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
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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
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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].
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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.
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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
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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
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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
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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
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(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.
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