One document matched: draft-ietf-radext-design-09.txt
Differences from draft-ietf-radext-design-08.txt
Network Working Group Alan DeKok (ed.)
INTERNET-DRAFT FreeRADIUS
Category: Best Current Practice G. Weber
<draft-ietf-radext-design-09.txt> Individual Contributor
Expires: April 12, 2010
12 October 2009
RADIUS Design Guidelines
draft-ietf-radext-design-09
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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 ............................................. 4
1.1. Terminology ......................................... 4
1.2. Requirements Language ............................... 4
1.3. Applicability ....................................... 5
2. RADIUS Data Model ........................................ 6
2.1. Standard Space ...................................... 6
2.1.1. Basic Data Types ............................... 6
2.1.2. Tagging Mechanism .............................. 8
2.1.3. Complex Attribute Usage ........................ 8
2.1.4. Complex Attributes and Security ................ 11
2.1.5. Service definitions and RADIUS ................. 11
2.2. Vendor Space ........................................ 12
3. Data Model Issues ........................................ 14
3.1. Vendor Space ........................................ 14
3.1.1. Interoperability Considerations ................ 16
3.1.2. Vendor Allocations ............................. 17
3.1.3. SDO Allocations ................................ 17
3.1.4. Publication of specifications .................. 18
3.2. Polymorphic Attributes .............................. 18
3.3. RADIUS Operational Model ............................ 19
4. IANA Considerations ...................................... 22
5. Security Considerations .................................. 22
6. References ............................................... 23
6.1. Normative References ................................ 23
6.2. Informative References .............................. 23
Appendix A - Design Guidelines ............................... 26
A.1. Types matching the RADIUS data model ................. 26
A.1.1. Transport of simple data ........................ 26
A.1.2. Transport of Authentication and Security Data ... 27
A.1.3. Opaque data types ............................... 27
A.2. Improper Data Types .................................. 27
A.2.1. Simple Data Types ............................... 28
A.2.2. Complex Data Types .............................. 29
A.3. Vendor-Specific formats .............................. 29
A.4. Changes to the RADIUS Operational Model .............. 29
A.5. Allocation of attributes ............................. 31
Appendix B - Complex Attributes .............................. 32
B.1. CHAP-Password ........................................ 32
B.2. CHAP-Challenge ....................................... 32
B.3. Tunnel-Password ...................................... 32
B.4. ARAP-Password ........................................ 33
B.5. ARAP-Features ........................................ 33
B.6. Connect-Info ......................................... 34
B.7. Framed-IPv6-Prefix ................................... 35
B.8. Egress-VLANID ........................................ 35
B.9. Egress-VLAN-Name ..................................... 36
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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 SDOs. By articulating
RADIUS design guidelines, it is hoped that this document will
encourage the development and publication of high quality RADIUS
attribute specifications.
However, the advice in this document will not be helpful unless it is
put to use. As with "Guidelines for Authors and Reviewers of MIB
Documents" [RFC4181], it is expected that this document will be used
by authors to check their document against the guidelines prior to
requesting review (such as an "Expert Review" described in
[RFC3575]). Similarly, it is expected that this document will used
by reviewers (such as WG participants or the AAA Doctors [DOCTORS]),
resulting in an improvement in 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.
1.1. 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, authorization, and/or accounting (AAA)
server is an entity that provides one or more AAA services to a
NAS.
RADIUS proxy
A RADIUS proxy acts as a RADIUS server to the NAS, and a RADIUS
client to the RADIUS server.
1.2. 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].
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1.3. Applicability
As RADIUS has become more widely accepted as a management protocol,
its usage has become more prevalent, both within the IETF as well as
within other SDOs. Given the expanded utilization of RADIUS, it has
become apparent that requiring SDOs to accomplish all their RADIUS
work within the IETF is inherently inefficient and unscalable. By
articulating guidelines for RADIUS attribute design, this document
enables SDOs out of the IETF to design their own RADIUS attributes
within the Vendor-Specific Attribute (VSA) space.
It is RECOMMENDED that SDOs follow the guidelines articulated in this
document. Doing so will ensure the widest possible applicability and
interoperability of the specifications, while requiring minimal
changes to existing systems. Specifications that do not follow the
guidelines articulated herein are NOT RECOMMENDED. However, we
recognize that there are some situations where SDOs or vendors
require the creation of specifications not following these
guidelines. We do not forbid these specifications, but it is
RECOMMENDED that they are created only if they have a limited scope
of applicability, and all attributes defined in those specifications
are VSAs, as discussed Appendix A.5, below.
It is RECOMMENDED that SDOs and vendors seek review of RADIUS
attribute specifications from the IETF. However, when specifications
are SDO specific, re-use existing data types, and follow these
guidelines, they do not require IETF review.
In order to enable IETF review of such specifications, the authors
recommend that:
* SDOs make their RADIUS attribute specifications publicly
available;
* SDOs request review of RADIUS attribute specifications by
sending email to the AAA Doctors [DOCTORS] or equivalent mailing
list;
* IETF and SDO RADIUS attribute specifications are reviewed
according to the guidelines proposed in this document;
* Reviews of specifications are posted to the RADEXT WG mailing
list, the AAA Doctors mailing list [DOCTORS] or another IETF
mailing list suggested by the Operations & Management Area
Directors of the IETF.
These reviews can assist with creation of specifications that meet
the SDO requirements, and which are also compatible with the
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traditional data model and uses of RADIUS. While these reviews
require access to public specifications, the review process does not
require publication of an IETF RFC.
The advice in this document applies to attributes used to encode
service-provisioning or authentication data. RADIUS protocol
changes, or specification of attributes (such as Service-Type) that
can be used to, in effect, provide new RADIUS commands require
greater expertise and deeper review, as do changes to the RADIUS
operational model as discussed below in Section 3.3. Such changes
MUST NOT be undertaken outside the IETF and when handled within the
IETF require "IETF Consensus" for adoption, as noted in [RFC3575]
Section 2.1.
2. RADIUS Data Model
The Remote Authentication Dial In User Service (RADIUS) defined in
[RFC2865] and [RFC2866] uses elements known as attributes in order to
represent authentication, authorization and accounting data.
Unlike SNMP, first defined in [RFC1157] and [RFC1155], RADIUS does
not define a formal data definition language. A handful of basic
data types are in common use, and a data type is associated with an
attribute when the attribute is defined.
Two distinct attribute spaces are defined: the standard space, and a
Vendor-Specific space. Attributes in the standard space generally
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). 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,
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RADIUS clients and servers can determine the data type based on pre-
configured entries within a data dictionary.
[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, in network byte order.
integer 32 bit unsigned value, in network byte order.
time 32 bit unsigned value, in network byte order.
-- 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, in network byte order.
IPv6 prefix 8 bits of reserved, 8 bits of prefix length, up to
128 bits of value, in network byte order.
integer64 64 bit unsigned value, in network byte order
This type has also been used to represent an IPv6
interface identifier.
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,
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].
Where the intent is to represent a specific IPv6 address, the IPv6
address type SHOULD be used. Although it is possible to use the IPv6
IPv6 Prefix type with a prefix length of 128 to represent an IPv6
address, this usage is NOT RECOMMENDED.
It is worth noting that since RADIUS only supports unsigned integers
of 32 or 64 bits, attributes using signed integer data types or
unsigned integer types of other sizes will require code changes, and
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SHOULD be avoided.
For [RFC2865] RADIUS VSAs, the length limitation of the String and
Text types is 247 octets instead of 253 octets, due to the additional
overhead of the Vendor-Specific Attribute.
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).
Other limitations of the tagging mechanism are that 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
having additional data types tagged-string and tagged-integer. These
types increase the complexity of implementing and managing RADIUS
systems.
For these reasons, the tagging scheme described in RFC 2868 is NOT
RECOMMENDED for use as a generic grouping mechanism.
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.
Attributes that use sub-fields instead of using a basic data type are
known as "complex attributes". As described below, the definition of
complex attributes can lead to interoperability and deployment
issues, so they need to be 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
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data type field. However, separating these values into different
attributes, each with its own type and length, would have the
following benefits:
* it is easier for the user to enter the data as well-known
types, rather than complex structures;
* it enables additional error checking by leveraging the
parsing and validation routines for well-known types;
* it simplifies 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 provide support for basic data types, and define attributes
in a data dictionary. This architecture enables a new attribute to
be supported by the 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. 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. As a result, a detailed understanding of the
new attribute is required on clients, and data dictionaries are less
useful on clients than on servers.
Given these considerations, the introduction of a new basic or
complex attribute will typically require code changes on the RADIUS
client. The magnitude of changes for the complex attribute could be
greater, due to the potential need for custom parsing logic.
The RADIUS server can be configured to send a new static attribute by
entering its type and data format in the RADIUS server dictionary,
and then filling in the value within a policy based on the attribute
name, data type and type-specific value. For complex attribute types
not supported by RADIUS server dictionaries, changes to the
dictionary code can be required in order to allow the new attribute
to be supported by and configured on the RADIUS server.
Code changes can also be required in policy management and in the
RADIUS server's receive path. These changes are 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. Many existing RADIUS policy languages
typically are not capable of parsing sub-elements, or providing
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sophisticated matching functionality.
Given these limitations, the introduction of complex attributes can
require code changes on the RADIUS server which would be unnecessary
if basic data types had been used instead. In addition, attribute-
specific parsing means more complex software to develop and maintain.
More complexity can lead to more error prone implementations,
interoperability problems, and even security vulnerabilities. These
issues 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, except in the case of
complex attributes involving authentication or security
functionality.
As can be seen in Appendix B, most of the existing complex attributes
involve authentication or security functionality. Supporting this
functionality 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 to represent these methods is
acceptable, and does not create additional interoperability or
deployment issues.
The only other exception to the recommendation against complex types
is for types that can be treated as opaque data by the RADIUS server.
For example, the EAP-Message attribute, defined in [RFC3579] Section
3.1 contains a complex data type that is an EAP packet. Since these
complex types do not need to be parsed by the RADIUS server, the
issues arising from policy language limitations do not arise.
Similarly, since attributes of these complex types can be configured
on the server using a data type of String, dictionary limitations are
also not encountered. Appendix A.1 below includes a series of
checklists that may be used to analyze a design for RECOMMENDED and
NOT RECOMMENDED behavior in relation to complex types.
If the RADIUS Server simply passes the contents of an attribute to
some non-RADIUS portion of the network, then the data is opaque, and
SHOULD be defined to be of type String. A concrete way of judging
this requirement is whether or not the attribute definition in the
RADIUS document contains delineated fields for sub-parts of the data.
If those fields need to be delineated in RADIUS, then the data is not
opaque, and it SHOULD be separated into individual RADIUS attributes.
An examination of existing RADIUS RFCs discloses a number of complex
attributes that have already been defined. Appendix B includes a
listing of complex attributes used within [RFC2865], [RFC2868],
[RFC2869], [RFC3162], [RFC4818], and [RFC4675]. The discussion of
these attributes includes reasons why a complex type is acceptable,
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or suggestions for how the attribute could have been defined to
follow the RADIUS data model.
In other cases, the data in the complex type 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, and use these data
types today by matching them to the attribute's textual description.
2.1.4. Complex Attributes and Security
The introduction of complex data types brings the potential for the
introduction of new security vulnerabilities. Experience shows that
the common data types have few security vulnerabilities, or else that
all known issues have been found and fixed. New data types require
new code, which may introduce new bugs, and therefore new attack
vectors.
RADIUS servers are highly valued targets, as they control network
access and interact with databases that store usernames and
passwords. An extreme outcome of a vulnerability due to a new,
complex type would be that an attacker is capable of taking complete
control over the RADIUS server.
The use of attributes representing opaque data does not reduce this
threat. The threat merely moves from the RADIUS server to the
application that consumes that opaque data.
The threat is particularly severe when the opaque data originates
from the user, and is not validated by the NAS. In those cases, the
RADIUS server is potentially exposed to attack by malware residing on
an unauthenticated host. Applications consuming opaque data that
reside on the RADIUS server SHOULD be properly isolated from the
RADIUS server, and SHOULD run with minimal privileges. Any potential
vulnerabilities in that application will then have minimal impact on
the security of the system as a whole.
2.1.5. Service definitions and RADIUS
RADIUS specifications define how an existing service or protocol can
be provisioned using RADIUS. Therefore, it is expected that a RADIUS
attribute specification will reference documents defining the
protocol or service to be provisioned. Within the IETF, a RADIUS
attribute specification SHOULD NOT be used to define the protocol or
service being provisioned. New services using RADIUS for
provisioning SHOULD be defined elsewhere and referenced in the RADIUS
specification.
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New attributes, or new values of existing attributes, SHOULD NOT be
used to define new RADIUS commands. RADIUS attributes are intended
to:
* authenticate users
* authorize users (i.e., service provisioning or changes to
provisioning)
* account for user activity (i.e., logging of session activity)
New commands (i.e., the Code field in the packet header) are
allocated only through "IETF Consensus" as noted in [RFC3575] Section
2.1. Specifications also SHOULD NOT use new attributes to modify the
interpretation of existing RADIUS commands.
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...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
The high-order octet of the Vendor-Id field is 0 and the low-order 3
octets are the Structure of Management Information (SMI) Network
Management Private Enterprise Code (PEC) 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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 VSA format is only
a single octet, like the RADIUS type field. While this limitation
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. The most interoperable way to address these issues is
for the vendor or SDO to request allocation of multiple Vendor
identifiers.
However, instead of doing this, vendors have defined the following
non-standard VSA formats:
* Vendor types of 16 bits, followed by an 8 bit length and
then attribute-specific data.
* Vendor types of 32 bits, followed by no length field, and
then attribute-specific data.
* Vendor types of the RFC format, but where some VSAs are
defined as "grouped" or TLV attributes. These attributes
are then used to carry sub-attributes.
* "Bare" ASCII strings that immediately follow the Vendor-Id,
without using a Vendor type or Vendor length.
All VSA schemes that do not follow the [RFC2865] recommendations are
NOT RECOMMENDED. These non-standard formats will typically not be
implementable without RADIUS server code changes. This includes all
the above formats, as well as Vendor types of more than 8 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.
Although [RFC2865] does not mandate it, implementations commonly
assume that the Vendor Id can be used as a key to determine the on-
the-wire format of a VSA. Vendors therefore SHOULD NOT use multiple
formats for VSAs that are associated with a particular Vendor Id. A
vendor wishing to use multiple VSA formats SHOULD request one Vendor
Id for each VSA format that they will use.
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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
and SDOs have evolved the data model to support new functions such as
attribute grouping and attribute fragmentation, with different groups
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 using sub-
elements could require allocation of several standard space
attributes using 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. It is possible that some of the enhancements
made within the vendor space may eventually become available within
the standard attribute space. However, the divergence of the
standard and vendor attribute spaces is most likely a permanent
feature, and should be recognized as such.
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
specific space in situations where interoperability is not only
useful, but is required. For example, SDOs outside the IETF (such as
the IEEE 802 and the 3rd Generation Partnership Project (3GPP)) have
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been assigned Vendor-Ids, enabling them to define their own VSA
format and assign Vendor types within their own space.
The use 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 Vendor-Specific RADIUS attributes has become a common
occurrence as part of standards activity outside the IETF. For
reasons of efficiency, it is easiest if the RADIUS attributes
required to manage a standard are 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 the IETF in order to complete the associated RADIUS
attributes specification.
Attribute scarcity
The standard RADIUS attribute space is limited to 255 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 Vendor 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 proven to be less of a constraint,
since it is still possible for a RADIUS client and server to mutually
indicate support for VSAs, after which behavior expectations can be
reset.
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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 with 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. We do not expect that all RADIUS attribute specifications
requiring interoperability will be developed within the IETF, and
allocated from 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.
3.1.1. Interoperability Considerations
Vendors and SDOs are reminded that the standard RADIUS attribute
space, and the enumerated value space for enumerated attributes, are
reserved for allocation through work published via the IETF, as noted
in [RFC3575] Section 2.1. Some vendors and SDOs have in the past
performed self-allocation by assigning vendor-specific meaning to
"unused" values from the standard RADIUS attribute ID or enumerated
value space. This self-allocation results in interoperability
issues, and is counter-productive. Similarly, the Vendor-Specific
enumeration practice discussed in [RFC2882] Section 2.2.1 is NOT
RECOMMENDED.
If it is not possible to follow the IETF process, vendors and SDOs
SHOULD self-allocate an attribute from their Vendor-Specific space,
and define an appropriate value for it.
As a side note, [RFC2865] Section 5.26 uses the term "Vendor-Specific
Attribute" to refer to an encoding format which can be used by
individual vendors to define attributes not suitable for general
usage. However, since then VSAs have also become widely used by SDOs
defining attributes intended for multi-vendor interoperability. As
such, these attributes are not specific to any single vendor, and the
term "Vendor-Specific" may be misleading. An alternate term which
better describes this use case is SDO-Specific Attribute (SSA).
The design and specification of VSAs for multi-vendor usage SHOULD be
undertaken with the same level of care as standard RADIUS attributes.
Specifically, the provisions of this document that apply to standard
RADIUS attributes also apply to SSAs or VSAs for multi-vendor usage.
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3.1.2. Vendor Allocations
Vendor RADIUS Attribute specifications SHOULD allocate attributes
from the vendor space, rather than requesting an allocation from the
RADIUS standard attribute space.
As discussed in [RFC2865] Section 5.26, the vendor space is intended
for vendors to support their own Attributes not suitable for general
use. However, it is RECOMMENDED that vendors follow the guidelines
outlined here, which are intended to enable maximum interoperability
with minimal changes to existing systems.
Following these guidelines means that RADIUS servers can be updated
to support the vendor's equipment by editing a RADIUS dictionary. If
these guidelines are not followed, then the vendor's equipment can
only be supported via code changes in RADIUS server implementations.
Such code changes add complexity and delay to implementations.
3.1.3. SDO Allocations
SDO RADIUS Attribute specifications SHOULD 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.
Any new RADIUS attributes or values intended for interoperable use
across a broad spectrum of the Internet Community SHOULD follow the
normal IETF process, and SHOULD result in allocations from the RADIUS
standard space.
The recommendation for SDOs to allocate attributes from a vendor
space rather than via the IETF process is a recognition that SDOs may
desire to assert change control over their own RADIUS specifications.
This change control can be obtained by requesting a PEC from the
Internet Assigned Number Authority (IANA), for use as a Vendor-Id
within a Vendor-Specific attribute. Further allocation of attributes
inside of the VSA space defined by that Vendor-Id is subject solely
to the discretion of the SDO. Similarly, the use of data types
(complex or not) within that VSA space is solely under the discretion
of the SDO. It is RECOMMENDED that SDOs follow the guidelines
outlined here, which are intended to enable maximum interoperability
with minimal changes to existing systems.
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It should be understood that SDOs do not have to rehost VSAs into the
standards space solely for the purpose of obtaining IETF review.
Rehosting puts pressure on the standards space, and may be harmful to
interoperability, since it can create two ways to provision the same
service. Rehosting may also require changes to the RADIUS data model
which will affect implementations that do not intend to support the
SDO specifications.
3.1.4. Publication of specifications
SDOs are encouraged to seek early review of SSA specifications by the
IETF. This review may be initiated by sending mail to the AAA
Doctors list [DOCTORS], with the understanding that this review is a
voluntary-based service offered on best-effort basis. Since the IETF
is not a membership organization, in order to enable the RADIUS SSA
specification to be reviewed, it is RECOMMENDED that it be made
publicly available; this also encourages interoperability. Where the
RADIUS SSA specification is embedded within a larger document which
cannot be made public, the RADIUS attribute and value definitions
SHOULD be published instead as an Informational RFC, as with
[RFC4679]. This process SHOULD be followed instead of requesting
IANA allocations from within the standard RADIUS attribute space.
Similarly, vendors are encouraged to make their specifications
publicly available, for maximum interoperability. However, it is not
necessary for them to request publication of their VSA specifications
as Informational RFCs by the IETF.
All other specifications, including new authentication,
authorization, and/or security mechanisms SHOULD be allocated via the
standard RADIUS space, as noted in [RFC3575] Section 2.1.
3.2. Polymorphic Attributes
A polymorphic attribute is one whose format or meaning 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 depend on earlier packets in a
sequence.
RADIUS server dictionary entries are typically static, enabling the
user to enter the contents of an attribute without support for
changing the format based on dynamic conditions. However, this
limitation on static types does not prevent implementations from
implementing policies that return different attributes based on the
contents of received attributes; this is a common feature of existing
RADIUS implementations.
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In general, polymorphism is NOT RECOMMENDED. Polymorphism rarely
enables capabilities that would not be available through use of
multiple attributes. Polymorphism requires code changes in the
RADIUS server in situations where attributes with fixed formats would
not require such changes. Thus, polymorphism increases complexity
while decreasing generality, without delivering any corresponding
benefits.
Note that changing an attribute's format dynamically is not the same
thing as using a fixed format and computing the attribute itself
dynamically. RADIUS authentication attributes such as User-Password,
EAP-Message, etc. while being computed dynamically, use a fixed
format.
3.3. RADIUS Operational Model
The RADIUS operational model includes several assumptions:
* The RADIUS protocol is stateless;
* Provisioning of services is not possible within an
Access-Reject;
* There is a distinction between authorization checks and user
authentication;
* The protocol provices for authentication and integrity
protection of packets;
* The RADIUS protocol is a Request/Response protocol;
* The protocol defines packet length restrictions.
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 [RFC5080]
Section 2.1.1 for a more in-depth discussion of the use of the State
Attribute.
As noted in [RFC5080] Section 2.6, the intent of an Access-Reject is
to deny access to the requested service. As a result, RADIUS does
not allow the provisioning of services within an Access-Reject.
Documents which include provisioning of services within an Access-
Reject are inherently incompatible with RADIUS, and need to be
redesigned.
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As noted in [RFC5080] Section 2.1.1, a RADIUS Access-Request may not
contain user authentication attributes or a State Attribute linking
the Access-Request to an earlier user authentication. Such an
Access-Request, known as an authorization check, provides no
assurance that it corresponds to a live user. RADIUS specifications
defining attributes containing confidential information (such as
Tunnel-Password) should be careful to prohibit such attributes from
being returned in response to an authorization check. Also,
[RFC5080] Section 2.1.1 notes that authentication mechanisms need to
tie a sequence of Access-Request/Access-Challenge packets together
into one authentication session. The State Attribute is RECOMMENDED
for this purpose.
While [RFC2865] did not require authentication and integrity
protection of RADIUS Access-Request packets, subsequent
authentication mechanism specifications such as RADIUS/EAP [RFC3579]
and Digest Authentication [RFC5090] have mandated authentication and
integrity protection for certain RADIUS packets. [RFC5080] Section
2.1.1 makes this behavior RECOMMENDED for all Access-Request packets,
including Access-Request packets performing authorization checks. It
is expected that specifications for new RADIUS authentication
mechanisms will continue this practice.
The RADIUS protocol as defined in [RFC2865] is a request-response
protocol spoken between RADIUS clients and servers. A single RADIUS
Access-Request packet will solicit in response at most a single
Access-Accept, Access-Reject or Access-Challenge packet, sent to the
IP address and port of the RADIUS Client that originated the Access-
Request. Similarly, a single Change-of-Authorization (CoA)-Request
packet [RFC5176] will solicit in response at most a single CoA-ACK or
CoA-NAK packet, sent to the IP address and port of the Dynamic
Authorization Client (DAC) that originated the CoA-Request. A single
Disconnect-Request packet will solicit in response at most a single
Disconnect-ACK or Disconnect-NAK packet, sent to the IP address and
port of the Dynamic Authorization Client (DAC) that originated the
Disconnect-Request. Changes to this model are likely to require
major revisions to existing implementations and so this practice is
NOT RECOMMENDED.
The Length field in the RADIUS packet header is defined in [RFC2865]
Section 3. It is noted there that the maximum length of a RADIUS
packet is 4096 octets. As a result, attribute designers SHOULD NOT
assume that a RADIUS implementation can successfully process RADIUS
packets larger than 4096 octets.
Even when packets are less than 4096 octets, they may be larger than
the Path Maximum Transmission Unit (PMTU). Any packet larger than
the PMTU will be fragmented, making communications more brittle as
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firewalls and filtering devices often discard fragments. Transport
of fragmented UDP packets appears to be a poorly tested code path on
network devices. Some devices appear to be incapable of transporting
fragmented UDP packets, making it difficult to deploy RADIUS in a
network where those devices are deployed. We RECOMMEND that RADIUS
messages be kept as small possible.
If a situation is envisaged where it may be necessary to carry
authentication, authorization or accounting data in a packet larger
than 4096 octets, then one of the following approaches is
RECOMMENDED:
1. Utilization of a sequence of packets.
For RADIUS authentication, a sequence of Access-Request/ Access-
Challenge packets would be used. For this to be feasible,
attribute designers need to enable inclusion of attributes that
can consume considerable space within Access-Challenge packets.
To maintain compatibility with existing NASes, either the use of
Access-Challenge packets needs to be permissible (as with
RADIUS/EAP, defined in [RFC3579]), or support for receipt of an
Access-Challenge needs to be indicated by the NAS (as in RADIUS
Location [RFC5580]). Also, the specification needs to clearly
describe how attribute splitting is to be signalled and how
attributes included within the sequence are to be interpreted,
without requiring stateful operation. Unfortunately, previous
specifications have not always exhibited the required foresight.
For example, even though very large filter rules are
conceivable, the NAS-Filter-Rule Attribute defined in [RFC4849]
is not permitted in an Access-Challenge packet, nor is a
mechanism specified to allow a set of NAS-Filter-Rule attributes
to be split across an Access-Request/Access-Challenge sequence.
In the case of RADIUS accounting, transporting large amounts of
data would require a sequence of Accounting-Request packets.
This is a non-trivial change to RADIUS, since RADIUS accounting
clients would need to be modified to split the attribute stream
across multiple Accounting-Requests, and billing servers would
need to be modified to re-assemble and interpret the attribute
stream.
2. Utilization of names rather than values.
Where an attribute relates to a policy that could conceivably be
pre-provisioned on the NAS, then the name of the pre-provisioned
policy can be transmitted in an attribute, rather than the
policy itself, which could be quite large. An example of this
is the Filter-Id Attribute defined in [RFC2865] Section 5.11,
which enables a set of pre-provisioned filter rules to be
referenced by name.
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3. Utilization of Packetization Layer Path MTU Discovery
techniques, as specified in [RFC4821]. As a last resort, where
the above techniques cannot be made to work, it may be possible
to apply the techniques described in [RFC4821] to discover the
maximum supported RADIUS packet size on the path between a
RADIUS client and a home server. While such an approach can
avoid the complexity of utilization of a sequence of packets,
dynamic discovery is likely to be time consuming and cannot be
guaranteed to work with existing RADIUS implementations. As a
result, this technique is not generally applicable.
4. IANA Considerations
This document does not require that the IANA update any existing
registry or create any new registry, but includes information that
affects the IANA, which:
* includes specific guidelines for Expert Reviewers appointed
under the IANA considerations of [RFC3575]
* includes guidelines that recommend against self allocation from
the RADIUS standard attribute space in other SDO RADIUS
Attribute specifications.
* recommends that SDOs request a Private Enterprise Code (PEC)
from the IANA, for use as a Vendor-Id within a Vendor-Specific
attribute.
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].
Obfuscation 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 and SHA-1 algorithms have recently become a focus of scrutiny
and concern in security circles, and as a result, the use of these
algorithms in new attributes is NOT RECOMMENDED. In addition,
previous documents referred to this method as generating "encrypted"
data. This terminology is no longer accepted within the
cryptographic community.
Where new RADIUS attributes use cryptographic algorithms, algorithm
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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
[FIPS].
Where new RADIUS attributes encapsulate complex data types, or
transport opaque data, the security considerations discussed in
Section 2.1.4 SHOULD be addressed.
Message authentication in RADIUS is provided largely via the Message-
Authenticator attribute. See [RFC3579] Section 3.2, and also
[RFC5080] 2.2.2, which says that client implementations SHOULD
include a Message-Authenticator attribute in every Access-Request.
In general, the security of the RADIUS protocol is poor. Robust
deployments SHOULD support a secure communications protocol such as
IPSec. See [RFC3579] Section 4, and [RFC3580] Section 5 for a more
in-depth explanation of these issues.
Implementations not following the suggestions outlined in this
document may be subject to a problems such as ambiguous protocol
decoding, packet loss leading to loss of billing information, and
denial of service attacks.
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.
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.
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INTERNET-DRAFT RADIUS Design Guidelines 12 October 2009
[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.
[RFC2882] Mitton, D, "Network Access Servers Requirements: Extended
RADIUS Practices", RFC 2882, July 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.
[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.
[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.
[RFC4679] Mammoliti, V., et al., "DSL Forum Vendor-Specific RADIUS
Attributes", RFC 4679, September 2006.
[RFC4818] Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix
Attribute", RFC 4818, April 2007.
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[RFC4821] Mathis, M. and Heffner, J, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4849] Congdon, P. et al, "RADIUS Filter-Rule Attribute", RFC 4849,
April 2007.
[RFC5080] Nelson, D. and DeKok, A, "Common Remote Authentication Dial In
User Service (RADIUS) Implementation Issues and Suggested
Fixes", RFC 5080, December 2007.
[RFC5090] Sterman, B. et al., "RADIUS Extension for Digest
Authentication", RFC 5090, February 2008.
[RFC5176] Chiba, M. et al., "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 5176,
January 2008.
[DOCTORS] AAA Doctors Mailing list <aaa-doctors@ops.ietf.org>
[FIPS] FIPS 140-3 (DRAFT), "Security Requirements for Cryptographic
Modules", http://csrc.nist.gov/publications/fips/fips140-3/
[IEEE-802.1Q]
IEEE Standards for Local and Metropolitan Area Networks: Draft
Standard for Virtual Bridged Local Area Networks,
P802.1Q-2003, January 2003.
[RFC5580] Tschofenig, H. (Ed.), "Carrying Location Objects in RADIUS and
Diameter", RFC 5580, August 2009.
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Appendix A - Design Guidelines
The following text provides guidelines for the design of attributes
used by the RADIUS protocol. Specifications that follow these
guidelines are expected to achieve maximum interoperability with
minimal changes to existing systems.
A.1. Types matching the RADIUS data model
A.1.1. Transport of simple data
Does the data fit within the existing RADIUS data model, as outlined
below? If so, it SHOULD be encapsulated in a [RFC2865] format RADIUS
attribute, or in a [RFC2865] format RADIUS VSA that uses one of the
existing RADIUS data types.
* 32-bit unsigned integer, in network byte order.
* Enumerated data types, represented as a 32-bit unsigned integer
with a list of name to value mappings. (e.g., Service-Type)
* 64-bit unsigned integer, in network byte order.
* IPv4 address in network byte order.
* IPv6 address in network byte order.
* IPv6 prefix.
* time as 32 bit unsigned value, in network byte order, and in
seconds since 00:00:00 UTC, January 1, 1970.
* string (i.e., binary data), totalling 253 octets or less in
length. This includes the opaque encapsulation of data
structures defined outside of RADIUS. See also Appendix A.1.3,
below.
* UTF-8 text, totalling 253 octets or less in length.
Note that the length limitations for VSAs of type String and Text are
less than 253 octets, due to the additional overhead of the Vendor-
Specific format.
The following data also qualifies as "simple data types":
* Attributes grouped into a logical container, using the
[RFC2868] tagging mechanism. This approach is NOT
RECOMMENDED (see Section 2.1.2), but is permissible where
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the alternatives are worse.
* Attributes requiring the transport of more than 247 octets of
Text or String data. This includes the opaque encapsulation
of data structures defined outside of RADIUS. See also Section
A.1.3, below.
Nested groups or attributes do not qualify as "simple data types",
and SHOULD NOT be used.
A.1.2. Transport of Authentication and Security Data
Does the data provide authentication and/or security capabilities, as
outlined below? If so, it SHOULD be encapsulated in a [RFC2865]
format RADIUS attribute. It SHOULD NOT be encapsulated in a
[RFC2865] format RADIUS VSA.
* Complex data types that carry authentication methods which
RADIUS servers are expected to parse and verify as part of
an authentication process.
* Complex data types that carry security information intended
to increase the security of the RADIUS protocol itself.
Any data type carrying authentication and/or security data that is
not meant to be parsed by a RADIUS server is an "opaque data type",
as defined below.
A.1.3. Opaque data types
Does the attribute encapsulate an existing data structure defined
outside of the RADIUS specifications? Can the attribute be treated
as opaque data by RADIUS servers (including proxies?) If both
questions can be answered affirmatively, a complex structure MAY be
used in a RADIUS specification.
The specification of the attribute SHOULD define the encapsulating
attribute to be of type String. The specification SHOULD refer to an
external document defining the structure. The specification SHOULD
NOT define or describe the structure, as discussed above in Section
2.1.3.
A.2. Improper Data Types
All data types other than the ones described above in Appendix A.1
SHOULD NOT be used. This section describes in detail a number of
data types that are NOT RECOMMENDED in new RADIUS specifications.
Where possible, replacement data types are suggested.
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A.2.1. Simple Data Types
Does the attribute use any of the following data types? If so, the
data type SHOULD be replaced with the suggested alternatives, or it
SHOULD NOT be used at all.
* Signed integers of any size.
SHOULD NOT be used. SHOULD be replaced with one or more
unsigned integer attributes. The definition of the attribute
can contain information that would otherwise go into the sign
value of the integer.
* 8 bit unsigned integers.
SHOULD be replaced with 32-bit unsigned integer. There is
insufficient justification to save three bytes.
* 16 bit unsigned integers.
SHOULD be replaced with 32-bit unsigned integer. There is
insufficient justification to save two bytes.
* Unsigned integers of size other than 32 or 64.
SHOULD be replaced by an unsigned integer of 32 or 64 bits.
There is insufficient justification to define a new size of
integer.
* Integers of any size in non-network byte order
SHOULD be replaced by unsigned integer of 32 or 64 bits,
in network byte order. There is no reason to transport integers
in any format other than network byte order.
* Tagged data types as described in [RFC2868].
These data types SHOULD NOT be used in new specifications.
* Complex data structures defined only within RADIUS.
SHOULD NOT be used. This recommendation does not apply to new
attributes for authentication or security, as described below
in Section A.2.2.
* Multi-field text strings.
Each field SHOULD be encapsulated in a separate attribute.
* Polymorphic attributes.
Multiple attributes, each with a static data type SHOULD be
defined instead.
* Nested AVPs.
Attributes should be defined in a flat typespace, possibly using
a 16-bit Vendor type field (see Section 2.3).
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A.2.2. Complex Data Types
Does the attribute:
* define a complex data type
* That a RADIUS server and/or client is expected to parse?
i.e. A type that cannot be treated as opaque data (Section A.1.3)
* for purposes other than authentication or security?
If so, this data type SHOULD be replaced with simpler types, as
discussed above in Appendix A.2.1. Also see Section 2.1.3 for a
discussion of why complex types are problematic.
A.3. Vendor-Specific formats
Does the specification contain Vendor-Specific attributes that match
any of the following criteria? If so, the data type should be
replaced with the suggested alternatives, or should not be used at
all.
* Vendor types of more than 8 bits.
SHOULD NOT be used. Vendor types of 8 bits SHOULD be used
instead.
* Vendor lengths of less than 8 bits. (i.e., zero bits)
SHOULD NOT be used. Vendor lengths of 8 bits SHOULD be used
instead.
* Vendor lengths of more than 8 bits.
SHOULD NOT be used. Vendor lengths of 8 bits SHOULD be used
instead.
* Vendor-Specific contents that are not in Type-Length-Value
format.
SHOULD NOT be used. Vendor-Specific attributes SHOULD be in
Type-Length-Value format.
In general, Vendor-Specific attributes SHOULD follow the [RFC2865]
suggested format. Vendor extensions to non-standard formats are NOT
RECOMMENDED as they can negatively affect interoperability.
A.4. Changes to the RADIUS Operational Model
Does the specification change the RADIUS operation model, as outlined
in the list below? If so, then another method of achieving the
design objectives SHOULD be used. Potential problem areas include:
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INTERNET-DRAFT RADIUS Design Guidelines 12 October 2009
* Defining new commands in RADIUS using attributes.
The addition of new commands to RADIUS MUST be handled via
allocation of a new Code, and not by the use of an attribute.
This restriction includes new commands created by overloading
the Service-Type attribute to define new values that modify
the functionality of Access-Request packets.
* Using RADIUS as a transport protocol for data unrelated to
authentication, authorization, or accounting. Using RADIUS to
transport authentication methods such as EAP is explicitly
permitted, even if those methods require the transport of
relatively large amounts of data. Transport of opaque data
relating to AAA is also permitted, as discussed above in
Section 2.1.3. However, if the specification does not relate
to AAA, then RADIUS SHOULD NOT be used.
* Assuming support for packet lengths greater than 4096 octets.
Attribute designers cannot assume that RADIUS implementations
can successfully handle packets larger than 4096 octets.
If a specification could lead to a RADIUS packet larger than
4096 octets, then the alternatives described in Section 3.3
SHOULD be considered.
* Stateless operation. The RADIUS protocol is stateless, and
documents which require stateful protocol behavior without the
use of the State Attribute need to be redesigned.
* Provisioning of service in an Access-Reject. Such provisioning
is not permitted, and MUST NOT be used. If limited access needs
to be provided, then an Access-Accept with appropriate
authorizations can be used instead.
* Lack of user authentication or authorization restrictions.
In an authorization check, where there is no demonstration of a
live user, confidential data cannot be returned. Where there
is a link to a previous user authentication, the State attribute
needs to be present.
* Lack of per-packet integrity and authentication.
It is expected that documents will support per-packet
integrity and authentication.
* Modification of RADIUS packet sequences.
In RADIUS, each request is encapsulated in it's own packet, and
elicits a single response that is sent to the requester. Since
changes to this paradigm are likely to require major
modifications to RADIUS client and server implementations, they
SHOULD be avoided if possible.
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INTERNET-DRAFT RADIUS Design Guidelines 12 October 2009
For further details, see Section 3.3.
A.5. Allocation of attributes
Does the attribute have a limited scope of applicability, as outlined
below? If so, then the attributes SHOULD be allocated from the
Vendor-Specific space.
* attributes intended for a vendor to support their own systems,
and not suitable for general usage
* 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.
Note that the points listed above do not relax the recommendations
discussed in this document. Instead, they recognize that the RADIUS
data model has limitations. In certain situations where
interoperability can be strongly constrained by the SDO or vendor, an
expanded data model MAY be used. We recommend, however, that the
RADIUS data model SHOULD be used, even if it is marginally less
efficient than alternatives.
When attributes are used primarily within a group of SDOs, and are
not applicable to the wider Internet community, we expect that one
SDO will be responsible for allocation from their own private space.
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Appendix B - Complex Attributes
This section summarizes RADIUS attributes with complex data types
that are defined in existing RFCs.
This appendix is published for informational purposes only, and
reflects the usage of attributes with complex data types at the time
of the publication of this document.
B.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 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 are required
on the RADIUS client and server to support it, regardless of the
attribute format. Therefore, this complex data type is acceptable in
this situation.
B.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.
B.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
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Tag | Salt
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Salt (cont) | String ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Since this is a security attribute and is encrypted, code changes are
required on the RADIUS client and server to support it, regardless of
the attribute format. Therefore, this complex data type is
acceptable in this situation.
B.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.
B.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
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Unlike the 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.
While the contents of this attribute is intended to be placed in an
ARAP packet, the fields need to be set by the RADIUS server. Using
standard RADIUS data types would have simplified RADIUS server
implementations, and subsequent management. The current form of the
attribute requires either the RADIUS server implementation, or the
RADIUS server administrator, to understand the internals of the ARAP
protocol.
B.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
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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, and is it not
directly involved in authentication. The individual sub-fields could
therefore have been encapsulated in separate attributes.
Since the form of the text string is well defined, there is no
benefit in using a text string. Instead, an integer attribute should
have been assigned for each of the transmit speed and the receive
speed. A separate enumerated integer should have been assigned for
the additional information, as is done for the NAS-Port-Type
attribute.
B.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.
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.
B.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
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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
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.
B.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
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document.
Authors' Addresses
Greg Weber
Knoxville, TN 37932
USA
Email: gdweber@gmail.com
Alan DeKok
The FreeRADIUS Server Project
http://freeradius.org/
Email: aland@freeradius.org
Weber, et al. Best Current Practice [Page 37]
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