RADIUS Extensions Working Group S. Winter Internet-Draft RESTENA Intended status: Experimental M. McCauley Expires:September 6, 2010January 10, 2012 OSCMarch 05, 2010July 09, 2011 NAI-based Dynamic Peer Discovery forRADIUS over TLSRADIUS/TLS andDTLS draft-ietf-radext-dynamic-discovery-02RADIUS/DTLS draft-ietf-radext-dynamic-discovery-03 Abstract This document specifies a means to find authoritativeAAARADIUS servers for a givenNAIrealm. It can be used in conjunction withRADIUS over TLSRADIUS/TLS andRADIUS over DTLS.RADIUS/DTLS. Status of This Memo This Internet-Draft is submittedto IETFin 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.(IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts.Drafts is at http://datatracker.ietf.org/drafts/current/. 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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 . . . . . . . . . . . . . . . . . . . .78 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8 5. Normative References . . . . . . . . . . . . . . . . . . . . . 8 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 aAAARADIUS server receives a request with aNAIrealm for which no homeAAARADIUS 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 exampleAccess-Challenges, Access-Accepts, Accounting Messages or Change-of-Authorisation messagesAccess-Challenges and Access-Accepts use thepreviously- establishedpreviously-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 definestwothree S-NAPTR servicetag: a general-purpose tag "nai-roaming"tags: "aaa+auth", "aaa+acct" anda special-purpose tag "eduroam" for the eduroam roaming consortium."aaa+dynauth". This specification defines two S-NAPTR protocol tags: "radius.tls" forRADIUS over TLSRADIUS/TLS [I-D.ietf-radext-radsec] and "radius.dtls" forRADIUS over DTLSRADIUS/DTLS [I-D.dekok-radext-dtls]. 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""aaa+auth:radius.tls" "" _radiustls._tcp.foobar.example.com._radiustls._tcp.example.com._radiustls._tcp.foobar.example.com. IN SRV 0 10 2083 radsec.example.com. b. The consortium "foo" provides roaming services for its members only. The realms used are of the form enterprise-name.example. The consortium operates a special purpose DNS server for the (private) TLD "example" which all 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""aaa+auth:radius.dtls" "" "very.bad.example" c.theThe 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""x-eduroam" for authentication purposes and eduroam AAA servers use this tag to look up other eduroam servers. An eduroam participant example.org which also providesgeneral-purposegeneral- purpose AAA on a different server uses the general"nai-roaming""aaa+auth" tag: example.org. IN NAPTR 50 50 "s""eduroam:radius.tls""x-eduroam:radius.tls" "" _radiustls._tcp.eduroam.example.org. example.org. IN NAPTR 50 50 "s""nai-roaming:radius.tls""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. 2.3. Realm to AAA server resolution algorithm Input I to the algorithm isathe RADIUS User-Nameinattribute with content of the formof"user@realm"; the literal @ sign being the separator between aNAI as definedlocal user identifier within a realm and its realm. The use of multiple literal @ signs in[RFC4282] as extracted from thea User-Nameattribute in an Access-Request.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. Output O of the algorithm is a set of hostname:port and an associatedorder/preference;order/ preference; the set can be empty. Note well: The attribute User-Namedoes not necessarilyis defined to containwell- formed NAIs andUTF-8 text. In practice, the content may or may noteven contain well-formed UTF-8 strings.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-formedNAIsrealms mapping to DNS domain names in UTF-8 encoding. The result of all other possible contents ofUser- NameUser-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 UTF-8 realms which fail the conversion rules as per [RFC5891] The algorithm to determine theAAARADIUS server to contact is as follows: 1. Determine P = (position offirstlast "@" 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. The name resolution library may need to convert R to a different respresentation, depending on the resolution backend used. 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)."aaa+auth", "aaa+acct", "aaa+dynauth" as appropriate. 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 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". The name resolution library on the RADIUS client uses DNS for name resolution. If DNS contains the following records: xn--tu-mnchen-t9a.example. IN NAPTR 50 50 "s""nai- roaming:radius.tls""aaa+ auth: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 2. R = "tu-m[U+00FC]nchen.example" 3. NOOP 4. [name resolution library converts R to xn--tu-mnchen- t9a.example] Query result: ( 50 50 "s""nai-roaming:radius.tls""aaa+auth: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""aaa+auth:radius.tls" ""_radiustls._tcp.xn--tu-mnchen-t9a.example._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 withoutsecurity,DNSSEC security extensions, 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 intendedtarget (TLS/DTLS: ceritifcatetarget: certificate verification orTLS 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 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.TLS-PSK keys. 4. IANA Considerations This document requests IANA registration of the following S-NAPTRparameters:parameter: o Application Service Tags *nai-roamingaaa+auth *eduroamaaa+acct * aaa+dynauth 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,[RFC5891] Klensin, J.,and P. Eronen, "The Network Access Identifier","Internationalized Domain Names in Applications (IDNA): Protocol", RFC4282, December 2005.5891, August 2010. [I-D.dekok-radext-dtls] DeKok, A., "DTLS as a Transport Layer for RADIUS",draft-dekok-radext-dtls-01draft-dekok-radext-dtls-03 (work in progress),June 2009.July 2010. [I-D.ietf-radext-radsec] Winter, S., McCauley, M., Venaas, S., and K. Wierenga, "TLS encryption forRADIUS over TCP", draft-ietf-radext-radsec-06RADIUS", draft-ietf-radext-radsec-09 (work in progress),March 2010.July 2011. 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.