Network Working Group G. Weber
INTERNET-DRAFT Cisco Systems
Category: Standards Track Alan DeKok (ed.)
FreeRADIUS
Expires: March 4, 2008
4 September 2007
RADIUS Design Guidelines
draft-ietf-radext-design-00.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document provides guidelines for the design of attributes used
by the Remote Authentication Dial In User Service (RADIUS) protocol.
It is expected that these guidelines will prove useful to authors and
reviewers of future RADIUS attribute specifications, both within the
IETF as well as other Standards Development Organizations (SDOs).
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Table of Contents
1. Introduction ............................................. 3
1.1. Applicability ....................................... 3
1.2. Terminology ......................................... 4
1.3. Requirements Language ............................... 4
2. RADIUS Data Model ........................................ 4
2.1. Standard Space ...................................... 5
2.1.1. Basic Data Types ............................... 5
2.1.2. Tagging Mechanism .............................. 6
2.1.3. Complex Attribute Usage ........................ 6
2.2. Vendor Space ........................................ 8
3. Data Model Issues ........................................ 10
3.1. Vendor Space ........................................ 10
3.2. Polymorphic Attributes .............................. 12
4. IANA Considerations ...................................... 13
5. Security Considerations .................................. 13
6. References ............................................... 13
6.1. Normative References ................................ 13
6.2. Informative References .............................. 14
Full Copyright Statement ..................................... 21
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1. Introduction
This document provides guidelines for the design of RADIUS attributes
both within the IETF as well as within other Standards Development
Organizations (SDOs).
As with "Guidelines for Authors and Reviewers of MIB Documents"
[RFC4181], it is expected that this document will enable authors to
check their document against the guidelines prior to requesting a
review (such an "Expert Review" described in [RFC3575]). Similarly,
it is hoped that this document will be of use to reviewers (such as
WG participants or the AAA Doctors) in improving the consistency of
reviews.
In order to meet these objectives, this document needs to cover not
only the science of attribute design, but also the art. As a result,
in addition to covering the most frequently encountered issues, this
document attempts to provide some of the considerations motivating
the guidelines.
In order to characterize current attribute usage, both the basic and
complex data types defined in the existing RADIUS RFCs are reviewed,
together with the ad-hoc extensions to that data model that have been
used in Vendor Specific Attributes (VSAs). In addition,
recommendations are made with respect to recommended VSA formats as
well as handling of RADIUS type 26 attributes within Diameter.
1.1. Applicability
The major goal of this document is to encourage the development and
publication of high quality RADIUS attribute specifications. By
articulating RADIUS design guidelines, it is hoped that this document
will be a step in that direction. However, the advice in this
document will not be helpful unless it is put to use. In particular,
the authors recommend:
o Development of a program to encourage SDOs to make their RADIUS
attribute specifications publicly available;
o Review of IETF and SDO specifications according to the
guidelines proposed in this document;
The advice in this document applies to attributes used to encode
data. RADIUS protocol changes, or specification of attributes that
can be used to provide new RADIUS commands (such as Service-Type) are
out of scope. Since protocol changes require greater expertise and
deeper review, such changes should not be undertaken outside the IETF
and when handled within the IETF require "IETF Consensus" for
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adoption, as noted in [RFC3575] Section 2.1.
As with protocol changes, this document does not provide guidance to
document authors seeking to change the RADIUS operational model.
While RADIUS server implementations may keep state, the RADIUS
protocol is stateless, although information may be passed from one
protocol transaction to another via the State Attribute. As a
result, documents which require stateful protocol behavior without
use of the State Attribute are inherently incompatible with RADIUS as
defined in [RFC2865], and need to be redesigned.
See [FIXES] Section 2.1.1 for a more in-depth discussion of the use
of the State Attribute.
1.2. Terminology
This document uses the following terms:
Network Access Server (NAS)
A device that provides an access service for a user to a network.
RADIUS server
A RADIUS authentication server is an entity that provides an
authentication service to a NAS.
RADIUS proxy
A RADIUS proxy acts as an authentication server to the NAS, and a
RADIUS client to the RADIUS server.
1.3. Requirements Language
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].
2. RADIUS Data Model
The Remote Authentication Dial In User Service (RADIUS) defined in
[RFC2865] [RFC2866] utilizes elements known as attributes, in order
to represent authentication, authorization and accounting (AAA) data.
Unlike SNMP, first defined in [RFC1157] [RFC1155], RADIUS does not
define a formal data definition language. A handful of basic data
types are provided, and a data type is associated with an attribute
when that attribute is defined.
Two distinct attribute spaces are defined: the standard space, and a
vendor specific space. Attributes in the standard space generally
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are composed of a type, length, value (TLV) triplet, although complex
attributes have also been defined. The vendor specific space is
encapsulated within a single attribute type (Vendor-Specific
Attribute or VSA). The format of this space is defined by individual
vendors, but the same TLV encoding used by the standard space is
recommended in [RFC2865] Section 5.26. The similarity between
attribute formats has enabled implementations to leverage common
parsing functionality, although in some cases the attributes in the
vendor specific space have begun to diverge from the common format.
2.1. Standard Space
The following subsections describe common data types and formats
within the RADIUS standard attribute space. Common exceptions are
identified.
2.1.1. Basic Data Types
The data type of RADIUS attributes is not transported on the wire.
Rather, the data type of a RADIUS attribute is fixed when that
attribute is defined. Based on the RADIUS attribute type code,
RADIUS clients and servers can determine the data type based on pre-
configured entries within a data dictionary.
RFC 2865 [RFC2865] defines the following data types:
text 1-253 octets containing UTF-8 encoded 10646 [RFC3629]
characters. Text of length zero (0) MUST NOT be sent;
omit the entire attribute instead.
string 1-253 octets containing binary data (values 0 through
255 decimal, inclusive). Strings of length zero (0)
MUST NOT be sent; omit the entire attribute instead.
IPv4 address 32 bit value, most significant octet first.
integer 32 bit unsigned value, most significant octet first.
time 32 bit unsigned value, most significant octet first
-- seconds since 00:00:00 UTC, January 1, 1970.
In addition to these data types, follow-on RADIUS specifications
define attributes using the following additional types:
IPv6 address 128 bit value, most significant octet first.
IPv6 prefix 8 bits of reserved, 8 bits of prefix length, up to
128 bits of value, most significant octet first.
integer64 64 bit unsigned value, most significant octet first.
Examples of the IPv6 address type include NAS-IPv6-Address defined in
[RFC3162] Section 2.1 and Login-IPv6-Host defined in [RFC3162]
Section 2.4. The IPv6 prefix type is used in [RFC3162] Section 2.3,
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and in [RFC4818] Section 3. The integer64 type is used for the ARAP-
Challenge-Response Attribute defined in [RFC2869] Section 5.15, and
the Framed-Interface-Id Attribute defined in [RFC3162] Section 2.2.
[RFC4675] Section 2.4 defines User-Priority-Table as 64-bits in
length, but denotes it as type "String".
Given that attributes of type IPv6 address, IPv6 prefix, and
integer64 are already in use, it is RECOMMENDED that RADIUS server
implementations include support for these additional basic types, in
addition to the types defined in [RFC2865].
It is worth noting that since RADIUS only supports unsigned integers
of 32 or 64 bits, attributes utilizing signed integer data types or
unsigned integer types of other sizes will require code changes, and
SHOULD be avoided.
2.1.2. Tagging Mechanism
[RFC2868] defines an attribute grouping mechanism based on the use of
a one octet tag value. Tunnel attributes that refer to the same
tunnel are grouped together by virtue of using the same tag value.
This tagging mechanism has some drawbacks. There are a limited
number of unique tags (31). The tags are not well suited for use
with arbitrary binary data values because it is not always possible
to tell if the first byte after the Length is the tag or the first
byte of the untagged value (assuming the tag is optional).
When integer values are tagged, the value portion is reduced to three
bytes meaning only 24-bit numbers can be represented.
The tagging mechanism does not offer an ability to create nested
groups of attributes.
Some RADIUS implementations treat tagged attributes as an additional
data type.
2.1.3. Complex Attribute Usage
The RADIUS attribute encoding is summarized in [RFC2865]:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Type | Length | Value ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
However, some standard attributes do not follow this format.
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Attributes that utilize sub-fields instead of utilizing a basic data
type are known as "complex attributes". As described below,
definition of complex attributes can lead to interoperability and
deployment issues, so that they need to introduced with care.
In general, complex attributes sent from the RADIUS server to the
client can be supported by concatenating the values into a String
data type field. However, separating these values into different
attributes, each with its own type and length, would make it easier
for the user to enter the data, would enable additional error
checking and would simplify implementations by eliminating special
case, attribute specific parsing.
One of the fundamental goals of the RADIUS protocol design was to
allow RADIUS servers to be configured to support new attributes
without requiring server code changes. RADIUS server implementations
typically utilize a data dictionary providing support for basic data
types, enabling a new attribute to be supported by addition of a
dictionary entry, without requiring RADIUS server code changes.
On the RADIUS client, code changes are typically required in order to
implement a new attribute, since the RADIUS client typically has to
compose the attribute dynamically when sending. When receiving, a
RADIUS client needs to be able to parse the attribute and carry out
the requested service, so that a detailed understanding of the new
attribute is required.
Given this, the introduction of a new basic or complex attribute will
typically require code changes on the RADIUS client, although the
magnitude of changes for the complex attribute could be greater, due
to the potential need for custom parsing logic.
However, the RADIUS server can be configured to send a new attribute
by entering its type and data format in the RADIUS server dictionary,
then filling in the value within a form based on the data type. For
complex attribute types not supported by RADIUS server dictionaries,
changes to the dictionary and forms code can be required in order to
allow the new attribute to be supported and configured by the RADIUS
server.
Code changes can also be required in the RADIUS server's receive
path, due to limitations in RADIUS server policy languages, which
typically only provide for limited operations (such as comparisons or
arithmetic operations) on the basic data types. Most existing RADIUS
policy languages typically are not capable of parsing sub-elements,
or providing sophisticated matching functionality.
Given these limitations, the introduction of complex attributes can
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require code changes on the RADIUS server which would be unnecessary
if basic data types were used instead. In addition, attribute-
specific parsing means more complex software to develop and maintain,
and more complexity can lead to more error prone implementations.
This can increase costs to network administrators as well as reducing
reliability and introducing deployment barriers. As a result, the
introduction of new complex data types within RADIUS attribute
specifications SHOULD be avoided.
The exception to this recommendation are attributes which can be
treated as opaque data, such as the EAP-Message attribute, defined in
[RFC3579] Section 3.1. Since these attributes do not need to be
parsed by the RADIUS server, the issues arising from policy language
limitations do not arise. Similarly, since these attributes can be
configured on the server using a data type of String, dictionary
limitations are also not encountered.
An examination of existing RADIUS RFCs discloses a number of complex
attributes that have already been defined. Appendix A includes a
listing of complex attributes utilized within [RFC2865], [RFC2868],
[RFC2869], [RFC3162], [RFC4818], and [RFC4675].
As can be seen in Appendix A, in most cases complex attributes
involve authentication or security functionality that requires code
changes on both the RADIUS client and server, regardless of the
attribute format. As a result, in most cases the use of complex
attributes did not create additional interoperability or deployment
issues.
In other cases the data are described textually. This is possible
because the data types are not sent within the attributes, but are a
matter for endpoint interpretation. An implementation can define
additional data types (e.g. IPv6 address), and use these data types
today by matching them to the attribute's textual description.
2.2. Vendor Space
As noted in [RFC2865] Section 5.26, the VSA format is defined as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Vendor-Id
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Vendor-Id (cont) | String...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
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The high-order octet of the Vendor-Id field is 0 and the low-order 3
octets are the SMI Network Management Private Enterprise Code of the
Vendor in network byte order.
While the format of the String field is defined by the vendor,
[RFC2865] Section 5.26 notes:
It SHOULD be encoded as a sequence of vendor type / vendor length
/ value fields, as follows. The Attribute-Specific field is
dependent on the vendor's definition of that attribute. An
example encoding of the Vendor-Specific attribute using this
method follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Vendor-Id
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Vendor-Id (cont) | Vendor type | Vendor length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute-Specific...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Multiple sub-attributes MAY be encoded within a single Vendor-
Specific attribute, although they do not have to be.
Note that the Vendor type field in the recommended format, like the
RADIUS type field, is only a single octet. While this results in an
efficient encoding, there are situations in which a vendor or SDO
will eventually wish to define more than 255 attributes. Also, an
SDO can be comprised of multiple subgroups, each of whom can desire
autonomy over the definition of attributes within their group. In
such a situation, a 16-bit Vendor type field would be more
appropriate:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Vendor-Id
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Vendor-Id (cont) | Vendor type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vendor length | Attribute-Specific...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Other attribute formats are NOT RECOMMENDED. Examples of NOT
RECOMMENDED formats include Vendor types of more than 16 bits, Vendor
lengths of less than 8 bits, Vendor lengths of more than 8 bits, and
Vendor-Specific contents that are not in Type-Length-Value format.
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3. Data Model Issues
Since the closure of the RADIUS Working Group, the popularity and
prevalence of RADIUS has continued to grow. In addition to
increasing demand for allocation of attributes within the RADIUS
standard attribute space, the number of vendors and SDOs creating new
attributes within the vendor-specific attribute space has grown, and
this has lead to some divergence in approaches to RADIUS attribute
design.
In general, standard RADIUS attributes have a more constrained data
model than attributes within the vendor space. For example, vendors
have evolved the data model to support new functions such as
attribute grouping and attribute fragmentation, with different
vendors taking different approaches.
Given these enhancements, it has become difficult for vendors or SDOs
to translate attributes from the vendor space to the more stringent
standards space. For example, a vendor-specific attribute utilizing
sub-elements could require allocation of several standard space
attributes utilizing basic data types. In this case not only would
translation require substantial additional work, it would further
deplete the RADIUS standard attribute space. Given these
limitations, translation of vendor attributes to the standards space
is not necessarily desirable, particularly if the VSA specification
is publicly available and can be implemented within existing RADIUS
clients and servers. In such situations the costs may substantially
outweigh the benefits. While it is possible that some of the
enhancements made within the vendor space may eventually become
available within the standard attribute space, the divergence of the
standard and vendor attribute spaces is most likely a permanent
feature, and should be recognized as such.
For future work, any extensions to the RADIUS data model should be
used to minimize the use of complex attributes.
3.1. Vendor Space
The usage model for RADIUS VSAs is described in [RFC2865] Section
6.2:
Note that RADIUS defines a mechanism for Vendor-Specific
extensions (Attribute 26) and the use of that should be encouraged
instead of allocation of global attribute types, for functions
specific only to one vendor's implementation of RADIUS, where no
interoperability is deemed useful.
Nevertheless, many new attributes have been defined in the vendor
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specific space in situations where interoperability is not only
useful, but is required. For example, Standards Development
Organizations (SDOs) outside the IETF (such as the IEEE 802 and the
3rd Generation Partnership Project (3GPP)) have been assigned Vendor-
Ids, enabling them to define their own VSA format and assign Vendor
types within their own space.
The utilization of VSAs by SDOs outside the IETF has gained in
popularity for several reasons:
Efficiency
As with SNMP, which defines an "Enterprise" Object Identifier (OID)
space suitable for use by vendors as well as other SDOs, the
definition of RADIUS attributes has become a common occurrence as
part of standards activity outside the IETF. For reasons of
efficiency, it is easiest for RADIUS attributes required to manage
a standard to be developed within the same SDO that develops the
standard itself. As noted in "Transferring MIB Work from IETF
Bridge MIB WG to IEEE 802.1 WG" [RFC4663], today few vendors are
willing to simultaneously fund individuals to participate within an
SDO to complete a standard, as well as to participate in IETF in
order to complete the associated RADIUS attributes specification.
Attribute scarcity
The standard RADIUS attribute space is limited to approximately 250
unique attributes; of these, only about half remain available for
allocation. In the vendor specific space, the number of attributes
available is a function of the format of the attribute (the size of
the type field).
Along with these advantages, some limitations of VSA usage are noted
in [RFC2865] Section 5.26:
This Attribute is available to allow vendors to support their own
extended Attributes not suitable for general usage. It MUST not
affect the operation of the RADIUS protocol.
Servers not equipped to interpret the vendor-specific information
sent by a client MUST ignore it (although it may be reported).
Clients which do not receive desired vendor-specific information
SHOULD make an attempt to operate without it, although they may do
so (and report they are doing so) in a degraded mode.
The limitation on changes to the RADIUS protocol effectively
prohibits VSAs from changing fundamental aspects of RADIUS operation,
such as modifying RADIUS packet sequences, or adding new commands.
However, the requirement for clients and servers to be able to
operate in the absence of VSAs has proved less of a constraint, since
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it is still possible for a RADIUS client and server to mutually
indicate support for VSAs, after which behavior expectations can be
reset.
Therefore, RFC 2865 provides considerable latitude for development of
new attributes within the vendor space, while prohibiting development
of protocol variants. This flexibility implies that RADIUS
attributes can often be developed within the vendor space without
loss (and possibly even gain) in functionality.
As a result, translation of RADIUS attributes developed within the
vendor space into the standard space may provide only modest
benefits, while accelerating the exhaustion of the standard attribute
space. Rather than expecting all RADIUS attribute specifications
requiring interoperability to be developed within the IETF and
expecting that they be allocated within the standards space, a more
scalable approach is to recognize the flexibility of the vendor space
while working toward improvements in the quality and availability of
RADIUS attribute specifications, regardless of where they are
developed.
In particular, it is RECOMMENDED that RADIUS Attribute specifications
allocate attributes from the vendor space, rather than requesting an
allocation from the RADIUS standard attribute space, for attributes
matching any of the following criteria:
* attributes relying on data types not defined within RADIUS *
attributes intended primarily for use within an SDO * attributes
intended primarily for use within a group of SDOs.
3.2. Polymorphic Attributes
A polymorphic attribute is one whose format is dynamic. For example,
rather than using a fixed data format, an attribute's format might
change based on the contents of another attribute. Or, the meaning
of an attribute may be dependent on earlier packets in a sequence.
Typically RADIUS server dictionary entries are static, enabling the
user to enter the contents of an attribute, without support for
changing the format based on dynamic conditions. However, this does
not prevent implementations from returning different attributes based
on the contents of received attributes; this is a common feature of
existing RADIUS implementations.
In general, polymorphism is NOT RECOMMENDED. Polymorphism rarely
enables capabilities that would not be available through use of
multiple attributes, while requiring code changes in the RADIUS
server in situations where attributes with fixed formats will not.
Thus, polymorphism increases complexity while decreasing generality,
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without delivering any corresponding benefits.
Note that changing an attribute's format dynamically is not the same
thing as utilizing a fixed format and computing the attribute itself
dynamically. RADIUS authentication attributes such as User-Password,
EAP-Message, etc. while being computed dynamically, utilize a fixed
format.
4. IANA Considerations
This document defines the use of a RADIUS type 26 attribute code in
the Diameter Protocol space as defined in [RFC3588] and [RFC4005].
5. Security Considerations
This specification provides guidelines for the design of RADIUS
attributes used in authentication, authorization and accounting.
Threats and security issues for this application are described in
[RFC3579] and [RFC3580]; security issues encountered in roaming are
described in [RFC2607].
Encryption of RADIUS attributes on a per-attribute basis is necessary
in some cases. The current standard mechanism for this is described
in [RFC2865] Section 5.2 (for obscuring User-Password values) and is
based on the MD5 algorithm specified in [RFC1321]. The MD5 algorithm
has recently become a focus of scrutiny and concern in security
circles, and as a result, the use of this technique in new attributes
is NOT RECOMMENDED.
Where new RADIUS attributes utilize cryptographic algorithms,
algorithm negotiation SHOULD be supported. Specification of a
mandatory-to-implement algorithm is REQUIRED, and it is RECOMMENDED
that the mandatory-to-implement algorithm be certifiable under FIPS
140.
6. References
6.1. 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.
[RFC3575] Aboba, B., "IANA Considerations for RADIUS (Remote
Authentication Dial In User Service)", RFC 3575, July 2003.
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[RFC4005] Calhoun, P., Zorn, G., Spence, D., and D. Mitton, "Diameter
Network Access Server Application", RFC 4005, August 2005.
6.2. Informative References
[RFC1155] Rose, M. and K. McCloghrie, "Structure and identification of
management information for TCP/IP-based internets", STD 16,
RFC 1155, May 1990.
[RFC1157] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple
Network Management Protocol (SNMP)", STD 15, RFC 1157, May
1990.
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M.,
and I. Goyret, "RADIUS Attributes for Tunnel Protocol
Support", RFC 2868, June 2000.
[RFC2869] Rigney, C., Willats, W., and P. Calhoun, "RADIUS Extensions",
RFC 2869, June 2000.
[RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC
3162, August 2001.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication Dial
In User Service) Support For Extensible Authentication
Protocol (EAP)", RFC 3579, September 2003.
[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G., Roese, J., "IEEE
802.1X Remote Authentication Dial In User Service (RADIUS)
Usage Guidelines", RFC3580, September 2003.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC 3629, November 2003.
[RFC4181] Heard, C., "Guidelines for Authors and Reviewers of MIB
Documents", RFC 4181, September 2005.
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[RFC4663] Harrington, D., "Transferring MIB Work from IETF Bridge MIB WG
to IEEE 802.1 WG", RFC 4663, September 2006.
[RFC4675] Congdon, P., Sanchez, M. and B. Aboba, "RADIUS Attributes for
Virtual LAN and Priority Support", RFC 4675, September 2006.
[RFC4818] Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix
Attribute", RFC 4818, April 2007.
[FIXES] Nelson, D. and DeKok, A, "Common Remote Authentication Dial In
User Service (RADIUS) Implementation Issues and Suggested
Fixes", RFC XXXX, DATE YYYY
[IEEE-802.1Q]
IEEE Standards for Local and Metropolitan Area Networks: Draft
Standard for Virtual Bridged Local Area Networks,
P802.1Q-2003, January 2003.
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Appendix A - Complex Attributes
This section summarizes RADIUS attributes with complex data types
that are defined with existing RFCs.
A.1 CHAP-Password
[RFC2865] Section 5.3 defines the CHAP-Password Attribute which is
sent from the RADIUS client to the RADIUS server in an Access-
Request. The the data type of the CHAP Identifier is not given, only
the one octet length:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
| Type | Length | CHAP Ident | String ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
Since this is an authentication attribute, code changes would have
been required on the RADIUS client and server, regardless of the
attribute format.
A.2 CHAP-Challenge
[RFC2865] Section 5.40 defines the CHAP-Challenge Attribute which is
sent from the RADIUS client to the RADIUS server in an Access-
Request. While the data type of the CHAP Identifier is given, the
text also says
If the CHAP challenge value is 16 octets long it MAY be placed in
the Request Authenticator field instead of using this attribute.
Defining attributes to contain values taken from the RADIUS packet
header is NOT RECOMMENDED. Attributes should have values that are
packed into a RADIUS AVP.
A.3 Tunnel-Password
[RFC2868] Section 3.5 defines the Tunnel-Password Attribute, which is
sent from the RADIUS server to the client in an Access-Accept. This
attribute includes Tag and Salt fields, as well as a String field
which consists of three logical sub-fields: the Data-Length (one
octet) and Password sub-fields (both of which are required), and the
optional Padding sub-field. The attribute appears as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Type | Length | Tag | Salt
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Salt (cont) | String ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Given that the String field is encrypted, this attribute would have
required code changes on the RADIUS client and server, regardless of
the format.
A.4 ARAP-Password
[RFC2869] Section 5.4 defines the ARAP-Password attribute, which is
sent from the RADIUS client to the server in an Access-Request. It
contains four 4 octet values, instead of having a single Value field:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value4
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As with the CHAP-Password attribute, this is an authentication
attribute which would have required code changes on the RADIUS client
and server regardless of format.
A.5 ARAP-Features
[RFC2869] Section 5.5 defines the ARAP-Features Attribute, which is
sent from the RADIUS server to the client in an Access-Accept or
Access-Challenge. It contains a compound string of two single octet
values, plus three 4-octet values, which the RADIUS client
encapsulates in a feature flags packet in the ARAP protocol:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value1 | Value2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value3 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| Value4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unlike previous attributes, this attribute contains no encrypted
component nor is it directly involved in authentication. The
individual sub-fields therefore could have been encapsulated in
separate attributes, although this would have required creation of an
8 bit data type.
A.6 Connect-Info
[RFC2869] Section 5.11 defines the Connect-Info attribute, which is
used to indicate the nature of the connection.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Text...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Even though the type is Text, the rest of the description indicates
that it is a complex attribute:
The Text field consists of UTF-8 encoded 10646 [8] characters.
The connection speed SHOULD be included at the beginning of the
first Connect-Info attribute in the packet. If the transmit and
receive connection speeds differ, they may both be included in the
first attribute with the transmit speed first (the speed the NAS
modem transmits at), a slash (/), the receive speed, then
optionally other information.
For example, "28800 V42BIS/LAPM" or "52000/31200 V90"
More than one Connect-Info attribute may be present in an
Accounting-Request packet to accommodate expected efforts by ITU
to have modems report more connection information in a standard
format that might exceed 252 octets.
This attribute contains no encrypted component nor is it directly
involved in authentication. The individual sub-fields therefore
could have been encapsulated in separate attributes
A.7 Framed-IPv6-Prefix
[RFC3162] Section 2.3 defines the Framed-IPv6-Prefix Attribute and
[RFC4818] Section 3 reuses this format for the Delegated-IPv6-Prefix
Attribute; these attributes are sent from the RADIUS server to the
client in an Access-Accept.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved | Prefix-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prefix
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prefix
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prefix
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Prefix |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The sub-fields encoded in these attributes are strongly related, and
there was no previous definition of this data structure that could be
referenced. Support for this attribute requires code changes on both
the client and server, due to a new data type being defined. In this
case it appears to be acceptable to encode them in one attribute.
A.8 Egress-VLANID
[RFC4675] Section 2.1 defines the Egress-VLANID Attribute which can
be sent by a RADIUS client or server.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Value
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Value (cont) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
While it appears superficially to be of type Integer, the Value field
is actually a packed structure, as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tag Indic. | Pad | VLANID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The length of the VLANID field is defined by the [IEEE-802.1Q]
specification. The Tag indicator field is either 0x31 or 0x32, for
compatibility with the Egress-VLAN-Name, as discussed below. The
complex structure of Egress-VLANID overlaps with that of the base
Integer data type, meaning that no code changes are required for a
RADIUS server to support this attribute. Code changes are required
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INTERNET-DRAFT RADIUS Design Guidelines 4 September 2007
on the NAS, if only to implement the VLAN ID enforcement.
Given the IEEE VLAN requirements and the limited data model of
RADIUS, the chosen method is likely the best of the possible
alternatives.
A.9 Egress-VLAN-Name
[RFC4675] Section 2.3 defines the Egress-VLAN-Name Attribute which
can be sent by a RADIUS client or server.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Tag Indic. | String...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Tag Indicator is either the character '1' or '2', which in ASCII
map to the identical values for Tag Indicator in Egress-VLANID,
above. The complex structure of this attribute is acceptable for
reasons identical to those given for Egress-VLANID.
Acknowledgments
We would like to acknowledge David Nelson, Bernard Aboba, Emile van
Bergen, Barney Wolff and Glen Zorn for contributions to this
document.
Authors' Addresses
Greg Weber
Cisco Systems
10850 Murdock Road
Knoxville, TN 37932
USA
Email: gdweber@cisco.com
Alan DeKok
The FreeRADIUS Server Project
http://freeradius.org/
Email: aland@freeradius.org
Weber, et al. Best Current Practice [Page 20]
INTERNET-DRAFT RADIUS Design Guidelines 4 September 2007
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
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OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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Acknowledgment
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Weber, et al. Best Current Practice [Page 21]
INTERNET-DRAFT RADIUS Design Guidelines 4 September 2007
Open issues
Open issues relating to this document are tracked on the following
web site:
http://www.drizzle.com/~aboba/RADEXT/
Weber, et al. Best Current Practice [Page 22]
