RADIUS Extensions Working Group                                S. Winter
Internet-Draft                                                   RESTENA
Intended status: Experimental                                M. McCauley
Expires: April 19, 2014                                              OSC
                                                        October 16, 2013

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


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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 19, 2014.

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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  DNS RR definition . . . . . . . . . . . . . . . . . . . .   3
       2.1.1.  S-NAPTR . . . . . . . . . . . . . . . . . . . . . . .   3
       2.1.2.  SRV . . . . . . . . . . . . . . . . . . . . . . . . .   8
       2.1.3.  Remarks . . . . . . . . . . . . . . . . . . . . . . .   8
     2.2.  Definition of the X.509 certificate property
           SubjectAltName:otherName:NAIRealm . . . . . . . . . . . .  10
   3.  DNS-based NAPTR/SRV Peer Discovery  . . . . . . . . . . . . .  11
     3.1.  Applicability . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Configuration Variables . . . . . . . . . . . . . . . . .  11
     3.3.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     3.4.  Realm to RADIUS server resolution algorithm . . . . . . .  12
       3.4.1.  Input . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.4.2.  Output  . . . . . . . . . . . . . . . . . . . . . . .  13
       3.4.3.  Algorithm . . . . . . . . . . . . . . . . . . . . . .  13
       3.4.4.  Validity of results . . . . . . . . . . . . . . . . .  15
       3.4.5.  Delay considerations  . . . . . . . . . . . . . . . .  16
       3.4.6.  Example . . . . . . . . . . . . . . . . . . . . . . .  16
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  20
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  22
   Appendix A.  Appendix A: ASN.1 Syntax of NAIRealm . . . . . . . .  23

1.  Introduction

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

   Where 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, each with their own forwarding
   servers, and who add or change their realm lists at their own
   discretion, there is additional complexity in synchronising the
   changed data across all branches.

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   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.

   This document also specifies various approaches for verifying that
   server information which was retrieved from DNS was from an
   authorised party; e.g. an organisation which 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",
   and "OPTIONAL" in this document are to be interpreted as described in
   RFC 2119.  [RFC2119]

1.2.  Terminology

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

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

   RADIUS/TLS node: a RADIUS/TLS client or server

2.  Definitions

2.1.  DNS RR definition

   DNS definitions of RADIUS/TLS servers can be either S-NAPTR records
   (see [RFC3958]) or SRV records.  When both are defined, the
   resolution algorithm prefers S-NAPTR results (see Section 3.4 below).

2.1.1.  S-NAPTR  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  |

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   |                 | defined in [RFC2865]                    |
   | - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
   | aaa+acct        | RADIUS Accounting, i.e. traffic as      |
   |                 | defined in [RFC2866]                    |
   | - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
   | aaa+dynauth     | RADIUS Dynamic Authorisation, i.e.      |
   |                 | traffic as defined in [RFC5176]         |
   +--------------- --+-----------------------------------------+

                      Figure 1: List of Service Tags

   This specification defines two S-NAPTR protocol tags:

   | Protocol Tag    | Use                                     |
   | radius.tls      | RADIUS transported over TLS as defined  |
   |                 | in [RFC6614]                            |
   | - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
   | radius.dtls     | RADIUS transported over DTLS as defined |
   |                 | in [I-D.ietf-radext-dtls]               |

                      Figure 2: List of Protocol Tags

   Note well:

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

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

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

   RADIUS is a time-critical protocol; RADIUS clients which 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.

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   As a general rule, only error conditions which generate an immediate
   response from the other end are eligible for a retry of a discovered
   target.  Any error condition involving time-outs, 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 or not.  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.

   If the TLS session setup to a discovered target does not succeed,
   that target (as identified by 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 has
   expired or until the entity which executes the algorithm changes its
   TLS context to either send a new client certificate or expect a
   different server certificate.  Server Identification and Handshake

   After the algorithm in this document has been executed, a RADIUS/TLS
   session as per [RFC6614] is established.  Since the algorithm does
   not allow to derive confidential keying material between the RADIUS/
   TLS client (i.e. the server which executes the discovery algorithm)
   and the RADIUS/TLS server which was discovered, TLS-PSK ciphersuites
   can not be used for the subsequent TLS handshake in the RADIUS/TLS
   conversation.  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 which allows to verify whether the contacted host
   is authoritative for a NAI realm or not.  It also gives one example
   of another mechanism which is currently in wide-spread deployment,
   and one possible approach based on DNSSEC which is yet unimplemented.  Mandatory-to-implement mechanism: Trust Roots + NAIRealm

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

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

   Step 2 is: compare the value of 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
   contacted RADIUS/TLS server's X.509 certificate property
   "subjectAlternativeName:otherName:NAIRealm" as defined in
   Section 2.2.  The comparison is a byte-by-byte comparison, except for
   dot-separated parts of the value whose content is a single "*"
   character; such labels match all strings in the same part of the NAI
   realm.  If at least one of the sAN:otherName:NAIRealm values matches
   the NAI realm, the server is considered authorized; if none matches,
   the server is considered unauthorized.


   | NAI realm           | NAIRealm          | MATCH?                |
   | foo.example         | foo.example       | YES                   |
   | foo.example         | *.example         | YES                   |
   | bar.foo.example     | *.example         | NO                    |
   | 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 3: Examples for NAI realm vs. certificate matching  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 wellformedness and validity
   as per [RFC5280] and whether it was issued from a root certificate
   which is deemed trustworthy by the RADIUS/TLS client.

   Step 2 is: compare the values of the contacted RADIUS/TLS server's
   X.509 certificate's extensions of type "Policy OID" to a list of

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   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 which 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

   It should be noted that these shortcomings can be mitigated by using
   the RADIUS infrastructure only with authentication payloads which
   provide mutual authentication; that way, the final EAP server that
   was reached can be validated by the EAP peer, and any improper
   redirections to a different server will be detected.  Other mechanism: DNSSEC / DANE

   Where DNSSEC is used, the results of the algorithm can be trusted;
   i.e. the entity which 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 that
   the server is also authorized (i.e. a recognised member of the
   roaming consortium).

   The authorization can be sketched using DNSSEC+DANE as follows: if
   DANE/TLSA records of all authorized servers are put into a DNSSEC
   zone with a common, consortium-specific branch of the DNS tree, then
   the entity executing the algorithm can retrieve TLSA RRs for the
   label "realm.commonroot" and verify that the presented server
   certificate during the RADIUS/TLS handshake matches the information
   in the TLSA record.


      Realm = "example.com"

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

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

      result of discovery algorithm for realm "example.com" =

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

   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.

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]                            |
   | - - - - - - - - | - - - - - - - - - - - - - - - - - - - - |
   | _radiustls._udp | RADIUS transported over DTLS as defined |
   |                 | in [I-D.ietf-radext-dtls]               |

                       Figure 4: 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 prioritised
   list.  [RFC2782] does not specify rules regarding "Definition of
   Conditions for Retry/Failure", nor "Server Identification and
   Handshake".  This specification defines that the rules for these two
   topics as defined in Section and Section 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.  Remarks

   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

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   However, arbitrary other labels or service tags may be used if, for
   example, a roaming consortium uses realm names which are not
   associated to DNS names or special-purpose consortia where a globally
   valid discovery is not a use case.  Such other labels require a
   consortium-wide agreement about the transformation from realm name to
   lookup label, and/or which service tag to use.


   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" ""

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

   b.  The consortium "foo" provides roaming services for its members
       only.  The realms used are of the form enterprise-name.example.
       The consortium operates a special purpose DNS server for the
       (private) TLD "example" which all RADIUS servers use to resolve
       realm names.  "Bad, Inc." is part of the consortium.  On the
       consortium's DNS server, realm bad.example might have the
       following DNS entries:

          bad.example IN NAPTR 50 50 "a" "aaa+auth:radius.dtls" ""

   c.  The eduroam consortium uses realms based on DNS, but provides its
       services to a closed community only.  However, a AAA domain
       participating in eduroam may also want to expose AAA services to
       other, general-purpose, applications (on the same or other 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 which also provides general-
       purpose AAA on a different server uses the general "aaa+auth"

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

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

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

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

2.2.  Definition of the X.509 certificate property

   This specification retrieves IP addresses and port numbers from the
   Domain Name System which are subsequently used to authenticate users
   via the RADIUS/TLS protocol.  Since the Domain Name System is not
   necessarily trustworthy (e.g. if DNSSEC is not deployed for the
   queried domain name), it is important to verify that the server which
   was contacted is authorized to service requests for the user which
   triggered the discovery process.

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

   Current subjectAltName fields do not semantically allow to express an
   NAI realm; 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 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 to lookup the SRV record for 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 do 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].

      id-on-nai OBJECT IDENTIFIER ::= { id-on XXX }

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      NAIRealm ::= UTF8String (SIZE (1..MAX))

   The NAIRealm, if present, MUST contain an NAI realm as defined in
   [I-D.ietf-radext-nai].  It MAY substitute labels on the leftmost dot-
   separated part 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 a *
   to indicate a common prefix inside the part, are not permitted.

   This subjectAltName MAY occur more than once in a certificate.

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

3.  DNS-based NAPTR/SRV Peer Discovery

3.1.  Applicability

   Dynamic server discovery as defined in this document is only
   applicable for AAA transactions where a RADIUS entity which 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.  RADIUS DynAuth server discovery

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
   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

      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

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3.3.  Terms

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

   Negative DNS response: a response which 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.

   DNS Error: Where the algorithm states "name resolution returns with
   an error", this shall mean that either the DNS request timed out, or
   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 its section "Usage Rules", final "else"

   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 being the
   separator between a local user identifier within a realm and its
   realm.  The use of multiple literal @ signs in a User-Name is
   strongly discouraged; but if present, the last @ sign is to be
   considered the separator.  All previous instances of the @ sign are
   to be considered part of the local user identifier.

   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

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   ([RFC5580], section 4.1).  Only Operator-Name values with the
   namespace "1" are supported by this algorithm - the input to the
   algorithm is the actual domain name, preceeded with an "@" (but
   without the "1" namespace identifier byte of that attribute).

   Note well: The attribute User-Name is defined to contain UTF-8 text.
   In practice, the content may or may not be UTF-8.  Even if UTF-8, it
   may or may not map to a domain name in the realm part.  Implementors
   MUST take possible conversion error paths into consideration when
   parsing incoming User-Name attributes.  This document describes
   server discovery only for well-formed realms mapping to DNS domain
   names in UTF-8 encoding.  The result of all other possible contents
   of User-Name is unspecified; this includes, but is not limited to:

      Usage of separators other than @

      Encoding of User-Name in local encodings

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

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

   For the last bullet point, "trailing dot", special precautions should
   be taken to avoid problems when resolving servers with the algorithm
   below: they may resolve to a 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; 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

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

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

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   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.

   6.   Using the host's name resolution library, perform a NAPTR query
        for R (see "Delay considerations" 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

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

   8.   If no records 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
        lookup 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; 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." or

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

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

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   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; order/preference; Effective TTL (
        all DNS TTLs that led to this result ) } for all hostnames).

   18.  Generate O-1 by resolving hostnames in O' into corresponding A
        and/or AAAA addresses: O-1 = (set of {IP address; port; 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 which
        triggered dynamic discovery was received on {IP address; port}.
        If yes, O-1 = { empty set }, O-2 = BACKOFF_TIME, log error,
        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, Terminate.

3.4.4.  Validity of results

   The dynamic discovery algorithm is used by servers which 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 this will
   either lead to a tight loop (if that DNS entry has topmost priority,
   the server would forward the request to itself, triggering dynamic
   discovery again in a perpetual loop), or lead to 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 recognise
   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: other reasons include the possiblity that a subsequent
   hop has a statically configured next-hop which leads to an earlier
   host in the loop; or the algorithm execution was executed with greedy
   result evaluation, and the own address was in a lower-priority branch
   of the result set which was not retrieved from DNS at all.

   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

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   conflicts with the possibility of changing device and network
   configurations on the receiving end.  Typically, TTL values for
   records in the name resolution system are used to indicate how long
   it is safe to rely on the results of the name resolution.  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 which was learned during
   discovering the server has not expired, subsequent new user sessions
   for the realm which corresponds to that open connection SHOULD re-use
   the existing connection and SHOULD NOT re-execute the dynamic
   discovery algorithm nor open a new connection.  To allow for a change
   of configuration, a RADIUS server SHOULD re-execute the dynamic
   discovery algorithm after the Effective TTL that is associated with
   this connection has expired.  The server MAY keep the session open
   during this re-assessment to avoid closure and immediate re-opening
   of the connection should the result not have 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 time-out of its name resolution
   algorithms.  The algorithm therefore control 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


      a user from the Technical University of Munich, Germany, has a
      RADIUS User-Name of "foobar@tu-m[U+00FC]nchen.example".

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      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.

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

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

   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" "" _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

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

      radsecserver.xn--tu-mnchen-t9a.example.  IN AAAA

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

      backupserver.xn--tu-mnchen-t9a.example.  IN A

   Then the algorithm executes as follows, with I =
   "foobar@tu-m[U+00FC]nchen.example", and no consortium name mangling
   in use:

   1.   P = 7

   2.   R = "tu-m[U+00FC]nchen.example"

   3.   NOOP

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   4.   name resolution library converts R to xn--tu-mnchen-t9a.example

   5.   TIMER starts.

   6.   Result:

           (TTL = 47) 50 50 "s" "aaa+auth:radius.tls" ""

           (TTL = 522) 50 50 "s" "fooservice:bar.dccp" ""

   7.   Result:

           (TTL = 47) 50 50 "s" "aaa+auth:radius.tls" ""

   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; 10; 60),

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

        } // minimum TTL is 47, up'ed 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; 10; 60),

           (; 2083; 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.

4.  Security Considerations

   The results from the execution of this algorithm are only trustworthy
   if each of the lookup steps by the name resolution library were
   cryptographically secured; i.e. if DNSSEC validation was turned on
   during the resolution AND all of the records were in a DNSSEC signed
   zone AND validation of all those records was successful.

   When using DNS without DNSSEC security extensions for at least one of
   the replies to NAPTR, SRV and A/AAAA requests as described in section
   Section 3, the result O 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 Section or equivalent
   MUST be used to verify authorization.

   The algorithm has a configurable completion time-out 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 which take a very long time to reply (e.g. due to a
   particularly byzantine tree structure, or artificial delays in

   To mitigate this DoS vector, implementations SHOULD consider rate-
   limiting either their amount of new executions of the dynamic
   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 to
   spent waiting for a result will influence the impact of a possible

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   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.  With the port for a TLS-based service open, the RADIUS
   server shares all the typical attack vectors for services based on
   TLS (such as HTTPS, SMTPS, ...).  Deployments of RADIUS/TLS with
   Dynamic Discovery should consider these attack vectors and take
   appropriate counter-measures (e.g. blacklisting known-bad IPs on a
   firewall, rate-limiting new connection attempts, etc.).

5.  Privacy Considerations

   The classic RADIUS operational model (known, pre-configured peers,
   shared secret security, 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

   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 which are
      cryptographically secured and do not use the attribute User-
      Password - such as certain EAP types.

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

   With RADIUS and dynamic discovery,

   o  Passive observers on the IP path cannot inspect any part of the
      RADIUS payload.  They can observe source and destination of the
      traffic flow, but can not easily use this information to create
      mobility profiles because the user who tries to authenticate is
      not identifiable due to the encrypted payload.

   o  Clearinghouses can be eliminated by RADIUS clients directly
      contacting the RADIUS home server, if this is desired.  The
      possibility of aggregation of user information in the
      clearinghouse thus does not manifest.  Note that despite the
      technical possibility of avoid clearinghouses, they may still
      remain in operation for other reasons.

   o  RADIUS clients which 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.

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

6.  IANA Considerations

   This document requests IANA registration of the following entries in
   existing registries:

   o  S-NAPTR Application Service Tags registry

      *  aaa+auth

      *  aaa+acct

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      *  aaa+dynauth

   o  S-NAPTR Application Protocol Tags registry

      *  radius.tls

      *  radius.dtls

   This document reserves the use of the "_radiustls" and "_radiusdtls"
   Service labels.

   This document requests the creation of a new IANA registry named
   "RADIUS/TLS SRV Protocol Registry" with the following initial

   o  _tcp

   o  _udp

   This specification allocates a X.509 certificate property "NAIRealm"
   as per section Section 2.2 above, see placeholders "XXX".  There is
   currently no IANA registry for the subjectAltName:otherName
   namespace.  The authority for this namespace appears to be the PKIX
   working group.  Before issuing the RFC, IANA should liaise with PKIX
   to ensure that a value for NAIRealm is issued; IANA should
   subsequently, prior to issuing the RFC, update the placeholders in
   said section.

7.  Normative References

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

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)", RFC
              2865, June 2000.

   [RFC2866]  Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

   [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
              Service Location Using SRV RRs and the Dynamic Delegation
              Discovery Service (DDDS)", RFC 3958, January 2005.

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   [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,
              January 2008.

   [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, May 2008.

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

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

              DeKok, A., "DTLS as a Transport Layer for RADIUS", draft-
              ietf-radext-dtls-05 (work in progress), April 2013.

   [RFC6614]  Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
              "Transport Layer Security (TLS) Encryption for RADIUS",
              RFC 6614, May 2012.

              DeKok, A., "The Network Access Identifier", draft-ietf-
              radext-nai-03 (work in progress), May 2013.

Appendix A.  Appendix A: ASN.1 Syntax of NAIRealm

      PKIXServiceNameSAN93 {iso(1) identified-organization(3) dod(6)
          internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
          id-mod-dns-srv-name-93(40) }



      -- EXPORTS ALL --


               FROM PKIX1Explicit88 { iso(1) identified-organization(3)
               dod(6) internet(1) security(5) mechanisms(5) pkix(7)

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               id-mod(0) id-pkix1-explicit(18) } ;
                -- from RFC 5280

      -- In the GeneralName definition using the 1993 ASN.1 syntax
      -- includes:


      -- Service Name Object Identifier

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

      id-on-nai OBJECT IDENTIFIER ::= { id-on XXX }

      -- Service Name

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

      NAIRealm ::= UTF8String (SIZE (1..MAX))


Authors' Addresses

   Stefan Winter
   Fondation RESTENA
   6, rue Richard Coudenhove-Kalergi
   Luxembourg  1359

   Phone: +352 424409 1
   Fax:   +352 422473
   EMail: stefan.winter@restena.lu
   URI:   http://www.restena.lu.

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   Mike McCauley
   Open Systems Consultants
   9 Bulbul Place
   Currumbin Waters  QLD 4223

   Phone: +61 7 5598 7474
   Fax:   +61 7 5598 7070
   EMail: mikem@open.com.au
   URI:   http://www.open.com.au.

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