One document matched: draft-dekok-radext-dtls-02.txt
Differences from draft-dekok-radext-dtls-01.txt
Network Working Group Alan DeKok
INTERNET-DRAFT FreeRADIUS
Category: Informational
<draft-dekok-radext-dtls-02.txt>
Expires: September 22, 2010
22 March 2010
DTLS as a Transport Layer for RADIUS
draft-dekok-radext-dtls-02
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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.
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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 (RTLS) ................. 6
2.2.1. Changes from RTLS to RDTLS ..................... 7
2.2.2. Reinforcement of RTLS .......................... 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 .................................. 13
7. IANA Considerations ...................................... 13
8. Security Considerations .................................. 14
8.1. Legacy RADIUS Security .............................. 14
9. References ............................................... 15
9.1. Normative references ................................ 15
9.2. Informative references .............................. 16
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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:
RDTLS
This term is a short-hand for "RADIUS over DTLS".
RDTLS client
This term refers both to RADIUS clients as defined in [RFC2865],
and to Dynamic Authorization clients as defined in [RFC5176], that
implement RDTLS.
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RDTLS server
This term refers both to RADIUS servers as defined in [RFC2865],
and to Dynamic Authorization servers as defined in [RFC5176], that
implement RDTLS.
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 (RTLS)
While this specification is largely RTLS over UDP instead of TCP,
there are some differences between the two methods.
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This section goes through the [RTLS] document in detail, explaining
the differences between RTLS and RDTLS. As most of [RTLS] also
applies to RDTLS, we highlight only the changes here, explaining how
to interpret [RTLS] for this specification:
* We replace references to "TCP" with "UDP"
* We replace references to "RTLS" with "RDTLS"
* 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 [RTLS], giving additional commentary only where
necessary.
2.2.1. Changes from RTLS to RDTLS
Section 2.1 does not apply to RDTLS. The relationship between RADIUS
packet codes and UDP ports in RDTLS is unchanged from RADIUS.
Section 2.2 applies also to RDTLS, except for the recommendation that
implementations "SHOULD" support TLS_RSA_WITH_RC4_128_SHA, which does
not apply to RDTLS.
Section 2.3 applies also to RTLS.
Section 2.4 does not apply to RDTLS. See the comments above on
Section 2.1. The relationship between RADIUS packet codes and UDP
ports in RDTLS is unchanged from RADIUS.
Section 3.3 item (1) does not apply to RDTLS. 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 RDTLS is unchanged from
RADIUS.
Section 3.3 item (4) does not apply to RDTLS. As RDTLS 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 RTLS
We wish to re-iterate that much of [RTLS] applies to this document.
Specifically, Section 4 and Section 6 of that document are applicable
in whole to RDTLS.
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. RDTLS clients SHOULD
use connected sockets, as discussed in Section X.Y, below.
RDTLS servers MUST maintain a boolean flag for each RADIUS client
that indicates whether or not it supports RDTLS. The interpretation
of this flag is as follows. If the flag is "false", then the client
may support RDTLS. Packets from the client need to be examined to
see if they are RADIUS or RDTLS. If the flag is "true" then the
client supports RDTLS, and all packets from that client MUST be
processed as RDTLS.
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 RDTLS 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 RDTLS.
We recognize, however, the upgrade path from RADIUS to RDTLS is
important. This path requires an RDTLS 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 RDTLS 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 RDTLS, packets from
that client may be, or may not be DTLS. In order to provide a robust
upgrade path, the RDTLS 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 [RTLS] 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 RDTLS 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.
RDTLS 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.
RDTLS 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, RDTLS 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.
RDTLS 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
RDTLS 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. RDTLS clients SHOULD NOT use
"unconnected" sockets, as it causes increased complexity in the
client application.
Once a DTLS session is established, an RDTLS client SHOULD use the
application-layer watchdog algorithm defined in [RFC3539] to
determine server responsiveness. The Status-Server packet defined in
[STATUS] MUST be used as the "watchdog packet" in the watchdog
algorithm.
RDTLS 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 ensures
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that clients do not waste resources on the server by causing it to
track idle sessions.
RDTLS clients SHOULD NOT send both normal RADIUS and RDTLS 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.
RDTLS 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
originating the packet was marked as supporting RDTLS.
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
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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 RDTLS server supports client
validation by methods other than source IP address.
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.
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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 RTLS
specifications. As a result, all security considerations for DTLS
apply to the DTLS portion of RDTLS. Similarly, the TLS and RADIUS
security issues discussed in [RTLS] 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 RDTLS 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
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 RDTLS. It may be tempting to use the shared secret as the
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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, RDTLS clients MUST support keys (i.e. shared
secrets) that are at least 32 characters in length.
RDTLS client implementors MUST expose a configuration that allows the
administrator to choose the cipher suite. RDTLS 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, RDTLS servers MUST support keys (i.e. shared
secrets) that are at least 32 characters in length. RDTLS 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.
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)
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Protocol Version 1.2", RFC 5246, August 2008.
[RTLS]
Winter. S, et. al., "TLS encryption for RADIUS over TCP", draft-
ietf-radext-radsec-06.txt, March 2010 (work in progress)
[STATUS]
DeKok, A., "Use of Status-Server Packets in the Remote
Authentication Dial In User Service (RADIUS) Protocol", draft-ietf-
radext-status-server-06.txt, February 2010 (work in progress).
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
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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
DeKok, Alan Informational [Page 17]
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