RADIUS Extensions Working Group S. Winter
Internet-Draft RESTENA
Intended status: Experimental M. McCauley
Expires: September 05, 2014 OSC
March 04, 2014

NAI-based Dynamic Peer Discovery for RADIUS/TLS and RADIUS/DTLS
draft-ietf-radext-dynamic-discovery-11

Abstract

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

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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on September 05, 2014.

Copyright Notice

Copyright (c) 2014 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

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

Where more than one administrative entity collaborates for RADIUS authentication of their respective customers (a "roaming consortium"), the Network Access Identifier (NAI) [I-D.ietf-radext-nai] is the suggested way of differentiating users between those entities; the part of a username to the right of the @ delimiter in a 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, 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 synchronising the changed data across all branches.

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", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", 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

[RFC4282] and its designated successor [I-D.ietf-radext-nai] define the terms NAI, realm, consortium. RFC Editor Note: please remove the RFC4282 reference here and in the Normative References section prior to publication and replace the draft NAI spec with its then-allocated RFC number.

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 NAPTR records in DNS domains which match NAI realms) is not of very experimental nature.

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

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

2.1.1.1. Registration of Application Service and Protocol 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 Authorisation, i.e.      |
|                 | traffic as defined in [RFC5176]         |
+--------------- --+-----------------------------------------+

Figure 1: List of Service Tags

This specification defines three S-NAPTR service 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

This specification defines two S-NAPTR 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 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 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.

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, 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.

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.

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 dynamic discover algorithm does not have provisions to establish 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-PKS ciphersuites cannot be used for 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 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.

2.1.1.3.1. Mandatory-to-implement mechanism: Trust Roots + NAIRealm

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 to 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.

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 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 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 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 member.

The shortcomings in server identification can be partially mitigated by using the RADIUS infrastructure only with authentication payloads which provide mutual authentication and credential protection (i.e. 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; 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.

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 Authorisation

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


+-----------------+-----------------------------------------+
| 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 3: List of SRV Labels

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

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 2.1.1.2 and Section 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 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.

Examples:

  1. 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.
    • _radiustls._tcp.foobar.example.com. IN SRV 0 10 2083 radsec.example.com.

  2. 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" "" roamserv.company.example

  3. 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" tag:
    • example.org. IN NAPTR 50 50 "s" "x-eduroam:radius.tls" "" _radiustls._tcp.eduroam.example.org.
    • example.org. IN NAPTR 50 50 "s" "aaa+auth:radius.tls" "" _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.

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 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 realm.

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]. [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.

  • id-on-nai OBJECT IDENTIFIER ::= { id-on XXX }
  • NAIRealm ::= UTF8String (SIZE (1..MAX))

The NAIRealm, if present, MUST contain an NAI realm as defined in

The comparison of a 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 matches the NAI realm, the server is considered authorized; if none matches, 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.


+---------------------+-------------------+-----------------------+
| 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 4: Examples for NAI realm vs. certificate matching

Examples:

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 new AAA transactions and per service (i.e. distinct discovery is needed for Authentication, Accounting, and Dynamic Authorization) 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

  1. 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.
  2. the first accounting ticket for a user session
  3. 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 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 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" 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 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 ([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; 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.
  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 terminate.
  7. Extract NAPTR records with service tag "aaa+auth", "aaa+acct", "aaa+dynauth" as appropriate. Keep note of the protocol tag and 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; 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" 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).
  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 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 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

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.
  • 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" "" _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.
  6. Result:
    • (TTL = 47) 50 50 "s" "aaa+auth:radius.tls" "" _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" "" _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, 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)
  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 being use of NAPTR vs. SRV; how to determine the authorization status of a contacted server for a given realm; which trust anchors to consider trustworthy for the RADIUS conversation setup.

Random parties which 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, backoffice compensations, procedures, ...).

A roaming consortium's roaming agreement must include a profile of which choice points of this document to use. So 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.

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 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 2.1.1.3 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 responses).

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 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.).

6. 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 consequence,

  • 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.
  • 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.
  • 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.
  • 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,

  • 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 exist reasons why clearinghouses might still be used. One reason to keep a clearinghouse is to act as a gateway for multiple backends in a company; another reason may be a requirement to sanitise RADIUS datagrams (filter attributes, tag requests with new attributes, ... ).
  • 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 which are only deployed for pure request routing reasons. This reduces the number of entities which can inspect the RADIUS traffic.
  • 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.
  • 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 which do not make use of this algorithm, and possibly using 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 authentications, while with dynamic discovery, many entities learn only very little about recently authenticated realms.

7. IANA Considerations

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

  • S-NAPTR Application Service Tags registry
    • aaa+auth
    • aaa+acct
    • aaa+dynauth

  • S-NAPTR Application Protocol Tags registry
    • radius.tls
    • radius.dtls

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

  • Service Name: radiustls; radiusdtls
  • Transport Protocols: TCP, UDP
  • 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 Editor Note: please insert the RFC number of this document. The protocol does not use broadcast, multicast or anycast communication.

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

  • _tcp
  • _udp

This document requires that a number of Object Identifiers be assigned. They re now under the control of IANA following [I-D.housley-pkix-oids].

IANA is requested to assign the following identifiers:

  • TBD99 is to be assigned from the "SMI Security for PKIX Module Identifier Registry". The suggested description is id-mod-nai-realm-08.
  • TBD98 is to be assigned from the "SMI Security for PKIX Other Name Forms Registry." The suggested description is id-on-nai.

RFC Editor Note: please replace the occurences of TBD98 and TBD99 in Appendix A of the document with the actually assigned numbers.

8. References

8.1. 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.
[RFC4282] Aboba, B., Beadles, M., Arkko, J. and P. Eronen, "The Network Access Identifier", RFC 4282, December 2005.
[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.
[I-D.ietf-radext-dtls] DeKok, A., "DTLS as a Transport Layer for RADIUS", Internet-Draft draft-ietf-radext-dtls-04, April 2013.
[RFC6614] Winter, S., McCauley, M., Venaas, S. and K. Wierenga, "Transport Layer Security (TLS) Encryption for RADIUS", RFC 6614, May 2012.
[I-D.ietf-radext-nai] DeKok, A., "The Network Access Identifier", Internet-Draft draft-ietf-radext-nai-03, May 2013.

8.2. Informative References

[RFC4017] Stanley, D., Walker, J. and B. Aboba, "Extensible Authentication Protocol (EAP) Method Requirements for Wireless LANs", RFC 4017, March 2005.
[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, August 2011.
[RFC6733] Fajardo, V., Arkko, J., Loughney, J. and G. Zorn, "Diameter Base Protocol", RFC 6733, October 2012.

Appendix A. 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 (TBD99) }

 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 RFC 5280, RFC 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 RFC 5280, RFC 5912
 ;


 -- Service Name Object Identifier

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

 id-on-nai OBJECT IDENTIFIER ::= { id-on TBD98 }

 -- Service Name

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

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

 END



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 Open Systems Consultants 9 Bulbul Place Currumbin Waters, QLD 4223 AUSTRALIA Phone: +61 7 5598 7474 Fax: +61 7 5598 7070 EMail: mikem@open.com.au URI: http://www.open.com.au.