RADIUS Extensions Working Group S. Winter
Internet-Draft RESTENA
Intended status: Experimental M. McCauley
Expires: January 14, 2010 OSC
July 13, 2009
NAI-based Dynamic Peer Discovery for RADIUS over TLS and DTLS
draft-ietf-radext-dynamic-discovery-01
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Abstract
This document specifies a means to find authoritative AAA servers for
a given NAI realm as defined in [RFC4282]. It can be used in
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conjunction with RADIUS over TLS and RADIUS over DTLS.
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 . . . . . . . . . . . . . . . . . . . . . . 7
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
RadSec node: a RadSec client or server
RadSec Client: a RadSec instance which initiates a new connection.
RadSec Server: a RadSec instance which listens on a RadSec port and
accepts new connections
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 RadSec servers can be either NAPTR records or SRV
records. When both are defined, the resolution algorithm prefers
NAPTR results (see section Section 2.3 below). The NAPTR service
field used is "AAAS+RADSECT". The SRV prefix used is "_radsec._tcp".
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
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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" "AAAS+RADSECT" ""
_radsec._tcp.foobar.example.com.
_radsec._tcp.example.com. IN SRV 0 10 2083
radsec.example.com.
b. Consortium "foo" provides roaming services for banks. The realms
used are of the form enterprise-name.foobankroam. The consortium
operates a special purpose DNS server for the (private) TLD
"foobankroam" which all AAA servers use to resolve realm names.
"Rupt, Inc." is part of the consortium. On the consortium's DNS
server, realm bank-rupt.foobankroam might have the following DNS
entries:
bank-rupt.foobankroam IN NAPTR 50 50 "a" "AAAS+RADSECT" ""
"triple-a.bank-rupt.com"
_radsec._tcp.bank-rupt.foobankroam IN SRV 0 10 2083 triple-a-
backup.bank-rupt.com"
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 labels
prefixed with "eduroam." and eduroam AAA servers use these labels
to look up servers. An eduroam participant which also provides
general-purpose AAA on a different server might have the
following DNS entries:
eduroam.restena.lu. IN NAPTR 50 50 "a" "AAAS+RADSECT" "" aaa-
eduroam.restena.lu
restena.lu. IN NAPTR 50 50 "a" "AAAS+RADSECT" "" aaa-
default.restena.lu
_radsec._tcp.eduroam.restena.lu. IN SRV 0 10 2083 aaa-
eduroam.restena.lu.
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_radsec._tcp.restena.lu. IN SRV 0 10 2083 aaa-
default.restena.lu.
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 assoiciated 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 service "AAAS+RADSECT" for R
5. If name resolution returns with error, O = { }. Terminate.
6. If no result, continue at step 9.
7. Evaluate NAPTR result, 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 "_radsec._tcp.")
10. Using the host's name resolution library, perform SRV lookup
with R' as label.
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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" "AAAS+RADSECT" ""
_radsec._tcp.xn--tu-mnchen-t9a.example.
_radsec._tcp.xn--tu-mnchen-t9a.example. IN SRV 0 10 2083
radsec.xn--tu-mnchen-t9a.example.
_radsec._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. Query result: ( 0 10 2083 radsec.xn--tu-mnchen-t9a.example. ; 0
20 2083 backup.xn--tu-mnchen-t9a.example. )
5. NOOP
6. NOOP
7. O = {(10,radsec.xn--tu-mnchen-t9a.example.:2083),(20,backup.xn--
tu-mnchen-t9a.example.:2083)}
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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
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 contains no actions for IANA. Maybe. Not sure about
the labels "AAAS+RADSECT" and "_radsec._tcp.".
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5. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
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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