |                        |
                  |                   |                        |
                  |                   |                        |
               +--+---+            +--+--+                +----+---+
               |      |            |     |                |        |
               | .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:

Winter & McCauley Experimental [Page 4] RFC 7585 RADIUS Peer Discovery October 2015

  1. ——–

/ \

  1. ——– ————

/ \

                       |    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].

Winter & McCauley Experimental [Page 5] RFC 7585 RADIUS Peer Discovery October 2015

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

Winter & McCauley Experimental [Page 6] RFC 7585 RADIUS Peer Discovery October 2015

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".

Winter & McCauley Experimental [Page 7] RFC 7585 RADIUS Peer Discovery October 2015

    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.

Winter & McCauley Experimental [Page 8] RFC 7585 RADIUS Peer Discovery October 2015

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

Winter & McCauley Experimental [Page 9] RFC 7585 RADIUS Peer Discovery October 2015

 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

Winter & McCauley Experimental [Page 10] RFC 7585 RADIUS Peer Discovery October 2015

 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.

Winter & McCauley Experimental [Page 11] RFC 7585 RADIUS Peer Discovery October 2015

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.

Winter & McCauley Experimental [Page 12] RFC 7585 RADIUS Peer Discovery October 2015

 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.

Winter & McCauley Experimental [Page 13] RFC 7585 RADIUS Peer Discovery October 2015

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].

Winter & McCauley Experimental [Page 14] RFC 7585 RADIUS Peer Discovery October 2015

    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.

Winter & McCauley Experimental [Page 15] RFC 7585 RADIUS Peer Discovery October 2015

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.

Winter & McCauley Experimental [Page 16] RFC 7585 RADIUS Peer Discovery October 2015

 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

Winter & McCauley Experimental [Page 17] RFC 7585 RADIUS Peer Discovery October 2015

 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.

Winter & McCauley Experimental [Page 18] RFC 7585 RADIUS Peer Discovery October 2015

 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).

Winter & McCauley Experimental [Page 19] RFC 7585 RADIUS Peer Discovery October 2015

 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

Winter & McCauley Experimental [Page 20] RFC 7585 RADIUS Peer Discovery October 2015

 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.

Winter & McCauley Experimental [Page 21] RFC 7585 RADIUS Peer Discovery October 2015

    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.

Winter & McCauley Experimental [Page 22] RFC 7585 RADIUS Peer Discovery October 2015

 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)

Winter & McCauley Experimental [Page 23] RFC 7585 RADIUS Peer Discovery October 2015

 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.

Winter & McCauley Experimental [Page 24] RFC 7585 RADIUS Peer Discovery October 2015

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.).

Winter & McCauley Experimental [Page 25] RFC 7585 RADIUS Peer Discovery October 2015

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

Winter & McCauley Experimental [Page 26] RFC 7585 RADIUS Peer Discovery October 2015

    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

Winter & McCauley Experimental [Page 27] RFC 7585 RADIUS Peer Discovery October 2015

 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.

Winter & McCauley Experimental [Page 28] RFC 7585 RADIUS Peer Discovery October 2015

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>.

Winter & McCauley Experimental [Page 29] RFC 7585 RADIUS Peer Discovery October 2015

 [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>.

Winter & McCauley Experimental [Page 30] RFC 7585 RADIUS Peer Discovery October 2015

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

Winter & McCauley Experimental [Page 31] RFC 7585 RADIUS Peer Discovery October 2015

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

Winter & McCauley Experimental [Page 32]

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