One document matched: draft-ietf-radext-dtls-02.txt
Differences from draft-ietf-radext-dtls-01.txt
Network Working Group Alan DeKok
INTERNET-DRAFT FreeRADIUS
Category: Informational
<draft-ietf-radext-dtls-02.txt>
Expires: May 16, 2013
16 July 2012
DTLS as a Transport Layer for RADIUS
draft-ietf-radext-dtls-02
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 "obfuscated"
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 May 16, 2013
Copyright Notice
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Copyright (c) 2012 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info/) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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. Similarities with RADIUS/TLS ........................ 7
2.2.1. Changes from RADIUS/TLS to RADIUS/DTLS ......... 7
2.2.2. Reinforcement of RADIUS/TLS .................... 8
3. Reception of Packets ..................................... 8
4. Connection Management .................................... 9
4.1. Server Connection Management ........................ 9
4.1.1. Table Management ............................... 10
4.1.2. Protocol Disambiguation ........................ 11
4.1.3. Processing Algorithm ........................... 12
4.2. Client Connection Management ........................ 13
5. Diameter Considerations .................................. 14
6. IANA Considerations ...................................... 14
7. Security Considerations .................................. 14
7.1. Legacy RADIUS Security .............................. 14
7.2. Resource Exhaustion ................................. 15
7.3. Network Address Translation ......................... 16
7.4. Wildcard Clients .................................... 16
8. References ............................................... 16
8.1. Normative references ................................ 16
8.2. Informative references .............................. 17
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1. Introduction
The RADIUS protocol as described in [RFC2865], [RFC2866], [RFC5176],
and others 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, some 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 [RFC6347] 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 shares with traditional
RADIUS the issues of 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.
RADIUS/UDP
RADIUS over UDP, as defined in [RFC2865].
RADIUS/TLS
RADIUS over TLS, as defined in [RFC6614].
silently discard
This means that the implementation discards the packet without
further processing. See Section X.Y for additional requirements on
packets being silently discarded.
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/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 "obfuscated" attributes such as User-Password
and Tunnel-Password.
* UDP port numbering and relationship between code and port
In short, the application creates a RADIUS packet as usual, and then
instead of sending it over a UDP socket, sends the packet to a DTLS
layer for encapsulation. DTLS then acts as a transport layer for
RADIUS, hence the names "RADIUS/UDP" and "RADIUS/DTLS".
The requirement that RADIUS remain largely unchanged ensures the
simplest possible implementation and widest interoperability of this
specification.
We note that the DTLS encapsulation of RADIUS means that the minimum
and maximum UDP packet sizes increase by the DTLS overhead.
Implementations should be aware of this, and take it into account
when allocating buffers to read and write RADIUS/DTLS packets.
The only changes made from RADIUS/UDP to RADIUS/DTLS 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.
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(2) The shared secret secret used to compute the MD5 integrity
checks and the attribute encryption MUST be "radius/dtls".
All other aspects of RADIUS are unchanged.
2.2. Similarities with RADIUS/TLS
While this specification can be thought of as RADIUS/TLS over UDP
instead of TCP, there are some differences between the two methods.
The bulk of [RFC6614] applies to this specification, so we do not
repeat it here.
This section explains the differences between RADIUS/TLS and
RADIUS/DTLS, as semantic "patches" to [RFC6614]. The changes are as
follows:
* 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 next section reviews some more
detailed changes from [RFC6614], 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. This recommendation is a historical
artifact of RADIUS/TLS, and does not apply to RADIUS/DTLS.
Section 2.3 does not apply to RADIUS/DTLS.
Section 2.4 does not apply to RADIUS/DTLS. 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,
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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 RADIUS/TDLS. 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.
2.2.2. Reinforcement of RADIUS/TLS
We re-iterate that much of [RFC6614] applies to this document.
Specifically, Section 4 and Section 6 of that document are applicable
in their entirety to RADIUS/DTLS.
3. Reception of Packets
As this specification permits implementations to to accept both
RADIUS/UDP and RADIUS/DTLS packets on the same port, we require a
method to disambiguate packets between the two protocols. This
method is applicable only to RADIUS/DTLS servers. RADIUS/DTLS
clients SHOULD use connected sockets, as discussed in Section X.Y,
below.
RADIUS/DTLS servers MUST maintain a boolean "DTLS Required" flag for
each client that indicates if it requires a client to use
RADIUS/DTLS. The interpretation of this flag is as follows. If the
flag is "true" then the client supports RADIUS/DTLS, and all packets
from that client MUST be processed as RADIUS/DTLS. If the flag is
"false", then the client supports RADIUS/UDP, but may still support
RADIUS/DTLS. Packets from the client need to be examined to see if
they are RADIUS/UDP or RADIUS/DTLS.
The "DTLS Required" flag MUST be exposed to administrators of the
server. As clients are upgraded, administrators can then manually
mark them as using RADIUS/DTLS.
Once a RADIUS/DTLS server has established a DTLS session with a
client that previously had the flag set to "false", the server MUST
set the "DTLS Required" flag to "true". This change requires 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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Note that this last requirement on servers can impose significant
changes for clients. Clients can no longer have multiple independent
RADIUS implementations or processes that originate RADIUS/UDP and
RADIUS/DTLS packets. Instead, they need to use only one transport
layer, either UDP or DTLS.
It is therefore RECOMMENDED that RADIUS/DTLS clients use a local
proxy which arbitrates all traffic between the client and any
servers. The proxy SHOULD accept traffic only from the authorized
subsystems on the client machine, and SHOULD proxy that traffic to
one or more known servers.
4. Connection Management
Where [RFC6614] 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
A RADIUS/DTLS server MUST maintain a table that tracks ongoing client
connections based on a key composed of the following 4-tuple:
* source IP address
* source port
* destination IP address
* destination port
Note that this table is independent of IP address version (IPv4 or
IPv6).
Each table entry contains the following information:
Protocol Type
A flag which is either "RADIUS/UDP" for old-style RADIUS traffic,
or "RADIUS/DTLS" for RADIUS/DTLS connections.
DTLS Data
An implementation-specific variable containing information about
the active DTLS connection. For non-DTLS connections, this
variable MUST be empty.
Last Packet
A variable containing a timestamp which indicates when the last
valid packet was received for this connection. Packets which are
"silently discarded" MUST NOT update this variable.
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Each entry may contain other information, such as idle timeouts,
connection lifetimes, and other implementation-specific data.
RADIUS/DTLS servers SHOULD NOT use connected sockets to read DTLS
packets from a client. This recommendation is because a connected
UDP socket will accept packets only from one source IP address and
port. This limitation would prevent the server from accepting
packets from multiple clients on the same port.
4.1.1. Table Management
This tracking table is subject to Denial of Service (DoS) attacks due
to the ability of an attacker to forge UDP traffic. RADIUS/DTLS
servers SHOULD use the stateless cookie tracking technique described
in [RFC6347] 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. The requirement to accept RADIUS/UDP and
RADIUS/DTLS on the same port makes this recommendation difficult to
implement in practice. Server implementation SHOULD therefore have a
way of tracking partially setup DTLS connections. Servers SHOULD
limit both the number and impact on resources of partial connections.
Entries in the tracking table MUST deleted when a TLS Closure Alert
([RFC5246] Section 7.2.1) or a TLS Error Alert ([RFC5246] Section
7.2.2) is received. Where the specifications require that a packet
received via a DTLS session 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. 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.
As UDP does not guarantee delivery of messages, RADIUS/DTLS servers
MUST also maintain a "Last Packet" 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 received a packet for a period of time, it is labelled
"idle". The server SHOULD delete idle DTLS session from the tracking
table after an "idle timeout". The server MAY cache the TLS session
parameters, in order to provide for fast session resumption.
This session "idle timeout" SHOULD be exposed to the administrator as
a 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.
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RADIUS/DTLS servers SHOULD also keep track of the total number of
sessions in the tracking table. They SHOULD stop the creating of new
sessions when a large number are already being tracked. This
"maximum sessions" number SHOULD be exposed to administrators as a
configurable setting.
4.1.2. Protocol Disambiguation
When the "DTLS Required" flag for a client is set to "false", the
client may, or may not be sending DTLS packets. For existing
connections, protocol disambiguation is simple, the "Protocol Type"
field in the tracking table entry is examined. New connections must
still be disambiguated.
In order to provide a robust upgrade path, the RADIUS/DTLS server
MUST examine the packet to see if it is RADIUS/UDP or RADIUS/DTLS.
This examination method is defined here.
We justify the examination methods by analysing the packet formats
for the two protocols. We assume that the server has a buffer in
which it has received a UDP packet matching no entry on the
conneciton tracking table. It must then analyse this buffer to
determine which protocol is used to process the packet.
The DTLS record format ([RFC6347] 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
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protocols. At first glance, it seems difficult for an application to
accept both protocols on the same port. However, this is not the
case.
The initial DTLS packet of a connection requires that the type field
(first octet) has value 22 (handshake). The first octet of a RADIUS
packet is the code field. The code value of 22 has been assigned as
Resource-Free-Response. That code is intended to be a response from
a server to a client, and will therefore never be sent by a client to
a server.
As a result, protocol disambiguation for new connections to a server
is straightforward. Only the first octet of the packet needs to be
examined to disambiguate RADIUS/DTLS from RADIUS/UDP. If that octet
has value 22, then the packet is likely to be RADIUS/DTLS.
Otherwise, the packet is likely to be RADIUS/UDP.
4.1.3. Processing Algorithm
When a RADIUS/DTLS server recieves a packet, it uses the following
algorithm to process that packet. As with RADIUS/UDP, packets from
unknown clients MUST be silently discarded.
The "DTLS Required" flag for that client is examined. If it is set
to "true", then the packet MUST be processed as RADIUS/DTLS.
If the "DTLS Required" flag is set to "false", the connection
tracking table is examined. Packets matching an existing entry MUST
be processed as defined by the "Protocol Type" field of that entry.
If the "DTLS Required" flag is set to "false" and no entry exists in
the connection tracking table, then the first octet of the packet is
examined. If it has value 22, then the packet MUST be processed as
RADIUS/DTLS. Otherwise, the packet MUST be processed as RADIUS/UDP.
In all cases, the packet MUST be checked for correctness. For
RADIUS/UDP, any packets which are silently discarded MUST NOT affect
the state of any variable in the session tracking table. For
RADIUS/DTLS, any packets which are discarded by the DTLS layer MUST
NOT affect the state of any variable in the session tracking table.
For RADIUS/DTLS, any RADIUS packets which are subsequently silently
discarded MUST result in the removal of the associated entry from the
connection tracking table.
When the packet matches an existing entry in the connection table,
and is accepted for processing by the server, the "Last Packet"
timestamp is updated. Where the packet does not match any entry in
the connection table, a new connection is created using the 4-tuple
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key defined above. The "Protocol Type" flag for that connection is
set to "RADIUS/DTLS", or "RADIUS/UDP", as determined by examining the
first octet of the packet.
When a server has the clients "DTLS Required" flag set to "false", it
MUST set the flag to "true" after establishing a DTLS session with
that client. It MUST NOT set the flag to "true" until a DTLS session
has been fully established. Doing so would mean that attackers could
perform a DoS attack by sending forged DTLS ClientHello packets to a
server.
4.2. Client Connection Management
Clients SHOULD use "connected" UDP sockets for RADIUS/DTLS traffic.
A connected socket will then rely on the operating system to perform
connection tracking. Clients SHOULD NOT use "unconnected" sockets
for RADIUS/DTLS traffic. Using unconnected sockets would require the
client to implement a connection tracking table, which is complex and
unnecessary.
Once a DTLS session is established, a 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. Clients SHOULD close a session when
no traffic other than watchdog packets and (possibly) watchdog
responses have been sent for three watchdog timeouts. This behavior
ensures that clients do not waste resources on the server by causing
it to track idle sessions.
RADIUS/DTLS clients MUST NOT send both RADIUS/UDP and RADIUS/DTLS
packets over the same key of (source IP, source port, destination IP,
destination port) as defined in Section 4.1, above . Doing so would
make it impossible to correctly process either kind of packet.
RADIUS/DTLS clients SHOULD NOT send both RADIUS/UDP and RADIUS/DTLS
packets to different servers 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 SHOULD use TLS session resumption. This practice
lowers the time and effort required to start a DTLS session with a
server, and increases network responsiveness.
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5. Diameter Considerations
This specification defines a transport layer for RADIUS. It makes no
other changes to the RADIUS protocol. As a result, there are no
Diameter considerations.
6. IANA Considerations
This specification does not create any new registries, nor does it
require assignment of any protocol parameters.
7. 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/UDP, 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 [RFC6614] 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 octet.
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. These packets will be silently ignored,
and will not change the security profile of the server.
7.1. Legacy RADIUS Security
We reiterate here the poor security of the legacy RADIUS protocol.
It is RECOMMENDED that all RADIUS clients and servers implement this
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specification. 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 which depends on RADIUS. Attackers could
obtain user passwords, and possibly gain complete network access. We
cannot 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/UDP MUST NOT be used for DTLS. Re-using a
shared secret between RADIUS/UDP and RADIUS/DTLS would negate all of
the benefits found by using DTLS.
RADIUS/DTLS client implementors MUST expose a configuration that
allows the administrator to choose the cipher suite. Where
certificates are used, RADIUS/DTLS client implementors MUST expose a
configuration which 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. These
keys SHOULD be able to contain arbitrary binary data. 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
octetss 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.
7.2. Resource Exhaustion
The use of DTLS allows DoS attacks, and resource exhaustion attacks
which were not possible in RADIUS/UDP. These attacks are the same as
described in [RFC6614] Section X.Y.
Use of the connection tracking table defined in Section X.Y can
result in resource exhaustion. Servers MUST therefore limit the
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absolute number of entries in the table. Servers MUST limit the
number of partially open DTLS sessions. These limits SHOULD be
exposed to the administrator as configurable settings.
7.3. Network Address Translation
Network Address Translation (NAT) is fundamentally incompatible with
RADIUS/UDP. RADIUS/UDP uses the source IP address to determine the
shared secret for the client, and NAT hides many clients behind one
source IP address.
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
multiple 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
behavior is 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. As discussed below, a method
for uniquely identifying each client MUST be used.
7.4. Wildcard Clients
Some RADIUS server implementations allow for "wildcard" clients.
That is, clients with an IPv4 netmask of other than 32, or an IPv6
netmask of other than 128. That practice is NOT RECOMMENDED for
RADIUS/UDP, as it means multiple clients use the same shared secret.
When a client is a "wildcard", then RADIUS/DTLS MUST be used.
Clients MUST be uniquely identified, and any certificate or PSK used
MUST be unique to each client.
8. References
8.1. Normative references
[RFC2865]
Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.
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INTERNET-DRAFT DTLS as a Transport Layer for RADIUS 16 July 2012
[RFC3539]
Aboba, B. et al., "Authentication, Authorization and Accounting
(AAA) Transport Profile", RFC 3539, June 2003.
[RFC5246]
Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS)
Protocol Version 1.2", RFC 5246, August 2008.
[RFC5997]
DeKok, A., "Use of Status-Server Packets in the Remote
Authentication Dial In User Service (RADIUS) Protocol", RFC 5997,
August 2010.
[RFC6347]
Rescorla E., and Modadugu, N., "Datagram Transport Layer Security",
RFC 6347, April 2006.
[RFC6614]
Winter. S, et. al., "TLS encryption for RADIUS over TCP", RFFC
6614, May 2012
8.2. 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.
[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.
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Acknowledgments
Parts of the text in Section 3 defining the Request and Response
Authenticators were taken with minor edits from [RFC2865] Section 3.
Authors' Addresses
Alan DeKok
The FreeRADIUS Server Project
http://freeradius.org
Email: aland@freeradius.org
DeKok, Alan Informational [Page 18]
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