Network Working Group Alan DeKok
INTERNET-DRAFT FreeRADIUS
Category: Informational
<draft-ietf-radext-dtls-01.txt>
Expires: April 12, 2011
12 October 2010
DTLS as a Transport Layer for RADIUS
draft-ietf-radext-dtls-01
Abstract
The RADIUS protocol [RFC2865] has limited support for authentication
and encryption of RADIUS packets. The protocol transports data "in
the clear", although some parts of the packets can have "hidden"
content. Packets may be replayed verbatim by an attacker, and
client-server authentication is based on fixed shared secrets. This
document specifies how the Datagram Transport Layer Security (DTLS)
protocol may be used as a fix for these problems. It also describes
how implementations of this proposal can co-exist with current RADIUS
systems.
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 April 12, 2011
Copyright Notice
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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
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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.
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Table of Contents
1. Introduction ............................................. 4
1.1. Terminology ......................................... 4
1.2. Requirements Language ............................... 5
2. Building on Existing Foundations ......................... 6
2.1. Changes to RADIUS ................................... 6
2.2. Changes from RADIUS over TLS (RADIUS/TLS) ........... 6
2.2.1. Changes from RADIUS/TLS to RADIUS/DTLS ......... 7
2.2.2. Reinforcement of RADIUS/TLS .................... 8
3. Reception of Packets ..................................... 8
3.1. Protocol Disambiguation ............................. 9
4. Connection Management .................................... 10
4.1. Server Connection Management ........................ 10
4.1.1. Table Management ............................... 10
4.2. Client Connection Management ........................ 11
5. Processing Algorithm ..................................... 12
6. Diameter Considerations .................................. 14
7. IANA Considerations ...................................... 14
8. Security Considerations .................................. 14
8.1. Legacy RADIUS Security .............................. 14
8.2. Network Address Translation ......................... 15
9. References ............................................... 16
9.1. Normative references ................................ 16
9.2. Informative references .............................. 17
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1. Introduction
The RADIUS protocol as described in [RFC2865], [RFC2866], and
[RFC5176] has traditionally used methods based on MD5 [RFC1321] for
per-packet authentication and integrity checks. However, the MD5
algorithm has known weaknesses such as [MD5Attack] and [MD5Break].
As a result, previous specifications such as [RFC5176] have
recommended using IPSec to secure RADIUS traffic.
While RADIUS over IPSec has been widely deployed, there are
difficulties with this approach. The simplest point against IPSec is
that there is no straightforward way for a RADIUS application to
control or monitor the network security policies. That is, the
requirement that the RADIUS traffic be encrypted and/or authenticated
is implicit in the network configuration, and is not enforced by the
RADIUS application.
This specification takes a different approach. We define a method
for using DTLS [RFC4347] as a RADIUS transport protocol. This
approach has the benefit that the RADIUS application can directly
monitor and control the security policies associated with the traffic
that it processes.
Another benefit is that RADIUS over DTLS continues to be a UDP-based
protocol. This continuity ensures that existing network-layer
infrastructure (firewall rules, etc.) does not need to be changed
when RADIUS clients and servers are upgraded to support RADIUS over
DTLS.
This specification does not, however, solve all of the problems
associated with RADIUS. The DTLS protocol does not add reliable or
in-order transport to RADIUS. DTLS also does not support
fragmentation of application-layer messages, or of the DTLS messages
themselves. This specification therefore continues to have all of
the issues that RADIUS currently has with order, reliability, and
fragmentation.
1.1. Terminology
This document uses the following terms:
RADIUS/DTLS
This term is a short-hand for "RADIUS over DTLS".
RADIUS/DTLS client
This term refers both to RADIUS clients as defined in [RFC2865],
and to Dynamic Authorization clients as defined in [RFC5176], that
implement RADIUS/DTLS.
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RADIUS/DTLS server
This term refers both to RADIUS servers as defined in [RFC2865],
and to Dynamic Authorization servers as defined in [RFC5176], that
implement RADIUS/DTLS.
silently discard
This means that the implementation discards the packet without
further processing. The implementation MAY provide the capability
of logging the error, including the contents of the silently
discarded packet, and SHOULD record the event in a statistics
counter.
1.2. 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
[RFC2119].
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2. Building on Existing Foundations
Adding DTLS as a RADIUS transport protocol requires a number of
changes to systems implementing standard RADIUS. This section
outlines those changes, and defines new behaviors necessary to
implement DTLS.
2.1. Changes to RADIUS
The RADIUS packet format is unchanged from [RFC2865], [RFC2866], and
[RFC5176]. Specifically, all of the following portions of RADIUS
MUST be unchanged when using RADIUS over DTLS:
* Packet format
* Permitted codes
* Request Authenticator calculation
* Response Authenticator calculation
* Minimum packet length
* Maximum packet length
* Attribute format
* Vendor-Specific Attribute (VSA) format
* Permitted data types
* Calculations of dynamic attributes such as CHAP-Challenge,
or Message-Authenticator.
* Calculation of "encrypted" attributes such as Tunnel-Password.
* UDP port numbering and usage
The RADIUS packets are encapsulated in DTLS, which acts as a
transport layer for it. The requirements above ensure the simplest
possible implementation and widest interoperability of this
specification.
The only changes made to RADIUS in this specification are the
following two items:
(1) The Length checks defined in [RFC2865] Section 3 MUST use the
length of the decrypted DTLS data instead of the UDP packet
length.
(2) The shared secret secret used to compute the MD5 integrity
checks and the attribute encryption MUST be "radsec".
All other portions of RADIUS are unchanged.
2.2. Changes from RADIUS over TLS (RADIUS/TLS)
While this specification is largely RADIUS/TLS over UDP instead of
TCP, there are some differences between the two methods.
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This section goes through the [RADIUS/TLS] document in detail,
explaining the differences between RADIUS/TLS and RADIUS/DTLS. As
most of [RADIUS/TLS] also applies to RADIUS/DTLS, we highlight only
the changes here, explaining how to interpret [RADIUS/TLS] for this
specification:
* We replace references to "TCP" with "UDP"
* We replace references to "RADIUS/TLS" with "RADIUS/DTLS"
* We replace references to "TLS" with "DTLS"
Those changes are sufficient to cover the majority of the differences
between the two specifications. The text below goes through some of
the sections of [RADIUS/TLS], giving additional commentary only where
necessary.
2.2.1. Changes from RADIUS/TLS to RADIUS/DTLS
Section 2.1 does not apply to RADIUS/DTLS. The relationship between
RADIUS packet codes and UDP ports in RADIUS/DTLS is unchanged from
RADIUS/UDP.
Section 2.2 applies also to RADIUS/DTLS, except for the
recommendation that implementations "SHOULD" support
TLS_RSA_WITH_RC4_128_SHA, which does not apply to RADIUS/DTLS.
Section 2.3 applies also to RADIUS/TLS.
Section 2.4 does not apply to RADIUS/DTLS. See the comments above on
Section 2.1. The relationship between RADIUS packet codes and UDP
ports in RADIUS/DTLS is unchanged from RADIUS/UDP.
Section 3.3 item (1) does not apply to RADIUS/DTLS. Each RADIUS
packet is encapsulated in one DTLS packet, and there is no "stream"
of RADIUS packets inside of a TLS session. Implementors MUST enforce
the requirements of [RFC2865] Section 3 for the RADIUS Length field,
using the length of the decrypted DTLS data for the checks. This
check replaces the RADIUS method of using the length field from the
UDP packet.
Section 3.3 item (3) does not apply to RTDLS. The relationship
between RADIUS packet codes and UDP ports in RADIUS/DTLS is unchanged
from RADIUS.
Section 3.3 item (4) does not apply to RADIUS/DTLS. As RADIUS/DTLS
still uses UDP for a transport, the use of negative ICMP responses is
unchanged from RADIUS.
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2.2.2. Reinforcement of RADIUS/TLS
We wish to re-iterate that much of [RADIUS/TLS] applies to this
document. Specifically, Section 4 and Section 6 of that document are
applicable in whole to RADIUS/DTLS.
3. Reception of Packets
As this specification permits implementations to to accept both
traditional RADIUS and DTLS packets on the same port, we define a
method to disambiguate between packets for the two protocols. This
method is applicable only to RADIUS servers. RADIUS/DTLS clients
SHOULD use connected sockets, as discussed in Section X.Y, below.
RADIUS/DTLS servers MUST maintain a boolean flag for each RADIUS
client that indicates whether or not it supports RADIUS/DTLS. The
interpretation of this flag is as follows. If the flag is "false",
then the client may support RADIUS/DTLS. Packets from the client
need to be examined to see if they are RADIUS or RADIUS/DTLS. If the
flag is "true" then the client supports RADIUS/DTLS, and all packets
from that client MUST be processed as RADIUS/DTLS.
Note that this last requirement can impose significant changes for
RADIUS clients. Clients can no longer have multiple independent
RADIUS implementations or processes that originate packets. We
RECOMMEND that RADIUS/DTLS clients implement a local RADIUS proxy
that arbitrates all RADIUS traffic.
This flag MUST be exposed to administrators of the RADIUS server. As
RADIUS clients are upgraded, administrators can then manually mark
them as supporting RADIUS/DTLS.
We recognize, however, the upgrade path from RADIUS to RADIUS/DTLS is
important. This path requires an RADIUS/DTLS server to accept
packets from a RADIUS client without knowing beforehand if they are
RADIUS or DTLS. The method to distinguish between the two is defined
in the next section.
Once an RADIUS/DTLS server has established a DTLS session with a
client that had the flag set to "false", it MUST set the flag to
"true". This change forces all subsequent traffic from that client
to use DTLS, and prevents bidding-down attacks. The server SHOULD
also notify the administrator that it has successfully established
the first DTLS session with that client.
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3.1. Protocol Disambiguation
When a RADIUS client is not marked as supporting RADIUS/DTLS, packets
from that client may be, or may not be DTLS. In order to provide a
robust upgrade path, the RADIUS/DTLS server MUST examine the packet
to see if it is RADIUS or DTLS. In order to justify the examination
methods, we first examine the packet formats for the two protocols.
The DTLS record format ([RFC4347] Section 4.1) is shown below, in
pseudo-code:
struct {
uint8 type;
uint16 version;
uint16 epoch;
uint48 sequence_number;
uint16 length;
uint8 fragment[DTLSPlaintext.length];
} DTLSPlaintext;
The RADIUS record format ([RFC2865] Section 3) is shown below, in
pseudo-code, with AuthVector.length=16.
struct {
uint8 code;
uint8 id;
uint16 length;
uint8 vector[AuthVector.length];
uint8 data[RadiusPacket.length - 20];
} RadiusPacket;
We can see here that a number of fields overlap between the two
protocols. The low byte of the DTLS version and the high byte of the
DTLS epoch overlap with the RADIUS length field. The DTLS length
field overlaps with the RADIUS authentication vector. At first
glance, it may be difficult for an application to accept both
protocols on the same port. However, this is not the case.
For the initial packet of a DTLS connection, the type field has value
22 (handshake), and the epoch and sequence number fields are
initialized to zero. The RADIUS code value of 22 has been assigned
as Resource-Free-Response, but it is not in wide use. In addition,
that packet code is a response packet, and would not be sent by a
RADIUS client to a server.
As a result, protocol disambiguation is straightforward. If the
first byte of the packet has value 22, it is a DTLS packet, and is a
DTLS connection initiation request. Otherwise, it is a RADIUS
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packet.
Once a DTLS session has been established, a separate tracking table
is used to disambiguate the protocols. The definition of this
tracking table is given in the next section.
The full processing algorithm is given below, in Section X.Y.
4. Connection Management
Where [RADIUS/TLS] can rely on the TCP state machine to perform
connection tracking, this specification cannot. As a result,
implementations of this specification will need to perform connection
management of the DTLS session in the application layer.
4.1. Server Connection Management
An RADIUS/DTLS server MUST maintain a table that tracks ongoing DTLS
sessions based on a key composed of the following 4-tuple:
* source IP address
* source port
* destination IP address
* destination port
The contents of the tracking table are a implementation-specific
value that describes an active DTLS session. This connection
tracking allows DTLS packets that have been received to be associated
with an active DTLS session.
RADIUS/DTLS servers SHOULD NOT use a "connect" API to manage DTLS
connections, as a connected UDP socket will accept packets only from
one source IP address and port. This limitation would prevent the
server from engaging in the normal RADIUS practice of accepting
packets from multiple clients on the same port.
Note that [RFC5080] Section 2.2.2 defines a duplicate detection cache
which tracks packets by key similar to that defined above.
4.1.1. Table Management
This tracking table is subject to Denial of Service (DoS) attacks.
RADIUS/DTLS servers SHOULD use the stateless cookie tracking
technique described in [RFC4347] Section 4.2.1. DTLS sessions SHOULD
NOT be added to the tracking table until a ClientHello packet has
been received with an appropriate Cookie value.
Entries in the tracking table MUST deleted when a TLS Closure Alert
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([RFC5246] Section 7.2.1) or a TLS Error Alert ([RFC5246] Section
7.2.2) is received. Where the RADIUS specifications require that a
RADIUS packet received via the DTLS session is to be "silently
discarded", the entry in the tracking table corresponding to that
DTLS session MUST also be deleted, the DTLS session MUST be closed,
and any TLS session resumption parameters for that session MUST be
discarded.
As UDP does not offer guaranteed delivery of messages, RADIUS/DTLS
servers MUST also maintain a timestamp per DTLS session. The
timestamp SHOULD be updated on reception of a valid DTLS packet. The
timestamp MUST NOT be updated in other situations. When a session
has not been used for a period of time, the server SHOULD pro-
actively close it, and delete the DTLS session from the tracking
table. The server MAY cache the TLS session parameters, in order to
provide for fast session resumption.
This session lifetime SHOULD be exposed as configurable setting. It
SHOULD NOT be set to less than 60 seconds, and SHOULD NOT be set to
more than 600 seconds (10 minutes). The minimum value useful value
for this timer is determined by the application-layer watchdog
mechanism defined in the following section.
RADIUS/DTLS servers SHOULD also keep track of the total number of
sessions in the tracking table, and refuse to create new sessions
when a large number are already being tracked. As system
capabilities vary widely, we can only recommend that this number
SHOULD be exposed as a configurable setting.
4.2. Client Connection Management
RADIUS/DTLS clients SHOULD use an operating system API to "connect" a
UDP socket. This "connected" socket will then rely on the operating
system to perform connection tracking, and will be simpler than the
method described above for servers. RADIUS/DTLS clients SHOULD NOT
use "unconnected" sockets, as it causes increased complexity in the
client application.
Once a DTLS session is established, an RADIUS/DTLS client SHOULD use
the application-layer watchdog algorithm defined in [RFC3539] to
determine server responsiveness. The Status-Server packet defined in
[RFC5997] MUST be used as the "watchdog packet" in the watchdog
algorithm.
RADIUS/DTLS clients SHOULD pro-actively close sessions when they have
been idle for a period of time. We RECOMMEND that a session be
closed when no traffic over than watchdog packets and (possibly)
responses have been sent for three watchdog timeouts. This behavior
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ensures that clients do not waste resources on the server by causing
it to track idle sessions.
RADIUS/DTLS clients SHOULD NOT send both normal RADIUS and
RADIUS/DTLS packets from the same source socket. This practice
causes increased complexity in the client application, and increases
the potential for security breaches due to implementation issues.
RADIUS/DTLS clients MUST NOT send both normal RADIUS and RADIUS/DTLS
packets over the same key as defined in Section 4.1, abovre (source
IP, source port, destination IP, destination port). Doing so would
require that servers perform RADIUS and RADIUS/DTLS determination for
every packet that has been received.
RADIUS/DTLS clients SHOULD use TLS session resumption, where
possible. This practice lowers the time and effort required to start
a DTLS session with a server, and increases network responsiveness.
5. Processing Algorithm
The following algorithm MUST be used by an implementation of this
protocol. This algorithm is used to route packets to the appropriate
destination. We assume the following variables:
D - implementation-specific handle to an existing DTLS session
P - UDP packet received from the network. This packet MUST
also contain information about source IP/port, and
destination IP/port.
R - a RADIUS packet
T - a tracking table used to manage ongoing DTLS sessions
We also presume the following functions or functionality exists:
receive_packet_from_network() - a function that reads a packet
from the network, and returns P as above. We presume also that
this function performs the normal RADIUS client validation, and
does not return P if the packet is from an unknown client.
lookup_dtls_session() - a function that takes a packet P, a table
T, and uses P to look up the corresponding DTLS session in T. It
returns either a session D, or a "null" indicator that no
corresponding session exists.
client_supports_rdtls() - a function that takes a packet P, and
returns a boolean value as to whether or not the client
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originating the packet was marked as supporting RADIUS/DTLS.
process_dtls_packet() - a function that takes a DTLS packet P, and
a DTLS session D. It performs all necessary steps to use D to
setup a DTLS session, and to decode P (where possible) into a
RADIUS packet. This function is also expected to perform checks
for TLS errors. On any fatal errors, it closes the session, and
deletes D from the tracking table T. If a RADIUS packet is
decoded from P, it is returned by the function as R, otherwise a
"null" indicator is returned.
process_dtls_clienthello() - a function that takes a DTLS packet
P, and initiates a DTLS session. If P contains a valid DTLS
Cookie, a DTLS session D is created, and stored in the tracking
table T. If P does not contain a DTLS Cookie, no session is
created, and instead a HelloVerifyRequest containing a cookie is
sent in response. Packets containing invalid cookies are
discarded.
process_radius_packet() - a function that takes a RADIUS packet P,
and processes it using the normal RADIUS methods.
The algorithm is as follows:
P = receive_packet_from_network()
D = lookup_dtls_session(T, P)
if (D || client_supports_rdtls(P)) {
R = process_dtls_packet(D, P)
if (R) {
process_radius_packet(R)
}
} else if (first_octet_of_packet_is_22(P)) {
process_dtls_clienthello(P)
} else {
process_radius_packet(P)
}
For simplicity, the timers necessary to perform expiry of "old"
sessions are not included in the above algorithm. This algorithm may
also need to be modified if the RADIUS/DTLS server supports client
validation by methods other than source IP address.
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6. Diameter Considerations
This specification is for a transport layer specific to RADIUS. As a
result, there are no Diameter considerations.
7. IANA Considerations
This specification does not create any new registries, nor does it
require assignment of any protocol parameters.
8. Security Considerations
This entire specification is devoted to discussing security
considerations related to RADIUS. However, we discuss a few
additional issues here.
This specification relies on the existing DTLS, RADIUS, and
RADIUS/TLS specifications. As a result, all security considerations
for DTLS apply to the DTLS portion of RADIUS/DTLS. Similarly, the
TLS and RADIUS security issues discussed in [RADIUS/TLS] also apply
to this specification. All of the security considerations for RADIUS
apply to the RADIUS portion of the specification.
However, many security considerations raised in the RADIUS documents
are related to RADIUS encryption and authorization. Those issues are
largely mitigated when DTLS is used as a transport method. The
issues that are not mitigated by this specification are related to
the RADIUS packet format and handling, which is unchanged in this
specification.
The only new portion of the specification that could have security
implications is a servers ability to accept both RADIUS and DTLS
packets on the same port. The filter that disambiguates the two
protocols is simple, and is just a check for the value of one byte.
We do not expect this check to have any security issues.
We also note that nothing prevents malicious clients from sending
DTLS packets to existing RADIUS implementations, or RADIUS packets to
existing DTLS implementations. There should therefore be no issue
with clients sending RADIUS/DTLS packets to legacy servers that do
not support the protocol.
8.1. Legacy RADIUS Security
We reiterate here the poor security of the legacy RADIUS protocol.
We RECOMMEND that all RADIUS clients and servers implement this
specification as soon as possible. New attacks on MD5 have appeared
over the past few years, and there is a distinct possibility that MD5
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may be completely broken in the near future.
The existence of fast and cheap attacks on MD5 could result in a loss
of all network security that depends on RADIUS. Attackers could
obtain user passwords, and possibly gain complete network access. It
is difficult to overstate the disastrous consequences of a successful
attack on RADIUS.
We also caution implementors (especially client implementors) about
using RADIUS/DTLS. It may be tempting to use the shared secret as
the basis for a TLS pre-shared key (PSK) method, and to leave the
user interface otherwise unchanged. This practice MUST NOT be used.
The administrator MUST be given the option to use DTLS. Any shared
secret used for RADIUS MUST NOT be used for DTLS. Re-using a shared
secret between RADIUS and DTLS would negate all of the benefits found
by using DTLS.
When using PSK methods, RADIUS/DTLS clients MUST support keys (i.e.
shared secrets) that are at least 32 characters in length.
RADIUS/DTLS client implementors MUST expose a configuration that
allows the administrator to choose the cipher suite. RADIUS/DTLS
client implementors SHOULD expose a configuration that allows an
administrator to configure all certificates necessary for
certificate-based authentication. These certificates include client,
server, and root certificates.
When using PSK methods, RADIUS/DTLS servers MUST support keys (i.e.
shared secrets) that are at least 32 characters in length.
RADIUS/DTLS server administrators MUST use strong shared secrets for
those PSK methods. We RECOMMEND using keys derived from a
cryptographically secure pseudo-random number generator (CSPRNG).
For example, a reasonable key may be 32 characters of a SHA-256 hash
of at least 64 bytes of data taken from a CSPRNG. If this method
seems too complicated, a certificate-based TLS method SHOULD be used
instead.
The previous RADIUS practice of using shared secrets that are minor
variations of words is NOT RECOMMENDED, as it would negate nearly all
of the security of DTLS.
8.2. Network Address Translation
Network Address Translation (NAT) is fundamentally incompatible with
RADIUS. RADIUS uses the source IP address to determine the shared
secret for the client, and NAT hides many clients behind one source
IP address.
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The migration flag described above in Section 3 is also tracked per
source IP address. Using a NAT in front of many RADIUS clients
negates the function of the flag, making it impossible to migrate
clients in a secure fashion.
In addition, port re-use on a NAT gateway means that packets from
different clients may appear to come from the same source port on the
NAT. That is, a RADIUS server may receive a RADIUS/DTLS packet from
a client IP/port combination, followed by the reception of a
RADIUS/UDP packet from that same client IP/port combination. If this
capability were allowed, it would permit a downgrade attack to occur,
and would negate all of the security added by RADIUS/DTLS.
As a result, RADIUS clients SHOULD NOT be located behind a NAT
gateway. If clients are located behind a NAT gateway, then a secure
transport such as DTLS MUST be used.
9. References
9.1. Normative references
[RFC2865]
Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.
[RFC3539]
Aboba, B. et al., "Authentication, Authorization and Accounting
(AAA) Transport Profile", RFC 3539, June 2003.
[RFC4347]
Rescorla E., and Modadugu, N., "Datagram Transport Layer Security",
RFC 4347, April 2006.
[RFC5246]
Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.2", RFC 5246, August 2008.
[RADIUS/TLS]
Winter. S, et. al., "TLS encryption for RADIUS over TCP", draft-
ietf-radext-radsec-06.txt, March 2010 (work in progress)
[RFC5997]
DeKok, A., "Use of Status-Server Packets in the Remote
Authentication Dial In User Service (RADIUS) Protocol", RFC 5997,
August 2010.
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9.2. Informative references
[RFC1321]
Rivest, R. and S. Dusse, "The MD5 Message-Digest Algorithm", RFC
1321, April 1992.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March, 1997.
[RFC2866]
Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[RFC5080]
Nelson, D. and DeKok, A, "Common Remote Authentication Dial In User
Service (RADIUS) Implementation Issues and Suggested Fixes", RFC
5080, December 2007.
[RFC5176]
Chiba, M. et al., "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 5176, January
2008.
[MD5Attack]
Dobbertin, H., "The Status of MD5 After a Recent Attack",
CryptoBytes Vol.2 No.2, Summer 1996.
[MD5Break]
Wang, Xiaoyun and Yu, Hongbo, "How to Break MD5 and Other Hash
Functions", EUROCRYPT. ISBN 3-540-25910-4, 2005.
Acknowledgments
Parts of the text in Section 3 defining the Request and Response
Authenticators were taken with minor edits from [RFC2865] Section 3.
The author would like to thank Mike McCauley of Open Systems
Consultants for making a Radiator server available for inter-
operability testing.
Authors' Addresses
Alan DeKok
The FreeRADIUS Server Project
http://freeradius.org
Email: aland@freeradius.org
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