RADIUS Extensions Working Group S. Winter Internet-Draft RESTENA Intended status: Experimental M. McCauley Expires: September 6, 2010 OSC March 05, 2010 NAI-based Dynamic Peer Discovery for RADIUS over TLS and DTLS draft-ietf-radext-dynamic-discovery-02 Abstract This document specifies a means to find authoritative AAA servers for a given NAI realm. It can be used in conjunction with RADIUS over TLS and RADIUS over DTLS. Status of This Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 6, 2010. Copyright Notice Copyright (c) 2010 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 Winter & McCauley Expires September 6, 2010 [Page 1] Internet-Draft RADIUS Peer Discovery March 2010 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 2. DNS-based NAPTR/SRV Peer Discovery . . . . . . . . . . . . . . 3 2.1. Applicability . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. DNS RR definition . . . . . . . . . . . . . . . . . . . . . 3 2.3. Realm to AAA server resolution algorithm . . . . . . . . . 5 3. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 5. Normative References . . . . . . . . . . . . . . . . . . . . . 8 Winter & McCauley Expires September 6, 2010 [Page 2] Internet-Draft RADIUS Peer Discovery March 2010 1. Introduction 1.1. Requirements Language In this document, several words are used to signify the requirements of the specification. The key words "MUST", "MUST NOT", "REQUIRED", "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 [I-D.ietf-radext-radsec] instance which initiates a new connection. RADIUS/TLS Server: a RADIUS/TLS [I-D.ietf-radext-radsec] instance which listens on a RADIUS/TLS port and accepts new connections RADIUS/TLS node: a RADIUS/TLS client or server 2. DNS-based NAPTR/SRV Peer Discovery 2.1. Applicability Dynamic server discovery as defined in this document is only applicable for AAA transactions where a AAA server receives a request with a NAI realm for which no home AAA server is known. I.e. where static server configuration does not contain a known home authentication server, or where the server configuration explicitly states that the realm destination is to be looked up dynamically. Furthermore, it is only applicable for new user sessions, i.e. for the initial Access-Request. Subsequent messages concerning this session, for example Access-Challenges, Access-Accepts, Accounting Messages or Change-of-Authorisation messages use the previously- established communication channel between client and server. 2.2. DNS RR definition DNS definitions of RADIUS/TLS servers can be either S-NAPTR records (see [RFC3958]) or SRV records. When both are defined, the resolution algorithm prefers S-NAPTR results (see section Section 2.3 below). This specification defines two S-NAPTR service tag: a general-purpose tag "nai-roaming" and a special-purpose tag "eduroam" for the eduroam roaming consortium. This specification defines two S-NAPTR protocol tags: "radius.tls" for RADIUS over TLS [I-D.ietf-radext-radsec] and "radius.dtls" for RADIUS over DTLS [I-D.dekok-radext-dtls]. Winter & McCauley Expires September 6, 2010 [Page 3] Internet-Draft RADIUS Peer Discovery March 2010 This specification defines the SRV prefix "_radiustls._tcp" for RADIUS over TLS [I-D.ietf-radext-radsec] and "_radiustls._udp" for RADIUS over DTLS [I-D.dekok-radext-dtls]. It is expected that in most cases, the label used for the records is the DNS representation (punycode) of the literal realm name for which the server is the AAA server. However, arbitrary other labels may be used if, for example, a roaming consortium uses realm names which are not associated to DNS names or special-purpose consortia where a globally valid discovery is not a use case. Such other labels require a consortium-wide agreement about the transformation from realm name to lookup label. Examples: a. A general-purpose AAA server for realm example.com might have DNS entries as follows: example.com. IN NAPTR 50 50 "s" "nai-roaming:radius.tls" "" _radiustls._tcp.foobar.example.com. _radiustls._tcp.example.com. IN SRV 0 10 2083 radsec.example.com. b. The consortium "foo" provides roaming services for its members only. The realms used are of the form enterprise-name.example. The consortium operates a special purpose DNS server for the (private) TLD "example" which all AAA servers use to resolve realm names. "Bad, Inc." is part of the consortium. On the consortium's DNS server, realm bad.example might have the following DNS entries: bad.example IN NAPTR 50 50 "a" "nai-roaming:radius.dtls" "" "very.bad.example" c. the eduroam consortium uses realms based on DNS, but provides its services to a closed community only. However, a AAA domain participating in eduroam may also want to expose AAA services to other, general-purpose, applications (on the same or other AAA servers). Due to that, the eduroam consortium uses the service tag "eduroam" and eduroam AAA servers use this tag to look up other eduroam servers. An eduroam participant example.org which also provides general-purpose AAA on a different server uses the general "nai-roaming" tag: example.org. IN NAPTR 50 50 "s" "eduroam:radius.tls" "" _radiustls._tcp.eduroam.example.org. Winter & McCauley Expires September 6, 2010 [Page 4] Internet-Draft RADIUS Peer Discovery March 2010 example.org. IN NAPTR 50 50 "s" "nai-roaming: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. 2.3. Realm to AAA server resolution algorithm Input I to the algorithm is a User-Name in the form of a NAI as defined in [RFC4282] as extracted from the User-Name attribute in an Access-Request. Output O of the algorithm is a set of hostname:port and an associated order/preference; the set can be empty. Note well: The attribute User-Name does not necessarily contain well- formed NAIs and may not even contain well-formed UTF-8 strings. This document describes server discovery only for well-formed NAIs in UTF-8 encoding. The result of all other possible contents of User- Name is unspecified; this includes, but is not limited to: Usage of separators other than @ Usage of multiple @ separators Encoding of User-Name in local encodings The algorithm to determine the AAA server to contact is as follows: 1. Determine P = (position of first "@" character) in I. 2. generate R = (substring from P+1 to end of I) 3. Optional: modify R according to agreed consortium procedures 4. Using the host's name resolution library, perform a NAPTR query for R. If no result, continue at step 9. If name resolution returns with error, O = { }. Terminate. 5. Extract NAPTR records with service tag "nai-roaming" (replace with other service tags where applicable). 6. If no result, continue at step 9. 7. Evaluate NAPTR result(s) for desired protocol tag, perform subsequent lookup steps until lookup yields one or more hostnames. O = (set of {Order/Preference, hostname:port} for Winter & McCauley Expires September 6, 2010 [Page 5] Internet-Draft RADIUS Peer Discovery March 2010 all lookup results). 8. Terminate. 9. Generate R' = (prefix R with "_radiustls._tcp." or "_radiustls._udp") 10. Using the host's name resolution library, perform SRV lookup with R' as label. 11. If name resolution returns with error, O = { }. Terminate. 12. If no result, O = {}; terminate. 13. Perform subsequent lookup steps until lookup yields one or more hostnames. O = (set of {Order/Preference, hostname} for all hostnames). Terminate. Example: Assume a user from the Technical University of Munich, Germany, has a RADIUS User-Name of "foobar@tu-m[U+00FC]nchen.example". If DNS contains the following records: xn--tu-mnchen-t9a.example. IN NAPTR 50 50 "s" "nai- roaming:radius.tls" "" _radiustls._tcp.xn--tu-mnchen-t9a.example. xn--tu-mnchen-t9a.example. IN NAPTR 50 50 "s" "fooservice: bar.dccp" "" _abc._def.xn--tu-mnchen-t9a.example. _radiustls._tcp.xn--tu-mnchen-t9a.example. IN SRV 0 10 2083 radsec.xn--tu-mnchen-t9a.example. _radiustls._tcp.xn--tu-mnchen-t9a.example. IN SRV 0 20 2083 backup.xn--tu-mnchen-t9a.example. radsec.xn--tu-mnchen-t9a.example. IN AAAA 2001:0DB8::202:44ff: fe0a:f704 radsec.xn--tu-mnchen-t9a.example. IN A 192.0.2.3 backup.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 Winter & McCauley Expires September 6, 2010 [Page 6] Internet-Draft RADIUS Peer Discovery March 2010 2. R = "tu-m[U+00FC]nchen.example" 3. NOOP 4. Query result: ( 50 50 "s" "nai-roaming:radius.tls" "" _radiustls._tcp.xn--tu-mnchen-t9a.example. ; 50 50 "s" "fooservice:bar.dccp" "" _abc._def.xn--tu-mnchen-t9a.example. ) 5. Result: 50 50 "s" "nai-roaming:radius.tls" "" _radiustls._tcp.xn--tu-mnchen-t9a.example. 6. NOOP 7. O = {(10,radsec.xn--tu-mnchen-t9a.example.:2083),(20,backup.xn-- tu-mnchen-t9a. example.:2083)} 8. Terminate. 9. (not executed) 10. (not executed) 11. (not executed) 12. (not executed) 13. (not executed) The implementation will then attempt to connect to two servers, with preference to radsec.xn--tu-mnchen-t9a.example.:2083, using either the AAAA or A addresses depending on the host configuration and its IP stack's capabilities. 3. Security Considerations When using DNS without security, the replies to NAPTR, SRV and A/AAAA requests as described in section Section 2 can not be trusted. RADIUS transports have an out-of-DNS-band means to verify that the discovery attempt led to the intended target (TLS/DTLS: ceritifcate verification or TLS shared secret ciphers; UDP/TCP: the RADIUS shared secret) and are safe from DNS-based redirection attacks. [Note: assuming here that a hypothetical RADIUS/UDP SRV discovery will NOT deliver the shared secret in the DNS response!] The discovery process is always susceptible to bidding down attacks if a realm has SRV records for RADIUS/UDP and/or RADIUS/TCP as well as for RADIUS/TLS and/or RADIUS/DTLS. While the SRV query will expose both transports, an attacker in the routing path might Winter & McCauley Expires September 6, 2010 [Page 7] Internet-Draft RADIUS Peer Discovery March 2010 suppress the subsequent A/AAAA results for the TLS or DTLS peer and trick the initiating peer into using the weakly protected UDP or TCP transports. The use of DNSSEC can not fully mitigate this attack, since it does not provide a means to detect packet suppression. The only way to disable such bidding down attacks is by intiating connections only to the peer(s) which match or exceed a configured minimum security level. All implementations SHOULD provide a means to configure the administratively desired minimum security level. 4. IANA Considerations This document requests IANA registration of the following S-NAPTR parameters: o Application Service Tags * nai-roaming * eduroam o Application Protocol Tags * radius.tls * radius.dtls 5. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3958] Daigle, L. and A. Newton, "Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS)", RFC 3958, January 2005. [RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The Network Access Identifier", RFC 4282, December 2005. [I-D.dekok-radext-dtls] DeKok, A., "DTLS as a Transport Layer for RADIUS", draft-dekok-radext-dtls-01 (work in progress), June 2009. [I-D.ietf-radext-radsec] Winter, S., McCauley, M., Venaas, S., and K. Wierenga, "TLS encryption for RADIUS over TCP", draft-ietf-radext-radsec-06 Winter & McCauley Expires September 6, 2010 [Page 8] Internet-Draft RADIUS Peer Discovery March 2010 (work in progress), March 2010. 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. Winter & McCauley Expires September 6, 2010 [Page 9]