RADIUS ALPN and removing MD5
draft-ietf-radext-radiusv11-10
| Document | Type | Active Internet-Draft (radext WG) | |
|---|---|---|---|
| Author | Alan DeKok | ||
| Last updated | 2024-07-01 | ||
| Replaces | draft-dekok-radext-radiusv11 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Experimental | ||
| Formats | |||
| Reviews |
GENART Last Call review
(of
-08)
by Christer Holmberg
Ready w/issues
OPSDIR Last Call Review due 2024-06-26
Incomplete
|
||
| Additional resources |
GitHub Repository
Mailing list discussion |
||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Jan-Frederik Rieckers | ||
| Shepherd write-up | Show Last changed 2024-03-17 | ||
| IESG | IESG state | Waiting for AD Go-Ahead | |
| Action Holder | |||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Paul Wouters | ||
| Send notices to | mrcullen42@gmail.com, rieckers@uni-bremen.de | ||
| IANA | IANA review state | Version Changed - Review Needed | |
| IANA expert review state | Expert Reviews OK |
draft-ietf-radext-radiusv11-10
RADEXT Working Group A. DeKok
Internet-Draft FreeRADIUS
Updates: 2865, 2866, 5176, 6613, 6614, 7360 (if 1 July 2024
approved)
Intended status: Experimental
Expires: 2 January 2025
RADIUS ALPN and removing MD5
draft-ietf-radext-radiusv11-10
Abstract
This document defines Application-Layer Protocol Negotiation
Extensions for use with RADIUS/TLS and RADIUS/DTLS. These extensions
permit the negotiation of an additional application protocol for
RADIUS over (D)TLS. No changes are made to RADIUS/UDP or RADIUS/TCP.
The extensions allow the negotiation of a transport profile where the
RADIUS shared secret is no longer used, and all MD5-based packet
signing and attribute obfuscation methods are removed.
This document updates RFC2865, RFC2866, RFC5176, RFC6613, RFC6614,
and RFC 7360.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-radext-radiusv11/.
Discussion of this document takes place on the RADEXT Working Group
mailing list (mailto:radext@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/radext/. Subscribe at
https://www.ietf.org/mailman/listinfo/radext/.
Source for this draft and an issue tracker can be found at
https://github.com/radext-wg/draft-ietf-radext-radiusv11.git.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 2 January 2025.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. The RADIUS/1.1 Transport profile for RADIUS . . . . . . . . . 8
3.1. ALPN Name for RADIUS/1.1 . . . . . . . . . . . . . . . . 8
3.2. Operation of ALPN . . . . . . . . . . . . . . . . . . . . 8
3.3. Configuration of ALPN for RADIUS/1.1 . . . . . . . . . . 9
3.3.1. Using Protocol-Error for Signaling ALPN Failure . . . 13
3.3.2. Tabular Summary . . . . . . . . . . . . . . . . . . . 14
3.4. Miscellaneous Items . . . . . . . . . . . . . . . . . . . 15
3.5. Session Resumption . . . . . . . . . . . . . . . . . . . 16
4. RADIUS/1.1 Packet and Attribute Formats . . . . . . . . . . . 17
4.1. RADIUS/1.1 Packet Format . . . . . . . . . . . . . . . . 17
4.2. The Token Field . . . . . . . . . . . . . . . . . . . . . 19
4.2.1. Sending Packets . . . . . . . . . . . . . . . . . . . 19
4.2.2. Receiving Packets . . . . . . . . . . . . . . . . . . 21
5. Attribute handling . . . . . . . . . . . . . . . . . . . . . 22
5.1. Obfuscated Attributes . . . . . . . . . . . . . . . . . . 22
5.1.1. User-Password . . . . . . . . . . . . . . . . . . . . 23
5.1.2. CHAP-Challenge . . . . . . . . . . . . . . . . . . . 23
5.1.3. Tunnel-Password . . . . . . . . . . . . . . . . . . . 24
5.1.4. Vendor-Specific Attributes . . . . . . . . . . . . . 24
5.2. Message-Authenticator . . . . . . . . . . . . . . . . . . 24
5.3. Message-Authentication-Code . . . . . . . . . . . . . . . 25
5.4. CHAP, MS-CHAP, etc. . . . . . . . . . . . . . . . . . . . 26
5.5. Original-Packet-Code . . . . . . . . . . . . . . . . . . 26
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6. Other Considerations when using ALPN . . . . . . . . . . . . 26
6.1. Protocol-Error . . . . . . . . . . . . . . . . . . . . . 27
6.2. Status-Server . . . . . . . . . . . . . . . . . . . . . . 28
6.3. Proxies . . . . . . . . . . . . . . . . . . . . . . . . . 29
7. Other RADIUS Considerations . . . . . . . . . . . . . . . . . 29
7.1. Crypto-Agility . . . . . . . . . . . . . . . . . . . . . 29
7.2. Error-Cause Attribute . . . . . . . . . . . . . . . . . . 30
7.3. Future Standards . . . . . . . . . . . . . . . . . . . . 31
8. Implementation Status . . . . . . . . . . . . . . . . . . . . 32
9. Privacy Considerations . . . . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
13. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 34
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
14.1. Normative References . . . . . . . . . . . . . . . . . . 37
14.2. Informative References . . . . . . . . . . . . . . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 40
1. Introduction
The RADIUS protocol [RFC2865] uses MD5 [RFC1321] to sign packets, and
to obfuscate certain attributes. Additional transport protocols were
defined for TCP ([RFC6613]), TLS ([RFC6614]), and DTLS ([RFC7360]).
However, those transport protocols still relied on MD5. That is, the
shared secret was used along with MD5, even when the RADIUS packets
were being transported in (D)TLS. At the time, the consensus of the
RADEXT working group was that this continued use of MD5 was
acceptable. TLS was seen as a simple "wrapper" around RADIUS, while
using a fixed shared secret. The intention at the time was to allow
the use of (D)TLS while making essentially no changes to the basic
RADIUS encoding, decoding, signing, and packet validation.
Issues of MD5 security have been known for decades, most most notably
in [RFC6151], and in [RFC6421], Section 3, among others. The
reliance on MD5 for security makes it impossible to use RADIUS in a
FIPS-140 compliant system, as FIPS-140 forbids systems from relying
on insecure cryptographic methods for security. In addition, the use
of MD5 in RADIUS/TLS and RADIUS/DLTS adds no security or privacy over
that provided by TLS.
This document defines an Application-Layer Protocol Negotiation
(ALPN) [RFC7301] extension for RADIUS over (D)TLS which removes the
need to use MD5 for (D)TLS. We make no changes to UDP or TCP
transport. This extension can best be understood as a transport
profile for RADIUS over TLS, rather than a whole-sale revision of the
RADIUS protocol.
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Systems which implement this transport profile can be more easily
verified to be FIPS-140 compliant. A preliminary implementation has
shown that only minor code changes are required to support RADIUS/1.1
on top of an existing RADIUS/TLS server implementation, which are:
* A method to set the list of supported ALPN protocols before the
TLS handshake starts
* After the TLS handshake has completed, a method to query if ALPN
has chosen a protocol, and if yes, which protocol was chosen.
* Changes to the packet encoder and decoder, so that the individual
packets are not signed, and no attribute is encoded with the
historic obfuscation methods.
That is, the bulk of the ALPN protocol can be left to the underlying
TLS implementation. This document discusses the ALPN exchange in
detail in order to give simplified descriptions for the reader, and
so that the reader does not have to read or understand all of
[RFC7301].
The detailed list of changes from historic TLS-based transports to
RADIUS/1.1 is as follows:
* ALPN is used for negotiation of this extension,
* TLS 1.3 or later is required,
* All uses of the RADIUS shared secret have been removed,
* The now-unused Request and Response Authenticator fields have been
repurposed to carry an opaque Token which identifies requests and
responses,
* The functionality of the Identifier field has been replaced by the
Token field, and the space previously taken by the Identifier
field is now reserved and unused,
* The Message-Authenticator attribute ([RFC3579], Section 3.2) is
not sent in any packet, and if received is ignored,
* Attributes such as User-Password, Tunnel-Password, and MS-MPPE
keys are sent encoded as "text" ([RFC8044], Section 3.4) or
"octets" ([RFC8044], Section 3.5), without the previous MD5-based
obfuscation. This obfuscation is no longer necessary, as the data
is secured and kept private through the use of TLS,
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* The conclusion of the efforts stemming from [RFC6421] is that
crypto-agility in RADIUS is best done via a TLS wrapper, and not
by extending the RADIUS protocol.
* [RFC5176] is updated to allow the Error-Cause attribute to appear
in Access-Reject packets.
The following items are left unchanged from traditional TLS-based
transports for RADIUS:
* The RADIUS packet header is the same size, and the Code and Length
fields ([RFC2865], Section 3) have the same meaning as before,
* The default 4K packet size is unchanged, although [RFC7930] can
still be leveraged to use larger packets,
* All attributes which have simple encodings (that is, attributes
which do not use MD5 obfuscation) have the same encoding and
meaning as before,
* As this extension is a transport profile for one "hop" (client to
server connection), it does not impact any other connection used
by a client or server. The only systems which are aware that this
transport profile is in use are the client and server who have
negotiated the use of this extension on a particular shared
connection,
* This extension uses the same ports (2083/tcp and 2083/udp) which
are defined for RADIUS/TLS [RFC6614] and RADIUS/DTLS [RFC7360].
A major benefit of this extension is that a home server which
implements it can also be more easily verified for FIPS-140
compliance. That is, a home server can remove all uses of MD4 and
MD5, which means that those algorithms are provably not used for
security purposes. In that case, however, the home server will not
support CHAP, MS-CHAP, or any authentication method which uses MD4 or
MD5. The choice of which authentication method to accept is always
left to the home server. This specification does not change any
authentication method carried in RADIUS, and does not mandate (or
forbid) the use of any authentication method for any system.
As for proxies, there was never a requirement that proxies implement
CHAP or MS-CHAP authentication. So far as a proxy is concerned,
attributes relating to CHAP and MS-CHAP are simply opaque data that
is transported unchanged to the next hop. It is therefore possible
for a FIPS-140 compliant proxy to transport authentication methods
which depend on MD4 or MD5, so long as that data is forwarded to a
home server which supports those methods.
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We reiterate that the decision to support (or not) any authentication
method is entirely site local, and is not a requirement of this
specification. The contents or meaning of any RADIUS attribute other
than Message-Authenticator (and similar attributes) are not modified.
The only change to the Message-Authenticator attribute is that it is
no longer used in RADIUS/1.1.
Unless otherwise described in this document, all RADIUS requirements
apply to this extension. That is, this specification defines a
transport profile for RADIUS. It is not an entirely new protocol,
and it defines only minor changes to the existing RADIUS protocol.
It does not change the RADIUS packet format, attribute format, etc.
This specification is compatible with all RADIUS attributes, past,
present, and future.
This specification is compatible with existing implementations of
RADIUS/TLS and RADIUS/DTLS. Systems which implement this standard
can fall back to historic RADIUS/TLS if no ALPN signaling is
performed, and the local configuration permits such fallback.
This specification is compatible with all existing RADIUS
specifications. There is no need for any RADIUS specification to
mention this transport profile by name, or to make provisions for
this specification. This document defines how to transform RADIUS
into RADIUS/1.1, and no further discussion of that transformation is
necessary.
We note that this document makes no changes to previous RADIUS
specifications. Existing RADIUS implementations can continue to be
used without modification. Where previous specifications are
explicitly mentioned and updated, those updates or changes apply only
when the RADIUS/1.1 transport profile is being used.
In short, when negotiated on a connection, the RADIUS/1.1 transport
profile permits implementations to avoid MD5 when signing packets, or
when obfuscating certain attributes.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The following list describes the terminology and abbreviations which
are used in this document.
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* ALPN
Application-Layer Protocol Negotiation, as defined in [RFC7301].
* RADIUS
The Remote Authentication Dial-In User Service protocol, as
defined in [RFC2865], [RFC2866], and [RFC5176] among others.
While this protocol can be viewed as "RADIUS/1.0", for simplicity
and historical compatibility, we keep the name "RADIUS".
* RADIUS/UDP
RADIUS over the User Datagram Protocol [RFC2865], [RFC2866],
[RFC5176], among others.
* RADIUS/TCP
RADIUS over the Transmission Control Protocol [RFC6613].
* RADIUS/TLS
RADIUS over the Transport Layer Security protocol [RFC6614].
* RADIUS/DTLS
RADIUS over the Datagram Transport Layer Security protocol
[RFC7360].
* RADIUS over TLS
Any RADIUS packets transported over TLS or DTLS. This terminology
is used instead of alternatives such as "RADIUS/(D)TLS", or
"either RADIUS/TLS or RADIUS/DTLS". This term is generally used
when referring to TLS-layer requirements for RADIUS packet
transport.
* historic RADIUS/TLS
RADIUS over (D)TLS as defined in [RFC6614] and [RFC7360]. This
term does not include the protocol defined in this specification.
* RADIUS/1.1
The transport profile defined in this document, which stands for
"RADIUS version 1.1". We use RADIUS/1.1 to refer interchangeably
to TLS and DTLS transport.
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* TLS
The Transport Layer Security protocol. Generally when we refer to
TLS in this document, we are referring interchangeably to TLS or
DTLS transport.
3. The RADIUS/1.1 Transport profile for RADIUS
This section describes the ALPN transport profile in detail. It
first gives the name used for ALPN, and then describes how ALPN is
configured and negotiated by client and server. It then concludes by
discussing TLS issues such as what to do for ALPN during session
resumption.
3.1. ALPN Name for RADIUS/1.1
The ALPN name defined for RADIUS/1.1 is as follows:
"radius/1.1"
The protocol defined by this specification.
Where ALPN is not configured or is not received in a TLS connection,
systems supporting ALPN MUST NOT use RADIUS/1.1.
Where ALPN is configured, the client signals support by sending ALPN
strings signaling which protocols it supports.. The server can
accept one of these proposals and reply with a matching ALPN string,
or reject this proposal, and not reply with any ALPN string. A full
walk-through of the protocol negotiation is given below.
Implementations MUST signal ALPN "radius/1.1" in order for it to be
used in a connection. Implementations MUST NOT have an
administrative flag which causes a connection to use "radius/1.1",
but which does not signal that protocol via ALPN.
The next step in defining RADIUS/1.1 is to review how ALPN works.
3.2. Operation of ALPN
In order to provide a high-level description of ALPN for readers who
are not familiar with the details of [RFC7301], we provide a brief
overview here.
Once a system has been configured to support ALPN, it is negotiated
on a per-connection basis as per [RFC7301]. The negotiation proceeds
as follows:
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1) The client sends an ALPN extension in the ClientHello. This
extension lists one or more application protocols by name. These
names are the protocols which the client is claiming to support.
2) The server receives the extension, and validates the application
protocol name(s) against the list it has configured.
If the server finds no acceptable common protocols (ALPN or
otherwise), it closes the connection.
3) Otherwise, the server returns a ServerHello with either no ALPN
extension, or an ALPN extension containing only one named application
protocol, which needs to be one of the names proposed by the client.
If the client did not signal ALPN, or the server does not accept
the ALPN proposal, the server does not reply with any ALPN name.
4) The client receives the ServerHello, validates the received
application protocol (if any) against the name(s) it sent, and
records which application protocol was chosen.
This check is necessary in order for the client to both know which
protocol the server has selected, and to validate that the
protocol sent by the server is one which is acceptable to the
client.
The next step in defining RADIUS/1.1 is to define how ALPN is
configured on the client and server, and to give more detailed
requirements on its configuration and operation.
3.3. Configuration of ALPN for RADIUS/1.1
Clients or servers supporting this specification can do so by
extending their TLS configuration through the addition of a new
configuration flag, called "Version" here. The exact name given
below does not need to be used, but it is RECOMMENDED that
administrative interfaces or programming interfaces use a similar
name in order to provide consistent terminology. This flag controls
how the implementation signals use of this protocol via ALPN.
When set, this flag contains the list of permitted RADIUS versions as
numbers, e.g. "1.0" or "1.1". The implementation may allow multiple
values in one variable, or allow multiple variables, or instead use
two configuration for "minimum" and "maximum" allowed versions. We
assume here that there is one variable, which can contain either no
value, or else a list of one or more versions which the current
implementation supports. In this specification, the possible values,
ALPN strings, and corresponding interpretations are:
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Flag Value | ALPN String(s) | Interpretation
------------------------------------------------------
unset | | no ALPN strings are sent.
1.0 | radius/1.0 | require historic RADIUS/TLS
1.0, 1.1 | radius/1.0, radius/1.1 | allow either historic
| | RADIUS/TLS or RADIUS/1.1.
1.1 | radius/1.1 | require RADIUS/1.1.
This configuration is also extensible to future RADIUS versions if
that extension becomes necessary. New flag values and ALPN names can
simply be added to the list. Implementations can then negotiate the
highest version which is supported by both client and server.
Implementations SHOULD support both historic RADIUS/TLS and
RADIUS/1.1. Such implementations MUST set the default value for this
configuratiun flag to "1.0, 1.1". This setting ensures that both
versions of RADIUS can be negotiated.
Implementations MAY support only RADIUS/1.1. In which case the
default value for this configuration flag MUST be "1.1". This
behavior is NOT RECOMMENDED, as it is incompatible with historic
RADIUS/TLS. This behavior can only be a reasonable default when all
(or nearly all) RADIUS clients have been updated to support
RADIUS/1.1.
A more detailed definition of the variable and the meaning of the
values is given below.
Configuration Flag Name
Version
Values
When the flag is unset, ALPN is not used.
Any connection MUST use historic RADIUS/TLS.
This flag is included here only for logical completeness.
Implementations of this specification SHOULD be configured to
always send one or more ALPN strings. This data signals that the
implementation is capable performing ALPN negotiation, even if it
is not currently configured to use RADIUS/1.1
Client Behavior
The client MUST NOT send any protocol name via ALPN.
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Server Behavior
The server MUST NOT signal any protocol name via ALPN.
If the server receives an ALPN name from the client, it MUST
NOT close the connection. Instead, it simply does not reply
with ALPN, and finishes the TLS connection setup as defined
for historic RADIUS/TLS.
Note that if a client sends "radius/1.1", the client will
see that the server failed to acknowledge this request, and
will close the connection. For any other client
configuration, the connection will use historic RADIUS/TLS.
Other values ("1.0", "1.0, 1.1", "1.1", etc.)
Client Behavior
The client MUST send the ALPN string(s) associated with the
configured version. e.g. For "1.0", send "radius/1.0".
The client will receive either no ALPN response from the
server, or an ALPN response of one version string with MUST
match one of the strings it sent, or else a TLS alert of
"no_application_protocol" (120).
If the connection remains open, the client MUST treat the
connection as using the matching ALPN version.
Server Behavior
If the server receives no ALPN name from the client, it MUST
use historic RADIUS/TLS.
If the server receives one or more ALPN names from the
client, it MUST reply with the highest mutually supported
version and then use the latest supported version for this
connection.
If the server receives one or more ALPN names from the
client, but none of the names match the versions supported
by (or configured on) the server, it MUST reply with a TLS
alert of "no_application_protocol" (120), and then MUST
close the TLS connection.
These requirements for negotiation are not specific to
RADIUS/1.1, and therefore can be used unchanged if any new
version of RADIUS is defined.
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By requiring the default configuration to allow historic RADIUS/TLS,
implementations will be able to negotiate both historic RADIUS/TLS
connections, and also RADIUS/1.1 connections. Any other recommended
default setting would prevent either the negotiation of historic
RADIUS/TLS, or prevent the negotiation of RADIUS/1.1.
Once administrators verify that both ends of a connection support
RADIUS/1.1, and that it has been negotiated successfully, the
configurations SHOULD be updated to require RADIUS/1.1. The
connections should be monitored after this change to ensure that the
systems continue to remain connected. If there are connection
issues, then the configuration should be reverted to using allow both
"radius/1.0" and "radius/1.1" ALPN strings, until such time as the
connection problems have been resolved.
We reiterate that systems implementing this specification, but which
are configured with setting that forbid RADIUS/1.1, will behave
largely the same as systems which do not implement this
specification. The only difference is that clients may send the ALPN
name "radius/1.0".
Systems implementing RADIUS/1.1 SHOULD NOT be configured by default
to forbid that protocol. That setting exists mainly for
completeness, and to give administrators the flexibility to control
their own deployments.
While [RFC7301] does not discuss the possibility of the server
sending a TLS alert of "no_application_protocol" (120) when the
client does not use ALPN, we believe that this behavior is useful.
As such, servers MAY send a a TLS alert of "no_application_protocol"
(120) when the client does not use ALPN.
However, some TLS implementations may not permit an application to
send a TLS alert of its choice, at a time of its choice. This
limitation means that it is not always possible for an application to
send the TLS alert as discussed in the previous section. The impact
is that an implementation may attempt to connect, and then see that
the connection fails, but not be able to determine why that failure
has occurred. Implementers and administrators should be aware that
unexplained connection failures may be due to ALPN negotiation
issues.
The server MAY send this alert during the ClientHello, if it requires
ALPN but does not receive it. That is, there may not always be a
need to wait for the TLS connection to be fully established before
realizing that no common ALPN protocol can be negotiated.
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Where the client does perform signaling via ALPN and the server
determines that there is no compatible application protocol name,
then as per [RFC7301], Section 3.2, it MUST send a TLS alert of
"no_application_protocol" (120).
Whether or not the server sent a TLS alert for no compatible ALPN, it
MUST close the connection. The above requirements on ALPN apply to
both new sessions, and to resumed sessions.
In contrast, there is no need for the client to signal that there are
no compatible application protocol names. The client sends zero or
more protocol names, and the server responds as above. From the
point of view of the client, the list it sent results in either a
connection failure, or a connection success.
It is RECOMMENDED that the server logs a descriptive error in this
situation, so that an administrator can determine why a particular
connection failed. The log message SHOULD include information about
the other end of the connection, such as IP address, certificate
information, etc. Similarly, when the client receives a TLS alert of
"no_application_protocol" it SHOULD log a descriptive error message.
Such error messages are critical for helping administrators to
diagnose connectivity issues.
3.3.1. Using Protocol-Error for Signaling ALPN Failure
When it is not possible to send a TLS alert of
"no_application_protocol" (120), then the only remaining method for
one party to signal the other is to send application data inside of
the TLS tunnel. Therefore, for the situation when a one end of a
connection determines that it requires ALPN while the other end does
not support ALPN, the end requiring ALPN MAY send a Protocol-Error
packet [RFC7930] inside of the tunnel, and then MUST close the
connection. If this is done, the Token field of the Protocol-Error
packet cannot be copied from any request, and therefore that field
MUST be set to all zeros.
The Protocol-Error packet SHOULD contain a Reply-Message attribute
with a textual string describing the cause of the error. The packet
SHOULD also contain an Error-Cause attribute, with value Unsupported
Extension (406). The packet SHOULD NOT contain other attributes.
An implementation sending this packet could bypass any RADIUS
encoder, and simply write this packet as a predefined, fixed set of
data to the TLS connection. That process would likely be simpler
than trying to call the normal RADIUS packet encoder to encode a
reply packet with no corresponding request packet.
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As this packet is an unexpected response packet, existing client
implementations of RADIUS over TLS will ignore it. They may either
log an error and close the connection, or they may discard the packet
and leave the connection open. If the connection remains open, the
end supporting ALPN will close the connection, so there will be no
side effects from sending this packet. Therefore, while using a
Protocol-Error packet in this way is unusual, it is both informative
and safe.
The purpose of this packet is not to have the other end of the
connection automatically determine what went wrong, and fix it.
Instead, the packet is intended to be (eventually) seen by an
administrator, who can then take remedial action.
3.3.2. Tabular Summary
The preceding text gives a large number of recommendations. In order
to give a simpler description of the outcomes, a table of possible
behaviors for client/server values of the Version flag is given
below. This table and the names given below are for informational
and descriptive purposes only.
Server
no ALPN | 1.0 | 1.0, 1.1 | 1.1
Client |--------------------------------------------
----------|
No ALPN | TLS TLS TLS Close-S
|
1.0 | TLS TLS TLS Alert
|
1.0, 1.1 | TLS TLS 1.1 1.1
|
1.1 | Close-C Alert 1.1 1.1
Figure 1: Possible outcomes for ALPN Negotiation
The table entries above have the following meaning:
Alert
The client sends ALPN, and the server does not agree to the
clients ALPN proposal. The server replies with a TLS alert of
"no_application_protocol" (120), and then closes the TLS
connection.
As the server replies with a TLS alert, the Protocol-Error
packet is not used here.
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Close-C
The client sends ALPN, but the server does not respond with
ALPN. The client closes the connection.
As noted in the previous section, the client MAY send a
Protocol-Error packet to the server before closing the
connection.
Close-S
The client does not send ALPN string(s), but the server
requires ALPN. The server closes the connection.
As noted in the previous section, the server MAY send a
Protocol-Error packet to the client before closing the
connection. The server MAY also send a TLS alert of
"no_application_protocol" (120) before closing the connection.
TLS
Historic RADIUS/TLS is used. The client either sends no ALPN
string, or sends "radius/1.0". The server either replies with
no ALPN string, or with "radius/1.0". The connection MUST use
historic RADIUS/TLS.
1.1
The client sends the ALPN string "radius/1.1. The server
acknowledges this negotiation with a reply of "radius/1.1", and
then RADIUS/1.1 is used.
Implementations should note that this table may be extended in future
specifications. The above text is informative, and does not mandate
that only the above ALPN strings are used. The actual ALPN
negotiation takes place as defined in the preceding sections of this
document, and in [RFC7301].
3.4. Miscellaneous Items
Implementations of this specification MUST require TLS version 1.3 or
later.
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The use of the ALPN string "radius/1.0" is technically unnecessary,
as it is largely equivalent to not sending any ALPN string. However,
that value is useful for RADIUS administrators. A system which sends
the ALPN string "radius/1.0" is explicitly signaling that it supports
ALPN negotiation, but that it is not currently configured to support
RADIUS/1.1. That information can be used by administrators to
determine which devices are capable of ALPN.
The use of the ALPN string "radius/1.0" also permits server
implementations to send a TLS alert of "no_application_protocol"
(120) when it cannot find a matching ALPN string. Experiments with
TLS library implementations suggest that in some cases it is possible
to send that TLS alert when ALPN is not used. However, such a
scenario is not discussed on [RFC7301], and is likely not universal.
As a result, ALPN as defined in [RFC7301] permits servers to send
that TLS alert in situations where it would be otherwise forbidden,
or perhaps unsupported.
Finally, defining ALPN strings for all known RADIUS versions will
make it easier to support additional ALPN strings if that
functionality is ever needed.
3.5. Session Resumption
[RFC7301], Section 3.1 states that ALPN is negotiated on each
connection, even if session resumption is used:
When session resumption or session tickets [RFC5077] are used, the
previous contents of this extension are irrelevant, and only the
values in the new handshake messages are considered.
In order to prevent down-bidding attacks, RADIUS systems which
negotiate the "radius/1.1" protocol MUST associate that information
with the session ticket, and enforce the use of "radius/1.1" on
session resumption. That is, if "radius/1.1" was negotiated for a
session, both clients and servers MUST behave as if the RADIUS/1.1
flag was set to "require" for that session.
A client which is resuming a "radius/1.1" connection MUST advertise
only the capability to do "radius/1.1" for the resumed session. That
is, even if the client configuration allows historic RADIUS/TLS for
new connections, it MUST signal "radius/1.1" when resuming a session
which had previously negotiated "radius/1.1".
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Similarly, when a server does resumption for a session which had
previously negotiated "radius/1.1". If the client attempts to resume
the sessions without signaling the use of RADIUS/1.1, the server MUST
close the connection. The server MUST send an appropriate TLS error,
and also SHOULD log a descriptive message as described above.
In contrast, there is no requirement for a client or server to force
the use of [RFC6614] RADIUS/TLS on session resumption. Clients are
free to signal support for "radius/1.1" on resumed sessions, even if
the original session did not negotiate "radius/1.1". Servers are
free to accept this request, and to negotiate the use of "radius/1.1"
for such sessions.
4. RADIUS/1.1 Packet and Attribute Formats
This section describes the application-layer data which is sent
inside of (D)TLS when using the RADIUS/1.1 protocol. Unless
otherwise discussed herein, the application-layer data is unchanged
from traditional RADIUS. This protocol is only used when
"radius/1.1" has been negotiated by both ends of a connection.
4.1. RADIUS/1.1 Packet Format
When RADIUS/1.1 is used, the RADIUS header is modified from standard
RADIUS. While the header has the same size, some fields have
different meaning. The Identifier and the Request / Response
Authenticator fields are no longer used in RADIUS/1.1. Any
operations which depend on those fields MUST NOT be performed. As
packet signing and security are handled by the TLS layer, RADIUS-
specific cryptographic primitives are no longer in RADIUS/1.1.
A summary of the RADIUS/1.1 packet format is shown below. The fields
are transmitted from left to right.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Reserved-1 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Reserved-2 |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attributes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-
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Figure 2: The RADIUS/1.1 Packet Format
Code
The Code field is one octet, and identifies the type of RADIUS
packet.
The meaning of the Code field is unchanged from previous RADIUS
specifications.
Reserved-1
The Reserved-1 field is one octet.
This field was previously used as the "Identifier" in historic
RADIUS/TLS. It is now unused, as the Token field replaces it both
as the way to identify requests, and to associate responses with
requests.
When sending packets, the Reserved-1 field MUST be set to zero.
The Reserved-1 field MUST be ignored when receiving a packet.
Length
The Length field is two octets.
The meaning of the Length field is unchanged from previous RADIUS
specifications.
Token
The Token field is four octets, and aids in matching requests and
replies, as a replacement for the Identifier field. The RADIUS
server can detect a duplicate request if it receives the same
Token value for two packets on a particular connection.
All values are possible for the Token field. Implementations MUST
treat the Token as an opaque blob when comparing Token values.
Further requirements are given below in Section 4.2.1 for sending
packets, and in Section 4.2.2 for receiving packets.
Reserved-2
The Reserved-2 field is twelve (12) octets in length.
These octets MUST be set to zero when sending a packet.
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These octets MUST be ignored when receiving a packet.
These octets are reserved for future protocol extensions.
4.2. The Token Field
This section describes in more detail how the Token field is used.
4.2.1. Sending Packets
The Token field MUST change for every new unique packet which is sent
on the same connection. For DTLS transport, it is possible to
retransmit duplicate packets, in which case the Token value MUST NOT
be changed when a duplicate packet is (re)sent. When the contents of
a retransmitted packet change for any reason (such changing Acct-
Delay-Time as discussed in [RFC2866], Section 5.2), the Token value
MUST be changed. Note that on reliable transports, packets are never
retransmitted, and therefore every new packet which is sent has a
unique Token value.
We note that in previous RADIUS specifications, the Identifier field
could have the same value for different packets on the same
connection. For example, Access-Request (Code 1) and Accounting-
Request (Code 4) packets could both use ID 3, and still be treated as
different packets. This overlap required that RADIUS clients and
servers track the Identifier field, not only on a per-connection
basis, but also on a per-Code basis. This behavior adds complexity
to implementations.
In contrast, the Token values MUST be generated from a 32-bit counter
which is unique to each connection. Such a counter SHOULD be
initialized to a random value, taken from a random number generator,
whenever a new connection is opened. The counter MUST then be
incremented for every unique new packet which is sent by the client.
Retransmissions of the same packet MUST use the same unchanged Token
value. As the Token value is mandated to be unique per packet, a
duplicate Token value is the only way that a server can detect
duplicate transmissions.
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This counter method ensures that the Tokens are unique, and are also
independent of any Code value in the RADIUS packet header. This
method is mandated because any other method of generating unique and
non-conflicting Token values is more complex, with no additional
benefit and only the likelihood of increased bugs and
interoperability issues. Any other method for generating Token
values would require substantially more resources to track
outstanding Token values and their associated expiry times. The
chance that initial values are re-used across two connections is one
in 2^32, which is acceptable.
The purpose for initializing the Token to a random counter is to aid
administrators in debugging systems. If the Token values always used
the same sequence, then it would easier for a person to confuse
different packets which have the same Token value. By instead
starting with a random value, those values are more evenly
distributed across the set of allowed values, and are therefore more
likely to be unique.
As there is no special meaning for the Token, there is no meaning
when a counter "wraps" around from a high value back to zero. The
originating system can simply continue to increment the Token value
without taking any special action in that situation.
Once a RADIUS response to a request has been received and there is no
need to track the packet any longer, the Token value can be reused.
This reuse happens only when the counter "wraps around" after 2^32
packets have been sent over one connection. This method of managing
the counter automatically ensures a long delay (i.e. 2^32 packets)
between multiple uses of the same Token value. This large number of
packets ensures that the only possible situation where there may be
conflict is when a client sends billions of packets a second across
one connection, or when a client sends billions of packets without
receiving replies. We suggest that such situations are vanishingly
rare. The best solution to those situations would be to limit the
number out outstanding packets over one connection to a number much
lower than billions.
If a RADIUS client has multiple independent subsystems which send
packets to a server, each subsystem MAY open a new connection which
is unique to that subsystem. There is no requirement that all
packets go over one particular connection. That is, despite the use
of a 32-bit Token field, RADIUS/1.1 clients are still permitted to
open multiple source ports as discussed in [RFC2865] Section 2.5.
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While multiple connections from client to server are allowed, We
reiterate the suggestion of [RFC3539], Section 3.3 that a single
connection is preferred to multiple connections. The use of a single
connection can improve throughput and latency, while simplifying the
clients efforts to determine server status.
4.2.2. Receiving Packets
A server which receives RADIUS/1.1 packets MUST perform packet
deduplication for all situations where it is required by RADIUS.
Where RADIUS does not require deduplication (e.g. TLS transport),
the server SHOULD NOT do deduplication. However, DTLS transport is
UDP-based, and therefore still requires deduplication.
When using RADIUS/1.1, implementations MUST do deduplication only on
the Token field, and not on any other field or fields in the packet
header. A server MUST treat the Token as being an opaque field with
no intrinsic meaning. This requirement makes the receiver behavior
independent of the methods by which the Counter is generated.
Where Token deduplication is done, it MUST be done on a per-
connection basis. If two packets which are received on different
connections contain the same Token value, then those packets MUST be
treated as distinct (i.e. different) packets. Systems performing
deduplication MAY still track the packet Code, Length, and Attributes
which is associated with a Token value. If it determines that the
sender is re-using Token values for distinct outstanding packets,
then an error should be logged, and the connection MUST be closed.
There is no way to negotiate correct behavior in the protocol.
Either the parties both operate normally and can communicate, or one
end misbehaves, and no communication is possible.
Once a reply has been sent, a system doing deduplication SHOULD cache
the replies as discussed in [RFC5080], Section 2.2.2:
Each cache entry SHOULD be purged after a period of time. This
time SHOULD be no less than 5 seconds, and no more than 30
seconds. After about 30 seconds, most RADIUS clients and end
users will have given up on the authentication request.
Therefore, there is little value in having a larger cache timeout.
This change from RADIUS means that the Identifier field is no longer
useful for RADIUS/1.1. The Reserved-1 field (previously used as the
Identifier) MUST be set to zero when encoding all RADIUS/1.1 packets.
Implementations of RADIUS/1.1 which receive packets MUST ignore this
field.
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5. Attribute handling
Most attributes in RADIUS have no special encoding "on the wire", or
any special meaning between client and server. Unless discussed in
this section, all RADIUS attributes are unchanged in this
specification. This requirement includes attributes which contain a
tag, as defined in [RFC2868].
5.1. Obfuscated Attributes
Since the (D)TLS layer provides for connection authentication,
integrity checks, and confidentiality, there is no need to hide the
contents of an attribute on a hop-by-hop basis. As a result, all
attributes defined as being obfuscated via the shared secret no
longer have the obfuscation step applied when RADIUS/1.1 is used.
Instead, those attributes MUST be encoded using the encoding for the
underlying data type, with any encryption / obfuscation step omitted.
For example, the User-Password attribute is no longer obfuscated, and
is instead sent as data type "text".
There are risks from sending passwords over the network, even when
they are protected by TLS. One such risk comes from the common
practice of multi-hop RADIUS routing. As all security in RADIUS is
on a hop-by-hop basis, every proxy which receives a RADIUS packet can
see (and modify) all of the information in the packet. Sites wishing
to avoid proxies SHOULD use dynamic peer discovery [RFC7585], which
permits clients to make connections directly to authoritative servers
for a realm.
There are others ways to mitigate these risks. The simplest is to
follow the requirements of [RFC6614], Section 2.4 item (3) and
[RFC7360], Section 10.4, which mandates that RADIUS over TLS
implementations validate the peer before sending any RADIUS traffic.
Another way to mitigate these risks is for the system being
authenticated to use an authentication protocol which never sends
passwords (e.g. EAP-pwd [RFC5931]), or which sends passwords
protected by a TLS tunnel (e.g. EAP-TTLS [RFC5281]). The processes
to choose and configuring an authentication protocol are strongly
site-dependent, so further discussion of these issues are outside of
the scope of this document. The goal here is to ensure that the
reader has enough information to make an informed decision.
We note that as the RADIUS shared secret is no longer used in this
specification, it is no longer possible or necessary for any
attribute to be obfuscated on a hop-by-hop basis using the previous
methods defined for RADIUS.
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5.1.1. User-Password
The User-Password attribute ([RFC2865], Section 5.2) MUST be encoded
the same as any other attribute of data type 'string' ([RFC8044],
Section 3.5).
The contents of the User-Password field MUST be at least one octet in
length, and MUST NOT be more than 128 octets in length. This
limitation is maintained from [RFC2865], Section 5.2 for
compatibility with historic transports.
Note that the User-Password attribute is not of data type 'text'.
The original reason in [RFC2865] was because the attribute was
encoded as an opaque and obfuscated binary blob. We maintain that
data type here, even though the attribute is no longer obfuscated.
The contents of the User-Password attribute do not have to be
printable text, or UTF-8 data as per the definition of the 'text'
data type in [RFC8044], Section 3.4.
However, implementations should be aware that passwords are often
printable text, and where the passwords are printable text, it can be
useful to store and display them as printable text. Where
implementations can process non-printable data in the 'text' data
type, they MAY use the data type 'text' for User-Password.
5.1.2. CHAP-Challenge
[RFC2865], Section 5.3 allows for the CHAP challenge to be taken from
either the CHAP-Challenge attribute ([RFC2865], Section 5.40), or the
Request Authenticator field. Since RADIUS/1.1 connections no longer
use a Request Authenticator field, it is no longer possible to use
the Request Authenticator field as the CHAP-Challenge when this
transport profile is used.
Clients which send CHAP-Password attribute ([RFC2865], Section 5.3)
in an Access-Request packet over a RADIUS/1.1 connection MUST also
include a CHAP-Challenge attribute ([RFC2865], Section 5.40).
Proxies may need to receive Access-Request packets over a non-
RADIUS/1.1 transport and then forward those packets over a RADIUS/1.1
connection. In that case, if the received Access-Request packet
contains a CHAP-Password attribute but no CHAP-Challenge attribute,
the proxy MUST create a CHAP-Challenge attribute in the proxied
packet using the contents from the incoming Request Authenticator of
the received packet.
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5.1.3. Tunnel-Password
The Tunnel-Password attribute ([RFC2868], Section 3.5) MUST be
encoded the same as any other attribute of data type 'string' which
contains a tag, such as Tunnel-Client-Endpoint ([RFC2868],
Section 3.3). Since the attribute is no longer obfuscated in
RADIUS/1.1, there is no need for a Salt field or Data-Length fields
as described in [RFC2868], Section 3.5, and the textual value of the
password can simply be encoded as-is.
Note that the Tunnel-Password attribute is not of data type 'text'.
The original reason in [RFC2868] was because the attribute was
encoded as an opaque and obfuscated binary blob. We maintain that
data type here, even though the attribute is no longer obfuscated.
The contents of the Tunnel-Password attribute do not have to be
printable text, or UTF-8 data as per the definition of the 'text'
data type in [RFC8044], Section 3.4.
However, implementations should be aware that passwords are often
printable text, and where the passwords are printable text, it can be
useful to store and display them as printable text. Where
implementations can process non-printable data in the 'text' data
type, they MAY use the data type 'text' for Tunnel-Password.
5.1.4. Vendor-Specific Attributes
Any Vendor-Specific attribute which uses similar obfuscation MUST be
encoded as per their base data type. Specifically, the MS-MPPE-Send-
Key and MS-MPPE-Recv-Key attributes ([RFC2548], Section 2.4) MUST be
encoded as any other attribute of data type 'string' ([RFC8044],
Section 3.4).
5.2. Message-Authenticator
The Message-Authenticator attribute ([RFC3579], Section 3.2) MUST NOT
be sent over a RADIUS/1.1 connection. That attribute is not used or
needed in RADIUS/1.1.
If the Message-Authenticator attribute is received over a RADIUS/1.1
connection, the attribute MUST be silently discarded, or treated as
an "invalid attribute", as defined in [RFC6929], Section 2.8. That
is, the Message-Authenticator attribute is no longer used to sign
packets for the RADIUS/1.1 transport. Its existence (or not) in this
transport is meaningless.
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A system which receives a Message-Authenticator attribute in a packet
MUST treat it as an "invalid attribute" as defined in [RFC6929],
Section 2.8. That is, the packet can still be processed, even if the
Message-Authenticator attribute is ignored.
For proxies, the Message-Authenticator attribute has always been
defined as being created and consumed on a "hop by hop" basis. That
is, a proxy which received a Message-Authenticator attribute from a
client would never forward that attribute as-is to another server.
Instead, the proxy would either suppress, or re-create, the Message-
Authenticator attribute in the outgoing request. This existing
behavior is leveraged in RADIUS/1.1 to suppress the use of Message-
Authenticator over a RADIUS/1.1 connection.
A proxy may receive an Access-Request packet over a RADIUS/1.1
connection, and then forward that packet over a RADIUS/UDP or a
RADIUS/TCP connection. In that situation, the proxy SHOULD add a
Message-Authenticator attribute to every Access-Request packet which
is sent over an insecure transport protocol.
The original text in [RFC3579], Section 3.3, "Note 1" paragraph
required that the Message-Authenticator attribute be present for
certain Access-Request packets. It also required the use of Message-
Authenticator when the Access-Request packet contained an EAP-Message
attribute. Experience has shown that some RADIUS clients never use
the Message-Authenticator, even for the situations where its use is
suggested.
When the Message-Authenticator attribute is missing from Access-
Request packets, it is often possible to trivially forge or replay
those packets. As such, this document RECOMMENDS that RADIUS clients
always include Message-Authenticator in Access-Request packets when
using UDP or TCP transport. As the scope of this document is limited
to defining RADIUS/1.1, we cannot mandate that behavior here.
Instead, we can note that there are no known negatives to this
behavior, and there are definite positives, such as increased
security.
5.3. Message-Authentication-Code
Similarly, the Message-Authentication-Code attribute defined in
[RFC6218], Section 3.3 MUST NOT be sent over a RADIUS/1.1 connection.
If it is received in a packet, it MUST be treated as "invalid
attribute" as defined in [RFC6929], Section 2.8.
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As the Message-Authentication-Code attribute is no longer used in
RADIUS/1.1, the related MAC-Randomizer attribute [RFC6218],
Section 3.2 MUST NOT be sent over a RADIUS/1.1 connection. If it is
received in a packet, it MUST be treated as "invalid attribute" as
defined in [RFC6929], Section 2.8.
5.4. CHAP, MS-CHAP, etc.
While some attributes such as CHAP-Password, etc. depend on insecure
cryptographic primitives such as MD5, these attributes are treated as
opaque blobs when sent between a RADIUS client and server. The
contents of the attributes are not obfuscated, and they do not depend
on the RADIUS shared secret. As a result, these attributes are
unchanged in RADIUS/1.1.
A server implementing this specification can proxy CHAP, MS-CHAP,
etc. without any issue. A home server implementing this
specification can authenticate CHAP, MS-CHAP, etc. without any issue.
5.5. Original-Packet-Code
[RFC7930], Section 4 defines an Original-Packet-Code attribute. This
attribute is needed because otherwise it is impossible to correlate
the Protocol-Error response packet with a particular request packet.
The definition in [RFC7930], Section 4 describes the reasoning behind
this need:
The Original-Packet-Code contains the code from the request that
generated the protocol error so that clients can disambiguate
requests with different codes and the same ID.
This attribute is no longer needed in RADIUS/1.1. The Identifier
field is unused, so it impossible for two requests to have the "same"
ID. Instead, the Token field permits clients and servers to
correlate requests and responses, independent of the Code value being
used.
Therefore, the Original-Packet-Code attribute ([RFC7930], Section 4)
MUST NOT be sent over a RADIUS/1.1 connection. If it is received in
a packet, it MUST be treated as "invalid attribute" as defined in
[RFC6929], Section 2.8.
6. Other Considerations when using ALPN
Most of the differences between RADIUS and RADIUS/1.1 are in the
packet header and attribute handling, as discussed above. The
remaining issues are a small set of unrelated topics, and are
discussed here.
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6.1. Protocol-Error
There are a number of situations where a RADIUS server is unable to
respond to a request. One situation is where the server depends on a
database, and the database is down. While arguably the server should
close all incoming connections when it is unable to do anything, this
action is not always effective. A client may aggressively try to
open new connections, or send packets to an unconnected UDP
destination where the server is not listening. Another situation
where the server is unable to respond is when the server is proxying
packets, and the outbound connections are either full or failed.
In all RADIUS spercifications prior to this one, there is no way for
the server to send a client the positive signal that it received a
request, but is unable to send a response. Instead, the server
usually just discards the request, which to the client is
indistinguishable from the situation where the server is down. This
failure case is made worse by the fact that perhaps some proxied
packets succeed while others fail. The client can only conclude then
that the server is randomly dropping packets, and is unreliable.
It would be very useful for servers to signal to clients that they
have received a request, but are unable to process it. This
specification uses the Protocol-Error packet [RFC7930], Section 4 as
that signal. The use of Protocol-Error allows for both hop-by-hop
signaling in the case of proxy forwarding errors, and also for end-
to-end signaling of server to client. Such signaling should greatly
improve the robustness of the RADIUS protocol.
When a RADIUS/1.1 server determines that it is unable to process an
Access-Request or Accounting-Request packet, it MUST respond with a
Protocol-Error packet containing an Error-Cause attribute. A proxy
which cannot forward the packet MUST respond with either 502 (Request
Not Routable (Proxy)), or 505 (Other Proxy Processing Error). This
requirement is to help distinguish failures in the proxy chain from
failures at the home server,
For a home server, if none of the Error-Cause values match the reason
for the failure, then the value 506 (Resources Unavailable) MUST be
used.
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When a RADIUS proxy receives a Protocol-Error reply, it MUST examine
the value of the Error-Cause attribute. If there is no Error-Cause
attribute, or its value is something other than 502 (Request Not
Routable (Proxy)), 505 (Other Proxy Processing Error), or 506
(Resources Unavailable), the proxy MUST return the Protocol-Error
response packet to the client, and include the Error-Cause attribute
from the response it received. This process allows for full "end to
end" signaling of servers to clients.
In all situations other then outlined in the preceding paragraph, a
client which receives a Protocol-Error reply MUST re-process the
original outgoing packet through the client forwarding algorithm.
This requirement includes both clients which originate RADIUS
traffic, and proxies which see an Error-Cause attribute of 502
(Request Not Routable (Proxy)), or 505 (Other Proxy Processing
Error).
The expected result of this processing is that the client forwards
the packet to a different server. Clients MUST NOT forward the
packet over the same connection, and SHOULD NOT forward it to over a
different connection to the same server.
This process may continue over multiple connections and multiple
servers, until the client either times out the request, or fails to
find a forwarding destination for the packet. A proxy which is
unable to forward a packet MUST reply with a Protocol-Error packet
containing Error-Cause, as defined above. A client which originates
packets MUST treat such a request as if it had received no response.
This behavior is intended to improve the stability of the RADIUS
protocol by addressing issues first raised in [RFC3539], Section 2.8.
6.2. Status-Server
[RFC6613], Section 2.6.5, and by extension [RFC7360], suggest that
the Identifier value zero (0) be reserved for use with Status-Server
as an application-layer watchdog. This practice MUST NOT be used for
RADIUS/1.1, as the Identifier field is not used in this transport
profile.
The rationale for reserving one value of the Identifier field was the
limited number of Identifiers available (256), and the overlap in
Identifiers between Access-Request packets and Status-Server packets.
If all 256 Identifier values had been used to send Access-Request
packets, then there would be no Identifier value available for
sending a Status-Server packet.
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In contrast, the Token field allows for 2^32 outstanding packets on
one RADIUS/1.1 connection. If there is a need to send a Status-
Server packet, it is nearly always possible to allocate a new value
for the Token field. If instead there are 2^32 outstanding packets
for one connection, then it is likely that something has gone
catastrophically wrong. In that case, the safest way forward is
likely to just close the connection.
6.3. Proxies
A RADIUS proxy normally decodes and then re-encodes all attributes,
included obfuscated ones. A RADIUS proxy will not generally rewrite
the content of the attributes it proxies (unless site-local policy
requires such a rewrite). While some attributes may be modified due
to administrative or policy rules on the proxy, the proxy will
generally not rewrite the contents of attributes such as User-
Password, Tunnel-Password, CHAP-Password, MS-CHAP-Password, MS-MPPE
keys, etc. All attributes are therefore transported through a
RADIUS/1.1 connection without changing their values or contents.
A proxy may negotiate RADIUS/1.1 (or not) with a particular client or
clients, and it may negotiate RADIUS/1.1 (or not) with a server or
servers it connect to, in any combination. As a result, this
specification is fully compatible with all past, present, and future
RADIUS attributes.
7. Other RADIUS Considerations
This section discusses issues in RADIUS which need to be addressed in
order to support ALPN, but which aren't direcly part of the
RADIUS/1.1 protocol.
7.1. Crypto-Agility
The crypto-agility requirements of [RFC6421] are addressed in
[RFC6614], Appendix C, and in [RFC7360], Section 10.1. This
specification makes no changes from, or additions to, those
specifications. The use of ALPN, and the removal of MD5 has no
impact on security or privacy of the protocol.
RADIUS/TLS has been widely deployed in at least eduroam [RFC7593] and
[EDUROAM] and in OpenRoaming [OPENROAMING]. RADIUS/DTLS has seen
less adoption, but it is known to be supported in many RADIUS clients
and servers.
It is RECOMMENDED that all implementations of historic RADIUS/TLS be
updated to support this specification. Where a system already
implements RADIUS over TLS, the additional effort to implement this
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specification is minimal. Once implementations support it,
administrators can gain the benefit of it with little or no
configuration changes. This specification is backwards compatible
with [RFC6614] and [RFC7360]. It is only potentially subject to
down-bidding attacks if implementations do not enforce ALPN
negotiation correctly on session resumption.
All crypto-agility needed or used by this specification is
implemented in TLS. This specification also removes all
cryptographic primitives from the application-layer protocol (RADIUS)
being transported by TLS. As discussed in the following section,
this specification also bans the development of all new cryptographic
or crypto-agility methods in the RADIUS protocol.
7.2. Error-Cause Attribute
The Error-Cause attribute is defined in [RFC5176]. The "Table of
Attributes" section given in [RFC5176], Section 3.6 permits that
attribute to appear in CoA-NAK and Disconnect-NAK packets. As no
other packet type is listed, the implication is that the Error-Cause
attribute cannot appear in any other packet. [RFC7930] also permits
Error-Cause to appear in Protocol-Error packets.
However, [RFC5080], Section 2.6.1 suggests that Error-Cause may
appear in Access-Reject packets. No explanation is given for this
change from [RFC5176]. There is not even an acknowledgment that this
suggestion is a change from any previous specification. We correct
that issue here.
This specification updates [RFC5176] to allow the Error-Cause
attribute to appear in Access-Reject packets. It is RECOMMENDED that
implementations include the Error-Cause attribute in Access-Reject
packets where appropriate.
That is, the reason for sending the Access-Reject packet (or
Protocol-Error packet) may match a defined Error-Cause value. In
that case, it is useful for implementations to send an Error-Cause
attribute with that value. This behavior can help RADIUS system
administrators debug issues in complex proxy chains.
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For example, a proxy may normally forward Access-Request packets
which contain EAP-Message attributes. The proxy can determine if the
contents of the EAP-Message are invalid, for example if the first
octet has value larger than 4. In that case, there may be no benefit
to forwarding the packet, as the home server will reject it. It may
then then possible for the proxy (with the knowledge and consent of
involved parties) to immediately reply with an Access-Reject
containing an Error-Cause attribute with value 202 for "Invalid EAP
Packet (Ignored)".
Another possibility is that if a proxy is configured to forward
packets for a particular realm, but it has determined that there are
no available connections to the next hop for that realm. In that
case, it may be possible for the proxy (again with the knowledge and
consent of involved parties) to reply with an Access-Reject
containing an Error-Cause attribute with value 502 for "Request Not
Routable (Proxy)"
These examples are given only for illustrative and informational
purposes. While it is useful to return an informative value for the
Error-Cause attribute, proxies can only modify the traffic they
forward with the explicit knowledge and consent of all involved
parties.
7.3. Future Standards
Future work may define new attributes, packet types, etc. It is
important to be able to do such work without requiring that every new
standard mention RADIUS/1.1 explicitly. This document defines
RADIUS/1.1 as having functional overlap with legacy RADIUS: the
protocol state machine is unchanged, the packet header Code field is
unchanged, and the attribute format is largely unchanged. As a
result, any new packet Code or attribute defined for RADIUS is
explicitly compatible with RADIUS/1.1: the field contents and
meanings are identical. The only difference between the two
protocols is that obfuscated attributes in RADIUS are not obfuscated
in RADIUS/1.1, and this document defines how that mapping is done.
Any future specification needs to mention RADIUS/1.1 only if it adds
fields to the RADIUS/1.1 packet header. Otherwise, transport
considerations for RADIUS/1.1 are identical to RADIUS over (D)TLS.
We reiterate that this specification defines a new transport profile
for RADIUS. It does not define a completely new protocol. Any
future specification which defines a new attribute MUST define it for
RADIUS/UDP first, after which those definitions can be applied to
this transport profile.
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New specifications MAY define new attributes which use the
obfuscation methods for User-Password as defined in [RFC2865],
Section 5.2, or for Tunnel-Password as defined in [RFC2868],
Section 3.5. There is no need for those specifications to describe
how those new attributes are transported in RADIUS/1.1. Since
RADIUS/1.1 does not use MD5, any obfuscated attributes will by
definition be transported as their underlying data type, "text"
([RFC8044], Section 3.4) or "string" ([RFC8044], Section 3.5).
New RADIUS specifications MUST NOT define attributes which can only
be transported via RADIUS over TLS. The RADIUS protocol has no way
to signal the security requirements of individual attributes. Any
existing implementation will handle these new attributes as "invalid
attributes" ([RFC6929], Section 2.8), and could forward them over an
insecure link. As RADIUS security and signaling is hop-by-hop, there
is no way for a RADIUS client or server to even know if such
forwarding is taking place. For these reasons and more, it is
therefore inappropriate to define new attributes which are only
secure if they use a secure transport layer.
The result is that specifications do not need to mention this
transport profile, or make any special provisions for dealing with
it. This specification defines how RADIUS packet encoding, decoding,
signing, and verification are performed when using RADIUS/1.1. So
long as any future specification uses the existing encoding, etc.
schemes defined for RADIUS, no additional text in future documents is
necessary in order to be compatible with RADIUS/1.1.
We note that it is theoretically possible for future standards to
define new cryptographic primitives for use with RADIUS/UDP. In that
case, those documents would likely have to describe how to transport
that data in RADIUS/1.1. We believe that such standards are unlikely
to be published, as other efforts in the RADEXT working group are
forbidding such updates to RADIUS.
8. Implementation Status
(This section to be removed by the RFC editor.)
This specification is being implemented (client and server) in the
FreeRADIUS project which is hosted on GitHub at
https://github.com/FreeRADIUS/freeradius-server/tree/v3.2.x The code
implementation "diff" is approximately 1,000 lines, including build
system changes and changes to configuration parsers.
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9. Privacy Considerations
This specification requires secure transport for RADIUS. RADIUS/1.1.
has all of the privacy benefits of RADIUS/TLS [RFC6614] and RADIUS/
DTLS [RFC7360], and none of the privacy or security issues of RADIUS/
UDP [RFC2865] or RADIUS/TCP [RFC6613].
10. Security Considerations
The primary focus of this document is addressing security
considerations for RADIUS. This specification relies on TLS and
associated ALPN negotiation for much of its security. We refer the
reader to [RFC8446] and [RFC7360] for discussions of the security of
those protocols. The discussion in this section is limited to issues
unique to this specification.
Implementations should rely on the underlying TLS library to perform
ALPN version negotiation. That is, implementations should supply a
list of permitted ALPN strings to the TLS library, and let it return
the negotiated value.
There are few other opportunities for security issues. If an
implementation gets ALPN negotiation wrong, then the wrong
application data will be transported inside of TLS. While RADIUS/1.0
and RADIUS/1.1 share similar packet formats, the protocols are not
mutually compatible.
RADIUS/1.0 requests sent over a RADIUS/1.1 connection may be accepted
by the RADIUS/1.1 server, as the server will ignore the ID field, and
try to use portions of the Request Authenticator as a Token.
However, the reply from the RADIUS/1.1 server will fail the Response
Authenticator validation by the RADIUS/1.0 client. The responses
will therefore be dropped. The client will generally log these
failures, and an administrator will address the issue.
RADIUS/1.1 requests sent over a RADIUS/1.0 connection will generally
be discarded the the RADIUS/1.0 server, as the packets will fail the
Request Authenticator checks. That is, all request packets such as
Accounting-Request, CoA-Request, and Disconnect-Request will be
discarded by the server. For Access-Request packets containing EAP-
Message, the packets will be missing Message-Authenticator, and will
therefore be discarded by the server. Other Access-Request packets
contain obfuscated attributes such as User-Password will have those
attributes decoded to nonsense, and will thus result in Access-Reject
responses.
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RADIUS/1.1 Access-Request packets containing non-obfuscated
attributes such as CHAP-Password may be accepted by a RADIUS/1.0
server but the response will contain a Response Authenticator (i.e.
MD5 hash), and not a Token which matches the Token in the request. A
similar analysis applies for Access-Request packets containing
Service-Type = Authorize-Only.
In conclusion, any mismatch of versions between client and server
will result in most request packets being discarded by the server,
and all response packets being discarded by the client. The two
protocols are therefore incompatible, and safe from misconfigurations
or erroneous implementations.
11. IANA Considerations
IANA is requested to update the "TLS Application-Layer Protocol
Negotiation (ALPN) Protocol IDs" registry with two new entries:
Protocol: RADIUS/1.0
Id. Sequence: 0x72 0x61 0x64 0x69 0x75 0x73 0x2f 0x31 0x2e 0x30
("radius/1.0")
Reference: This document
Protocol: RADIUS/1.1
Id. Sequence: 0x72 0x61 0x64 0x69 0x75 0x73 0x2f 0x31 0x2e 0x31
("radius/1.1")
Reference: This document
12. Acknowledgments
In hindsight, the decision to retain MD5 for historic RADIUS/TLS was
likely wrong. It was an easy decision to make in the short term, but
it has caused ongoing problems which this document addresses.
Thanks to Bernard Aboba, Karri Huhtanen, Heikki Vatiainen, Alexander
Clouter, Michael Richardson, Hannes Tschofenig, Matthew Newton, and
Josh Howlett for reviews and feedback.
13. Changelog
(This section to be removed by the RFC editor.)
draft-dekok-radext-sradius-00
Initial Revision
draft-dekok-radext-radiusv11-00
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Use ALPN from RFC 7301, instead of defining a new port. Drop the
name "SRADIUS".
Add discussion of Original-Packet-Code
draft-dekok-radext-radiusv11-01
Update formatting.
draft-dekok-radext-radiusv11-02
Add Flag field and description.
Minor rearrangements and updates to text.
draft-dekok-radext-radiusv11-03
Remove Flag field and description based on feedback and expected
use-cases.
Use "radius/1.0" instead of "radius/1"
Consistently refer to the specification as "RADIUSv11", and
consistently quote the ALPN name as "radius/1.1"
Add discussion of future attributes and future crypto-agility
work.
draft-dekok-radext-radiusv11-04
Remove "radius/1.0" as it is unnecessary.
Update Introduction with more historical background, which
motivates the rest of the section.
Change Identifier field to be reserved, as it is entirely unused.
Update discussion on clear text passwords.
Clarify discussion of Status-Server, User-Password, and Tunnel-
Password.
Give high level summary of ALPN, clear up client / server roles,
and remove "radius/1.0" as it is unnecessary.
Add text on RFC6421.
draft-dekok-radext-radiusv11-05
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Clarify naming. "radius/1.1" is the ALPN name. "RADIUS/1.1" is
the transport profile.
Clarify that future specifications do not need to make provisions
for dealing with this transport profile.
Typos and word smithing.
Define and use "RADIUS over TLS" instead of RADIUS/(D)TLS.
Many cleanups and rework based on feedback from Matthew Newton.
draft-ietf-radext-radiusv11-00
No changes from previous draft.
draft-ietf-radext-radiusv11-01
Move to "experimental" based on WG feedback.
Many cleanups based on review from Matthew Newton
Removed requirement for supporting TLS-PSK.
This document does not deprecate new cryptographic work in RADIUS.
The "deprecating insecure transports" document does that.
draft-ietf-radext-radiusv11-02
Note that we also update RFC 7930
Minor updates to text.
Add text explaining why "allow" is the default, and how to upgrade
to "require"
Discuss the use of the TLS alert "no_application_protocol" (120),
and its limitations.
Suggest the use of Protocol-Error as an application signal when it
is not possible to send a "no_application_protocol" TLS alert.
Update discussion of Message-Authenticator, and suggest that
RADIUS/1.1 proxies always add Message-Authenticator to Access-
Request packets being sent over UDP or TCP.
Add term "historic RADIUS/TLS" as it is simpler than more awkward
"6614 or 7360".
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Re-add ALPN "radius/1.0" based on comments from Heikki. It
signals that the system is ALPN-capable, among other.
draft-ietf-radext-radiusv11-03
Rename to "RADIUS ALPN and removing MD5"
Add a few things missed when re-adding "radius/1.0"
Clarify wording in a number of places.
draft-ietf-radext-radiusv11-04
Address github issues 3..9
draft-ietf-radext-radiusv11-05
Add discussion of Protocol-Error as per-link signalling.
draft-ietf-radext-radiusv11-06
Review from Paul Wouters
Clarify client handling of Protocol-Error
draft-ietf-radext-radiusv11-07
Clarifications as requested by Jan-Frederik
draft-ietf-radext-radiusv11-08
Review from Claudio Allocchio
draft-ietf-radext-radiusv11-09
Review from Barry Lieba
14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
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[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, DOI 10.17487/RFC2865, June 2000,
<https://www.rfc-editor.org/rfc/rfc2865>.
[RFC6421] Nelson, D., Ed., "Crypto-Agility Requirements for Remote
Authentication Dial-In User Service (RADIUS)", RFC 6421,
DOI 10.17487/RFC6421, November 2011,
<https://www.rfc-editor.org/rfc/rfc6421>.
[RFC6614] Winter, S., McCauley, M., Venaas, S., and K. Wierenga,
"Transport Layer Security (TLS) Encryption for RADIUS",
RFC 6614, DOI 10.17487/RFC6614, May 2012,
<https://www.rfc-editor.org/rfc/rfc6614>.
[RFC6929] DeKok, A. and A. Lior, "Remote Authentication Dial In User
Service (RADIUS) Protocol Extensions", RFC 6929,
DOI 10.17487/RFC6929, April 2013,
<https://www.rfc-editor.org/rfc/rfc6929>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.
[RFC7360] DeKok, A., "Datagram Transport Layer Security (DTLS) as a
Transport Layer for RADIUS", RFC 7360,
DOI 10.17487/RFC7360, September 2014,
<https://www.rfc-editor.org/rfc/rfc7360>.
[RFC8044] DeKok, A., "Data Types in RADIUS", RFC 8044,
DOI 10.17487/RFC8044, January 2017,
<https://www.rfc-editor.org/rfc/rfc8044>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
14.2. Informative References
[EDUROAM] eduroam, "eduroam", n.d., <https://eduroam.org>.
[OPENROAMING]
Alliance, W. B., "OpenRoaming: One global Wi-Fi network",
n.d., <https://wballiance.com/openroaming/>.
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[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/rfc/rfc1321>.
[RFC2548] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes",
RFC 2548, DOI 10.17487/RFC2548, March 1999,
<https://www.rfc-editor.org/rfc/rfc2548>.
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866,
DOI 10.17487/RFC2866, June 2000,
<https://www.rfc-editor.org/rfc/rfc2866>.
[RFC2868] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege,
M., and I. Goyret, "RADIUS Attributes for Tunnel Protocol
Support", RFC 2868, DOI 10.17487/RFC2868, June 2000,
<https://www.rfc-editor.org/rfc/rfc2868>.
[RFC3539] Aboba, B. and J. Wood, "Authentication, Authorization and
Accounting (AAA) Transport Profile", RFC 3539,
DOI 10.17487/RFC3539, June 2003,
<https://www.rfc-editor.org/rfc/rfc3539>.
[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
Dial In User Service) Support For Extensible
Authentication Protocol (EAP)", RFC 3579,
DOI 10.17487/RFC3579, September 2003,
<https://www.rfc-editor.org/rfc/rfc3579>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/rfc/rfc5077>.
[RFC5080] Nelson, D. and A. DeKok, "Common Remote Authentication
Dial In User Service (RADIUS) Implementation Issues and
Suggested Fixes", RFC 5080, DOI 10.17487/RFC5080, December
2007, <https://www.rfc-editor.org/rfc/rfc5080>.
[RFC5176] Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
Aboba, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 5176,
DOI 10.17487/RFC5176, January 2008,
<https://www.rfc-editor.org/rfc/rfc5176>.
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[RFC5281] Funk, P. and S. Blake-Wilson, "Extensible Authentication
Protocol Tunneled Transport Layer Security Authenticated
Protocol Version 0 (EAP-TTLSv0)", RFC 5281,
DOI 10.17487/RFC5281, August 2008,
<https://www.rfc-editor.org/rfc/rfc5281>.
[RFC5931] Harkins, D. and G. Zorn, "Extensible Authentication
Protocol (EAP) Authentication Using Only a Password",
RFC 5931, DOI 10.17487/RFC5931, August 2010,
<https://www.rfc-editor.org/rfc/rfc5931>.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/rfc/rfc6151>.
[RFC6218] Zorn, G., Zhang, T., Walker, J., and J. Salowey, "Cisco
Vendor-Specific RADIUS Attributes for the Delivery of
Keying Material", RFC 6218, DOI 10.17487/RFC6218, April
2011, <https://www.rfc-editor.org/rfc/rfc6218>.
[RFC6613] DeKok, A., "RADIUS over TCP", RFC 6613,
DOI 10.17487/RFC6613, May 2012,
<https://www.rfc-editor.org/rfc/rfc6613>.
[RFC7585] Winter, S. and M. McCauley, "Dynamic Peer Discovery for
RADIUS/TLS and RADIUS/DTLS Based on the Network Access
Identifier (NAI)", RFC 7585, DOI 10.17487/RFC7585, October
2015, <https://www.rfc-editor.org/rfc/rfc7585>.
[RFC7593] Wierenga, K., Winter, S., and T. Wolniewicz, "The eduroam
Architecture for Network Roaming", RFC 7593,
DOI 10.17487/RFC7593, September 2015,
<https://www.rfc-editor.org/rfc/rfc7593>.
[RFC7930] Hartman, S., "Larger Packets for RADIUS over TCP",
RFC 7930, DOI 10.17487/RFC7930, August 2016,
<https://www.rfc-editor.org/rfc/rfc7930>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/rfc/rfc8446>.
Author's Address
Alan DeKok
FreeRADIUS
Email: aland@freeradius.org
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