RADIUS Extensions Working Group S. Winter
Internet-Draft RESTENA
Intended status: Experimental M. McCauley
Expires: April 27, 2009 OSC
S. Venaas
UNINETT
K. Wierenga
Cisco
October 24, 2008
TLS encryption for RADIUS over TCP (RadSec)
draft-ietf-radext-radsec-02
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Abstract
This document specifies security on the transport layer (TLS) for the
RADIUS protocol [RFC2865] when transmitted over TCP
[I-D.dekok-radext-tcp-transport]. This enables dynamic trust
relationships between RADIUS servers.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Normative: Transport Layer Security for RADIUS over TCP . . . 4
2.1. TCP port and packet types . . . . . . . . . . . . . . . . 4
2.2. Connection Setup . . . . . . . . . . . . . . . . . . . . . 4
2.3. RADIUS Datagrams . . . . . . . . . . . . . . . . . . . . . 5
3. Informative: Design Decisions . . . . . . . . . . . . . . . . 6
3.1. X.509 Certificate Considerations . . . . . . . . . . . . . 6
3.2. Ciphersuites and Compression Negotiation Considerations . 8
3.3. RADIUS Datagram Considerations . . . . . . . . . . . . . . 8
4. Diameter Compatibility . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Appendix A. DNS NAPTR Peer Discovery . . . . . . . . . . . . . . 12
Appendix B. Implementation Overview: Radiator . . . . . . . . . . 13
Appendix C. Implementation Overview: radsecproxy . . . . . . . . 14
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1. Introduction
The RADIUS protocol [RFC2865] is a widely deployed authentication and
authorisation protocol. The supplementary RADIUS Accounting
specification [RFC2866] also provides accounting mechanisms, thus
delivering a full AAA solution. However, RADIUS is experiencing
several shortcomings, such as its dependency on the unreliable
transport protocol UDP and the lack of security for large parts of
its packet payload. RADIUS security is based on the MD5 algorithm,
which has been proven to be insecure.
The main focus of RadSec is to provide a means to secure the
communication between RADIUS/TCP peers on the transport layer. The
most important use of RadSec lies in roaming environments where
RADIUS packets need to be transferred through different
administrative domains and untrusted, potentially hostile networks.
An example for a world-wide roaming environment that uses RadSec to
secure communication is "eduroam", see [eduroam].
There are multiple known attacks on the MD5 algorithm which is used
in RADIUS to provide integrity protection and a limited
confidentiality protection. RadSec wraps the entire RADIUS packet
payload into a TLS stream and thus mitigates the risk of attacks on
MD5.
Because of the static trust establishment between RADIUS peers (IP
address and shared secret) the only scalable way of creating a
massive deployment of RADIUS-servers under control by different
administrative entities is to introduce some form of a proxy chain to
route the access requests to their home server. This creates a lot
of overhead in terms of possible points of failure, longer
transmission times as well as middleboxes through which
authentication traffic flows. These middleboxes may learn privacy-
relevant data while forwarding requests. The new features in RadSec
obsolete the use of IP addresses and shared MD5 secrets to identify
other peers and thus allow the dynamic establishment of connections
to peers that are not previously configured, and thus makes it
possible to avoid intermediate aggregation proxies. The definition
of lookup mechanisms is out of scope of this document, but an
implementation of a DNS NAPTR lookup based mechanism exists and is
described as an example lookup mechanism in Appendix A.
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
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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. Normative: Transport Layer Security for RADIUS over TCP
2.1. TCP port and packet types
The default destination port number for RadSec is TCP/2083. There
are no separate ports for authentication, accounting and dynamic
authorisation changes. The source port is arbitrary.
2.2. Connection Setup
RadSec nodes
1. establish TCP connections as per [I-D.dekok-radext-tcp-transport]
2. negotiate TLS sessions according to [RFC5246] or its predecessor
TLS 1.1. The following restrictions apply:
* The authentication MUST be mutual, i.e. both the RadSec server
and the RadSec client authenticate each other.
* When using X.509 certificates, RadSec servers SHOULD indicate
their acceptable Certification Authorities as per section
7.4.4 of [RFC5246] (see Section 3.1 (1) )
* When using X.509 certificates, the TLS Extension "Trusted CA
Indication" from [RFC5246] or its TLS 1.1 predecessor SHOULD
be used to indicate trusted CAs for the client (see
Section 3.1 (2) )
* When using X.509 certificates, certificate validation is
performed as per [RFC5280] or its predecessor. The client MAY
perform additional checks to accomodate for different trust
models.
* The client MUST NOT negotiate cipher suites which only provide
integrity protection.
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* The cipher suite TLS_RSA_WITH_3DES_EDE_CBC_SHA MUST be
supported.
* The cipher suites TLS_RSA_WITH_AES_128_CBC_SHA and
TLS_RSA_WITH_RC4_128_SHA SHOULD be supported. (see Section 3.2
(1) )
3. start exchanging RADIUS datagrams. Note Section 3.3 (1) ). The
shared secret to compute the (obsolete) MD5 integrity checks and
attribute encryption MUST be "radsec" (see Section 3.3 (2) ).
2.3. RADIUS Datagrams
Authentication, Accounting and Authorization packets are sent
according to the following rules:
RadSec clients handle the following packet types from [RFC2865],
[RFC2866], [RFC5176] on the connection they initiated (see
Section 3.3 (3) and (4) ):
o send Access-Request
o send Accounting-Request
o send Status-Server
o send Disconnect-ACK
o send Disconnect-NAK
o send CoA-ACK
o send CoA-NAK
o receive Access-Challenge
o receive Access-Accept
o receive Access-Reject
o receive Accounting-Response
o receive Disconnect-Request
o receive CoA-Request
RadSec servers handle the following packet types from [RFC2865],
[RFC2866], [RFC5176] on the connections they serve to clients:
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o receive Access-Request
o receive Accounting-Request
o receive Status-Server
o receive Disconnect-ACK
o receive Disconnect-NAK
o receive CoA-ACK
o receive CoA-NAK
o send Access-Challenge
o send Access-Accept
o send Access-Reject
o send Accounting-Response
o send Disconnect-Request
o send CoA-Request
3. Informative: Design Decisions
This section explains the design decisions that led to the rules
defined in the previous section.
3.1. X.509 Certificate Considerations
(1) If a RadSec client is in possession of multiple certificates from
different CAs (i.e. is part of multiple roaming consortia) and
dynamic discovery is used, the discovery mechanism possibly does not
yield sufficient information to identify the consortium uniquely
(e.g. DNS discovery). Subsequently, the client may not know by
itself which client certificate to use for the TLS handshake. Then
it is necessary to for the server to signal which consortium it
belongs to, and which certificates it expects. If there is no risk
of confusing multiple roaming consortia, providing this information
in the handshake is not crucial.
(2) If a RadSec server is in possession of multiple certificates from
different CAs (i.e. is part of multiple roaming consortia), it will
need to select one of its certificates to present to the RadSec
client. If the client sends the Trusted CA Indication, this hint can
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make the server select the appropriate certificate and prevent a
handshake failure. Omitting this indication makes it impossible to
deterministically select the right certificate in this case. If
there is no risk of confusing multiple roaming consortia, providing
this indication in the handshake is not crucial.
(3) When using X.509 certificate validation, peer validation always
includes a check on whether the DNS name or the IP address of the
server that is contacted matches its certificate. When a RadSec peer
establishes a new connection (acts as a client) to another peer, the
following rules of precedence are used during validation:
o If the client expects a certain fully qualified domain name (FQDN)
and the presented certificate contains both at least one instance
of the subjectAltName field with type dNSName and a Common Name,
then the certificate is considered a match if any one of those
subjectAltName fields matches the expected FQDN. The Common Name
field is not evaluated in this case.
o If the client expects a certain fully qualified domain name (FQDN)
and the presented certificate does not contain any subjectAltName
field with type dNSName, then the certificate is considered a
match if the Common Name field matches the expected FQDN.
o If the client expects a certain IP address and the presented
certificate contains both at least one instance of the
subjectAltName field with type iPAddr and a Common Name, then the
certificate is considered a match if any one of those
subjectAltName fields matches the expected IP address. The Common
Name field is not evaluated in this case.
o If the client expects a certain IP address and the presented
certificate does not contain any subjectAltName field with type
iPAddr, then the certificate is considered a match if the Common
Name field matches the expected IP address.
(4) If dynamic peer resolution is used, the above verification steps
may not be sufficient to ensure that a connecting peer is authorised
to perform user authentications. In these cases, the trust fabric
cannot depend on peer authentication methods like DNSSEC to identify
RadSec nodes. The RadSec nodes also need to be properly authorised.
Operators of a RadSec infrastructure should define their own
authorisation trust model and apply this model to the certificates
after they have passed the standard validity checks above. Current
RadSec implementations offer varying degrees of versatility for
defining criteria which certificates to accept.
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3.2. Ciphersuites and Compression Negotiation Considerations
RadSec implementations need not necessarily support all TLS
ciphersuites listed in [RFC5246]. Not all TLS ciphersuites
are supported by available TLS tool kits and licenses may be required
in some cases. The existing implementations of RadSec use OpenSSL as
cryptographic backend, which supports all of the ciphersuites listed
in the rules in the normative section.
The TLS cphersuite TLS_RSA_WITH_3DES_EDE_CBC_SHA is mandatory-to-
implement according to [RFC5246] and thus has to be supported by
RadSec nodes.
The two other ciphersuites in the normative section
(TLS_RSA_WITH_RC4_128_SHA and TLS_RSA_WITH_AES_128_CBC_SHA) are
widely implemented in TLS toolkits and are considered good practice
to implement.
TLS also supports compression. Compression is an optional
feature. During the RadSec conversation the client and server may
negotiate compression, but must continue the conversation even if the
other peer rejects compression.
3.3. RADIUS Datagram Considerations
(1) After the TLS session is established, RADIUS packet payloads are
exchanged over the encrypted TLS tunnel. In plain RADIUS, the packet
size can be determined by evaluating the size of the datagram that
arrived. Due to the stream nature of TCP and TLS, this does not hold
true for RadSec packet exchange. Instead, packet boundaries of
RADIUS packets that arrive in the stream are calculated by evaluating
the packet's Length field. Special care needs to be taken on the
packet sender side that the value of the Length field is indeed
correct before sending it over the TLS tunnel, because incorrect
packet lengths can no longer be detected by a differing datagram
boundary.
(2) Within RADIUS [RFC2865], a shared secret is used for hiding
of attributes such as User-Password, as well as in computation of
the Response Authenticator. In RADIUS accounting [RFC2866], the
shared secret is used in computation of both the Request
Authenticator and the Response Authenticator. Since TLS provides
integrity protection and encryption sufficient to substitute for
RADIUS application-layer security, it is not necessary to configure a
RADIUS shared secret. The use of a fixed string for the obsolete
shared secret eliminates possible node misconfigurations.
(3) RADIUS [RFC2865] uses different UDP ports for authentication,
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accounting and dynamic authorisation changes. RadSec allocates a
single port for all RADIUS packet types. Nevertheless, in RadSec the
notion of a client which sends authentication requests and processes
replies associated with it's users' sessions and the notion of a
server which receives requests, processes them and sends the
appropriate replies is to be preserved. The normative rules about
acceptable packet types for clients and servers mirror the packet
flow behaviour from RADIUS.
(4) RADIUS [RFC2865] used negative ICMP responses to a newly
allocated UDP port to signal that a peer RADIUS server does not
support reception and processing of the packet types in [RFC5176].
These packet types are listed as to be received in RadSec
implementations. Note well: it is not required for an implementation
to actually process these packet types. It is sufficient that upon
receiving such a packet, an unconditional NAK is sent back to
indicate that the action is not supported.
4. Diameter Compatibility
Since RadSec is only a new transport profile for RADIUS,
compatibility of RadSec - Diameter [RFC3588] vs. RADIUS [RFC2865] -
Diameter [RFC3588] is identical. The considerations regarding
payload size in [I-D.dekok-radext-tcp-transport] apply.
5. Security Considerations
The computational resources to establish a TLS tunnel are
significantly higher than simply sending mostly unencrypted UDP
datagrams. Therefore, clients connecting to a RadSec node will more
easily create high load conditions and a malicious client might
create a Denial-of-Service attack more easily.
In the case of dynamic peer discovery, a RadSec node needs to be able
to accept connections from a large, not previously known, group of
hosts, possibly the whole internet. In this case, the server's
RadSec port can not be protected from unauthorised connection
attempts with measures on the network layer, i.e. access lists and
firewalls. This opens more attack vectors for Distributed Denial of
Service attacks, just like any other service that is supposed to
serve arbitrary clients (like for example web servers).
In the case of dynamic peer discovery, X.509 certificates are the
only proof of authorisation for a connecting RadSec nodes. Special
care needs to be taken that certificates get verified properly
according to the chosen trust model (particularly: consulting CRL
lists, checking critical extensions, checking subjectAltNames etc.)
to prevent unauthorised connections.
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Some TLS ciphersuites only provide integrity validation of their
payload, and provide no encryption. This specification forbids the
use of such ciphersuites. Since the RADIUS payload's shared secret
is fixed and well-known, failure to comply with this requirement will
expose the entire datagram payload in plain text, including User-
Password, to intermediate IP nodes.
6. IANA Considerations
This document has no actions for IANA. The TCP port 2083 was already
previously assigned by IANA for RadSec. No new RADIUS attributes or
packet codes are defined.
7. Acknowledgements
RadSec version 1 was first implemented by Open Systems Consultants,
Currumbin Waters, Australia, for their "Radiator" RADIUS server
product (see [radsec-whitepaper]).
Funding and input for the development of this Internet Draft was
provided by the European Commission co-funded project "GEANT2"
[geant2] and further feedback was provided by the TERENA Task Force
Mobility [terena].
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.
[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.
[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.
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[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.
[RFC5246] Dierks, T. and E. Rescorla, "The
Transport Layer Security (TLS)
Protocol Version 1.2", RFC 5246,
August 2008.
[I-D.dekok-radext-tcp-transport] DeKok, A., "RADIUS Over TCP",
draft-dekok-radext-tcp-transport-00
(work in progress), July 2008.
8.2. Informative References
[RFC3588] Calhoun, P., Loughney, J., Guttman,
E., Zorn, G., and J. Arkko,
"Diameter Base Protocol", RFC 3588,
September 2003.
[radsec-whitepaper] Open System Consultants, "RadSec -
a secure, reliable RADIUS
Protocol", May 2005, <http://
www.open.com.au/radiator/
radsec-whitepaper.pdf>.
[radiator-manual] Open System Consultants, "Radiator
Radius Server - Installation and
Reference Manual", 2006, <http://
www.open.com.au/radiator/ref.html>.
[radsecproxy-impl] Venaas, S., "radsecproxy Project
Homepage", 2007, <http://
software.uninett.no/radsecproxy/>.
[eduroam] Trans-European Research and
Education Networking Association,
"eduroam Homepage", 2007,
<http://www.eduroam.org/>.
[geant2] Delivery of Advanced Network
Technology to Europe, "European
Commission Information Society and
Media: GEANT2", 2008,
<http://www.geant2.net/>.
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[terena] TERENA, "Trans-European Research
and Education Networking
Association", 2008,
<http://www.terena.org/>.
Appendix A. DNS NAPTR Peer Discovery
The following text is paraphrased from the file goodies/dnsroam.cfg
in the Radiator distribution; further documentation of the <AuthBy
DNSROAM> feature in Radiator can be found at [radiator-manual]. It
describes an algorithm to retrieve the RadSec route information from
the global DNS using NAPTR and SRV records. The input of the
algorithm is the realm part of the user name.
The following algorithm is used to discover a target server from a
Realm using DNS:
1. Look for NAPTR records for the Realm. If found, continue at step
2, otherwise continue at step 4.
2. For each NAPTR found record, examine the Service field and use
that to determine the transport protocol and TLS requirements for
the server. The Service field starts with 'AAA' for insecure and
'AAAS' for TLS secured. The Service field contains '+RADSECS'
for RadSec over SCTP, '+RADSECT' for RadSec over TCP or '+RADIUS'
for RADIUS protocol over UDP. The most common Service field you
will see will be 'AAAS+RADSECT' for TLS secured RadSec over TCP.
3.
A. If the NAPTR has the 'S' flag, look for SRV records for the
name. For each SRV record found, note the Port number and
then look for A and AAAA records corresponding to the name in
the SRV record.
B. If the NAPTR has the 'A' flag, look for a A and AAAA records
for the name.
4. All A and AAAA records found are ordered according to their Order
and Preference fields. The most preferable server address is
used as the target server address, along with any other server
attributes discovered from DNS. If no SRV record was found for
the address, the DNSROAM configured Port is used. Algorithm
terminates.
5. Look for A and AAAA records on the literal realm name, preceded
by "_radsec._tcp.". For example, if the realm is 'example.com',
it looks for the record '_radsec._tcp.example.com'. If more than
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one result is returned, no ordering is assumed. Algorithm
terminates.
For example, if the User-Name realm was 'example.com', and DNS
contained the following records:
example.com. IN NAPTR 50 50 "s" "AAAS+RADSECT" ""
_radsec._tcp.example.com.
_radsec._tcp.example.com. IN SRV 0 10 2083 radsec.example.com.
radsec.example.com. IN AAAA 2001:0DB8::202:44ff:fe0a:f704
Then the target selected would be a RadSec server on port 2083 at
IPv6 address 2001:0DB8::202:44ff:fe0a:f704. The connection would be
made over TCP/IP, and TLS encryption would be used. This complete
specification of the realm is the most flexible and is recommended.
Appendix B. Implementation Overview: Radiator
Radiator implements the RadSec protocol for proxying requests with
the <Authby RADSEC> and <ServerRADSEC> clauses in the Radiator
configuration file.
The <AuthBy RADSEC> clause defines a RadSec client, and causes
Radiator to send RADIUS requests to the configured RadSec server
using the RadSec protocol.
The <ServerRADSEC> clause defines a RadSec server, and causes
Radiator to listen on the configured port and address(es) for
connections from <Authby RADSEC> clients. When an <Authby RADSEC>
client connects to a <ServerRADSEC> server, the client sends RADIUS
requests through the stream to the server. The server then services
the request in the same was as if the request had been received from
a conventional UDP RADIUS client.
Radiator is compliant to version 2 of RadSec if the following options
are used:
<AuthBy RADSEC>
* Protocol tcp
* UseTLS
* TLS_CertificateFile
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* Secret radsec
<ServerRADSEC>
* Protocol tcp
* UseTLS
* TLS_RequireClientCert
* Secret radsec
As of Radiator 3.15, the default shared secret for RadSec connections
is configurable and defaults to "mysecret" (without quotes). For
compliance with this document, this setting needs to be configured
for the shared secret "radsec". The implementation uses TCP
keepalive socket options, but does not send Status-Server packets.
Once established, TLS connections are kept open throughout the server
instance lifetime.
Appendix C. Implementation Overview: radsecproxy
The RADIUS proxy named radsecproxy was written in order to allow use
of RadSec in current RADIUS deployments. This is a generic proxy
that supports any number and combination of clients and servers,
supporting RADIUS over UDP and RadSec. The main idea is that it can
be used on the same host as a non-RadSec client or server to ensure
RadSec is used on the wire, however as a generic proxy it can be used
in other circumstances as well.
The configuration file consists of client and server clauses, where
there is one such clause for each client or server. In such a clause
one specifies either "type tls" or "type udp" for RadSec or UDP
transport. For RadSec the default shared secret "mysecret" (without
quotes), the same as Radiator, is used. A secret may be specified by
putting say "secret somesharedsecret" inside a client or server
clause.
In order to use TLS for clients and/or servers, one must also specify
where to locate CA certificates, as well as certificate and key for
the client or server. This is done in a TLS clause. There may be
one or several TLS clauses. A client or server clause may reference
a particular TLS clause, or just use a default one. One use for
multiple TLS clauses may be to present one certificate to clients and
another to servers.
If any RadSec (TLS) clients are configured, the proxy will at startup
listen on port 2083, as assigned by IANA for the OSC RadSec
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implementation. An alternative port may be specified. When a client
connects, the client certificate will be verified, including checking
that the configured FQDN or IP address matches what is in the
certificate. Requests coming from a RadSec client are treated
exactly like requests from UDP clients.
The proxy will at startup try to establish a TLS connection to each
(if any) of the configured RadSec (TLS) servers. If it fails to
connect to a server, it will retry regularly. There is some back-off
where it will retry quickly at first, and with longer intervals
later. If a connection to a server goes down it will also start
retrying regularly. When setting up the TLS connection, the server
certificate will be verified, including checking that the configured
FQDN or IP address matches what is in the certificate. Requests are
sent to a RadSec server just like they would to a UDP server.
The proxy supports Status-Server messages. They are only sent to a
server if enabled for that particular server. Status-Server requests
are always responded to.
This RadSec implementation has been successfully tested together with
Radiator. It is a freely available open-source implementation. For
source code and documentation, see [radsecproxy-impl].
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.
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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.
Stig Venaas
UNINETT
Abels gate 5 - Teknobyen
Trondheim 7465
NORWAY
Phone: +47 73 55 79 00
Fax: +47 73 55 79 01
EMail: stig.venaas@uninett.no
URI: http://www.uninett.no.
Klaas Wierenga
Cisco Systems International BV
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The Netherlands
Phone: +31 (0)20 3571752
Fax:
EMail: kwiereng@cisco.com
URI: http://www.cisco.com.
Winter, et al. Expires April 27, 2009 [Page 16]
Internet-Draft RADIUS over TCP/TLS (RadSec) October 2008
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