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Network Working Group D. Nelson
Request for Comments: 5080 Elbrys Networks, Inc
Updates: 2865, 2866, 2869, 3579 A. DeKok
Category: Standards Track FreeRADIUS
December 2007
Common Remote Authentication Dial In User Service (RADIUS)
Implementation Issues and Suggested Fixes
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document describes common issues seen in Remote Authentication
Dial In User Service (RADIUS) implementations and suggests some
fixes. Where applicable, ambiguities and errors in previous RADIUS
specifications are clarified.
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Table of Contents
1. Introduction ....................................................2
1.1. Terminology ................................................3
1.2. Requirements Language ......................................3
2. Issues ..........................................................3
2.1. Session Definition .........................................3
2.1.1. State Attribute .....................................3
2.1.2. Request-ID Supplementation ..........................6
2.2. Overload Conditions ........................................7
2.2.1. Retransmission Behavior .............................7
2.2.2. Duplicate Detection and Orderly Delivery ...........10
2.2.3. Server Response to Overload ........................11
2.3. Accounting Issues .........................................12
2.3.1. Attributes Allowed in an Interim Update ............12
2.3.2. Acct-Session-Id and Acct-Multi-Session-Id ..........12
2.3.3. Request Authenticator ..............................13
2.3.4. Interim-Accounting-Interval ........................13
2.3.5. Counter Values in the RADIUS Management
Information Base (MIB) .............................14
2.4. Multiple Filter-ID Attributes .............................15
2.5. Mandatory and Optional Attributes .........................16
2.6. Interpretation of Access-Reject ...........................18
2.6.1. Improper Use of Access-Reject ......................18
2.6.2. Service Request Denial .............................19
2.7. Addressing ................................................20
2.7.1. Link-Local Addresses ...............................20
2.7.2. Multiple Addresses .................................20
2.8. Idle-Timeout ..............................................21
2.9. Unknown Identity ..........................................21
2.10. Responses After Retransmissions ..........................22
2.11. Framed-IPv6-Prefix .......................................23
3. Security Considerations ........................................24
4. References .....................................................25
4.1. Normative References ......................................25
4.2. Informative References ....................................25
1. Introduction
The last few years have seen an increase in the deployment of RADIUS
clients and servers. This document describes common issues seen in
RADIUS implementations and suggests some fixes. Where applicable,
ambiguities and errors in previous RADIUS specifications are
clarified.
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1.1. Terminology
This document uses the following terms:
Network Access Server (NAS)
The device providing access to the network. Also known as the
Authenticator in IEEE 802.1X or Extensible Authentication Protocol
(EAP) terminology, or RADIUS client.
service
The NAS provides a service to the user, such as network access via
802.11 or Point to Point Protocol (PPP).
session
Each service provided by the NAS to a peer constitutes a session,
with the beginning of the session defined as the point where
service is first provided, and the end of the session is defined
as the point where service is ended. A peer may have multiple
sessions in parallel or series if the NAS supports that, with each
session generating a separate start and stop accounting record.
silently discard
This means the implementation discards the packet without further
processing. The implementation SHOULD provide the capability of
logging the error, including the contents of the silently
discarded packet, and SHOULD record the event in a statistics
counter.
1.2. Requirements Language
In this document, several words are used to signify the requirements
of the specification. The key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
2. Issues
2.1. Session Definition
2.1.1. State Attribute
Regarding the State attribute, [RFC2865] Section 5.24 states:
This Attribute is available to be sent by the server to the client
in an Access-Challenge and MUST be sent unmodified from the client
to the server in the new Access-Request reply to that challenge,
if any.
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This Attribute is available to be sent by the server to the client
in an Access-Accept that also includes a Termination-Action
Attribute with the value of RADIUS-Request. If the NAS performs
the Termination-Action by sending a new Access-Request upon
termination of the current session, it MUST include the State
attribute unchanged in that Access-Request.
Some RADIUS client implementations do not properly use the State
attribute in order to distinguish a restarted EAP authentication
process from the continuation of an ongoing process (by the same user
on the same NAS and port). Where an EAP-Message attribute is
included in an Access-Challenge or Access-Accept attribute, RADIUS
servers SHOULD also include a State attribute. See Section 2.1.2 on
Request ID supplementation for additional benefits to using the State
attribute in this fashion.
As defined in [RFC2865] Table 5.44, Access-Request packets may
contain a State attribute. The table does not qualify this
statement, while the text in Section 5.24 (quoted above) adds other
requirements not specified in that table.
We extend the requirements of [RFC2865] to say that Access-Requests
that are part of an ongoing Access-Request / Access-Challenge
authentication process SHOULD contain a State attribute. It is the
responsibility of the server, to send a State attribute in an
Access-Challenge packet, if that server needs a State attribute in a
subsequent Access-Request to tie multiple Access-Requests together
into one authentication session. As defined in [RFC2865] Section
5.24, the State MUST be sent unmodified from the client to the server
in the new Access-Request reply to that challenge, if any.
While most server implementations require the presence of a State
attribute in an Access-Challenge packet, some challenge-response
systems can distinguish the initial request from the response to the
challenge without using a State attribute to track an authentication
session. The Access-Challenge and subsequent Access-Request packets
for those systems do not need to contain a State attribute.
Other authentication mechanisms need to tie a sequence of Access-
Request / Access-Challenge packets together into one ongoing
authentication session. Servers implementing those authentication
mechanisms SHOULD include a State attribute in Access-Challenge
packets.
In general, if the authentication process involves one or more
Access-Request / Access-Challenge sequences, the State attribute
SHOULD be sent by the server in the Access-Challenge packets. Using
the State attribute to create a multi-packet session is the simplest
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method available in RADIUS today. While other methods of creating
multi-packet sessions are possible (e.g., [RFC3579] Section 2.6.1),
those methods are NOT RECOMMENDED.
The only permissible values for a State attribute are values provided
in an Access-Accept, Access-Challenge, CoA-Request or Disconnect-
Request packet. A RADIUS client MUST use only those values for the
State attribute that it has previously received from a server. An
Access-Request sent as a result of a new or restarted authentication
run MUST NOT include the State attribute, even if a State attribute
has previously been received in an Access-Challenge for the same user
and port.
Access-Request packets that contain a Service-Type attribute with the
value Authorize Only (17) MUST contain a State attribute. Access-
Request packets that contain a Service-Type attribute with value Call
Check (10) SHOULD NOT contain a State attribute. Any other Access-
Request packet that performs authorization checks MUST contain a
State attribute. This last requirement often means that an Access-
Accept needs to contain a State attribute, which can then be used in
a later Access-Request that performs authorization checks.
The standard use case for Call Check is pre-screening authentication
based solely on the end-point identifier information, such as phone
number or Media Access Control (MAC) address in Calling-Station-ID
and optionally Called-Station-ID. In this use case, the NAS has no
way to obtain a State attribute suitable for inclusion in an Access-
Request. Other, non-standard, uses of Call Check may require or
permit the use of a State attribute, but are beyond the scope of this
document.
In an Access-Request with a Service-Type Attribute with value Call
Check, it is NOT RECOMMENDED for the User-Name and User-Password
attributes to contain the same values (e.g., a MAC address).
Implementing MAC address checking without using a Service-Type of
Call Check is NOT RECOMMENDED. This practice gives an attacker both
the clear-text and cipher-text of the User-Password field, which
permits many attacks on the security of the RADIUS protocol. For
example, if the Request Authenticator does not satisfy the [RFC2865]
requirements on global and temporal uniqueness, the practice
described above may lead to the compromise of the User-Password
attribute in other Access-Requests for unrelated users. Access to
the cipher-text enables offline dictionary attacks, potentially
exposing the shared secret and compromising the entire RADIUS
protocol.
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Any Access-Request packet that performs authorization checks,
including Call Check, SHOULD contain a Message-Authenticator
attribute. Any response to an Access-Request performing an
authorization check MUST NOT contain confidential information about
any user (such as Tunnel-Password), unless that Access-Request
contains a State attribute. The use of State here permits the
authorization check to be tied to an earlier user authentication. In
that case, the server MAY respond to the NAS with confidential
information about that user. The server MUST NOT respond to that
authorization check with confidential information about any other
user.
For an Access-Request packet performing an authorization check that
does not contain a State attribute, the server MUST respond with an
Access-Reject.
2.1.2. Request-ID Supplementation
[RFC3579] Section 2.6.1 states:
In EAP, each session has its own unique Identifier space. RADIUS
server implementations MUST be able to distinguish between EAP
packets with the same Identifier existing within distinct
sessions, originating on the same NAS. For this purpose, sessions
can be distinguished based on NAS and session identification
attributes. NAS identification attributes include NAS-Identifier,
NAS-IPv6-Address and NAS-IPv4-Address. Session identification
attributes include User-Name, NAS-Port, NAS-Port-Type, NAS-Port-
Id, Called-Station-Id, Calling-Station-Id and Originating-Line-
Info.
There are issues with the suggested algorithm. Since proxies may
modify Access-Request attributes such as NAS-IP-Address, depending on
any attribute under control of the NAS to distinguish request
identifiers can result in deployment problems.
The FreeRADIUS implementation does not track EAP identifiers by NAS-
IP-Address or other non-EAP attributes sent by the NAS. Instead, it
uses the EAP identifier, source Internet Protocol (IP) address, and
the State attribute as a "key" to uniquely identify each EAP session.
Since the State attribute is under the control of the RADIUS server,
the uniqueness of each session is controlled by the server, not the
NAS. The algorithm used in FreeRADIUS is as follows:
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if (EAP start, or EAP identity) {
allocate unique State Attribute
insert session into "active session" table with
key=(EAP identifier, State, source IP)
} else {
look up active session in table, with above key
}
This algorithm appears to work well in a variety of situations,
including situations where home servers receive messages via
intermediate RADIUS proxies.
Implementations that do not use this algorithm are often restricted
to having an EAP Identifier space per NAS, or perhaps one that is
global to the implementation. These restrictions are unnecessary
when the above algorithm is used, which gives each session a unique
EAP Identifier space. The above algorithm SHOULD be used to track
EAP sessions in preference to any other method.
2.2. Overload Conditions
2.2.1. Retransmission Behavior
[RFC2865] Section 2.4 describes the retransmission requirements for
RADIUS clients:
At one extreme, RADIUS does not require a "responsive" detection
of lost data. The user is willing to wait several seconds for the
authentication to complete. The generally aggressive Transmission
Control Protocol (TCP) retransmission (based on average round trip
time) is not required, nor is the acknowledgment overhead of TCP.
At the other extreme, the user is not willing to wait several
minutes for authentication. Therefore the reliable delivery of
TCP data two minutes later is not useful. The faster use of an
alternate server allows the user to gain access before giving up.
Some existing RADIUS clients implement excessively aggressive
retransmission behavior, utilizing default retransmission timeouts of
one second or less without support for congestive backoff. When
deployed at a large scale, these implementations are susceptible to
congestive collapse. For example, as the result of a power failure,
a network with 3,000 NAS devices with a fixed retransmission timer of
one second will continuously generate 3,000 RADIUS Access-Requests
per second. This is sufficient to overwhelm most RADIUS servers.
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Suggested solutions include:
[a] Jitter. To avoid synchronization, a RADIUS client SHOULD
incorporate induced jitter within its retransmission
algorithm, as specified below.
[b] Congestive backoff. While it is not necessary for RADIUS
client implementations to implement complex retransmission
algorithms, implementations SHOULD support congestive
backoff.
RADIUS retransmission timers are based on the model used in Dynamic
Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315]. Variables
used here are also borrowed from this specification. RADIUS is a
request/response-based protocol. The message exchange terminates
when the requester successfully receives the answer, or the message
exchange is considered to have failed according to the RECOMMENDED
retransmission mechanism described below. Other retransmission
mechanisms are possible, as long as they satisfy the requirements on
jitter and congestive backoff.
The following algorithms apply to any client that originates RADIUS
packets, including but not limited to Access-Request, Accounting-
Request, Disconnect-Request, and CoA-Request [RFC3576].
The retransmission behavior is controlled and described by the
following variables:
RT Retransmission timeout
IRT Initial retransmission time (default 2 seconds)
MRC Maximum retransmission count (default 5 attempts)
MRT Maximum retransmission time (default 16 seconds)
MRD Maximum retransmission duration (default 30 seconds)
RAND Randomization factor
With each message transmission or retransmission, the sender sets RT
according to the rules given below. If RT expires before the message
exchange terminates, the sender re-computes RT and retransmits the
message.
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Each of the computations of a new RT include a randomization factor
(RAND), which is a random number chosen with a uniform distribution
between -0.1 and +0.1. The randomization factor is included to
minimize the synchronization of messages.
The algorithm for choosing a random number does not need to be
cryptographically sound. The algorithm SHOULD produce a different
sequence of random numbers from each invocation.
RT for the first message transmission is based on IRT:
RT = IRT + RAND*IRT
RT for each subsequent message retransmission is based on the
previous value of RT:
RT = 2*RTprev + RAND*RTprev
MRT specifies an upper bound on the value of RT (disregarding the
randomization added by the use of RAND). If MRT has a value of 0,
there is no upper limit on the value of RT. Otherwise:
if (RT > MRT)
RT = MRT + RAND*MRT
MRD specifies an upper bound on the length of time a sender may
retransmit a message. The message exchange fails once MRD seconds
have elapsed since the client first transmitted the message. MRD
MUST be set, and SHOULD have a value between 5 and 30 seconds. These
values mirror the values for a server's duplicate detection cache, as
described in the next section.
MRC specifies an upper bound on the number of times a sender may
retransmit a message. If MRC is zero, the message exchange fails
once MRD seconds have elapsed since the client first transmitted the
message. If MRC is non-zero, the message exchange fails when either
the sender has transmitted the message MRC times, or when MRD seconds
have elapsed since the client first transmitted the message.
For Accounting-Request packets, the default values for MRC, MRD, and
MRT SHOULD be zero. These settings will enable a RADIUS client to
continue sending accounting requests to a RADIUS server until the
request is acknowledged. If any of MRC, MRD, or MRT are non-zero,
then the accounting information could potentially be discarded
without being recorded.
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2.2.2. Duplicate Detection and Orderly Delivery
When packets are retransmitted by a client, the server may receive
duplicate requests. The limitations of the transport protocol used
by RADIUS, the User Datagram Protocol (UDP), means that the Access-
Request packets may be received, and potentially processed, in an
order different from the order in which the packets were sent.
However, the discussion of the Identifier field in Section 3 of
[RFC2865] says:
The RADIUS server can detect a duplicate request if it has the
same client source IP address and source UDP port and Identifier
within a short span of time.
Also, Section 7 of [RFC4669] defines a
radiusAuthServDupAccessRequests object as:
The number of duplicate Access-Request packets received.
This text has a number of implications. First, without duplicate
detection, a RADIUS server may process an authentication request
twice, leading to an erroneous conclusion that a user has logged in
twice. That behavior is undesirable, so duplicate detection is
desirable. Second, the server may track not only the duplicate
request, but also the replies to those requests. This behavior
permits the server to send duplicate replies in response to duplicate
requests, increasing network stability.
Since Access-Request packets may also be sent by the client in
response to an Access-Challenge from the server, those packets form a
logically ordered stream, and, therefore have additional ordering
requirements over Access-Request packets for different sessions.
Implementing duplicate detection results in new packets being
processed only once, ensuring order.
RADIUS servers MUST therefore implement duplicate detection for
Access-Request packets, as described in Section 3 of [RFC2865].
Implementations MUST also cache the Responses (Access-Accept,
Access-Challenge, or Access-Reject) that they send in response to
Access-Request packets. If a server receives a valid duplicate
Access-Request for which it has already sent a Response, it MUST
resend its original Response without reprocessing the request. The
server MUST silently discard any duplicate Access-Requests for which
a Response has not yet been sent.
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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.
Cache entries MUST also be purged if the server receives a valid
Access-Request packet that matches a cached Access-Request packet in
source address, source port, RADIUS Identifier, and receiving socket,
but where the Request Authenticator field is different from the one
in the cached packet. If the request contains a Message-
Authenticator attribute, the request MUST be processed as described
in [RFC3580] Section 3.2. Packets with invalid Message-
Authenticators MUST NOT affect the cache in any way.
However, Access-Request packets not containing a Message-
Authenticator attribute always affect the cache, even though they may
be trivially forged. To avoid this issue, server implementations may
be configured to require the presence of a Message-Authenticator
attribute in Access-Request packets. Requests not containing a
Message-Authenticator attribute MAY then be silently discarded.
Client implementations SHOULD include a Message-Authenticator
attribute in every Access-Request to further help mitigate this
issue.
When sending requests, RADIUS clients MUST NOT reuse Identifiers for
a source IP address and source UDP port until either a valid response
has been received, or the request has timed out. Clients SHOULD
allocate Identifiers via a least-recently-used (LRU) method for a
particular source IP address and source UDP port.
RADIUS clients do not have to perform duplicate detection. When a
client sends a request, it processes the first response that has a
valid Response Authenticator as defined in [RFC2865] Section 3. Any
later responses MUST be silently discarded, as they do not match a
pending request. That is, later responses are treated exactly the
same as unsolicited responses, and are silently discarded.
2.2.3. Server Response to Overload
Some RADIUS server implementations are not robust in response to
overload, dropping packets with even probability across multiple
sessions. In an overload situation, this results in a high failure
rate for multi-round authentication protocols such as EAP [RFC3579].
Typically, users will continually retry in an attempt to gain access,
increasing the load even further.
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A more sensible approach is for a RADIUS server to preferentially
accept RADIUS Access-Request packets containing a valid State
attribute, so that multi-round authentication conversations, once
begun, will be more likely to succeed. Similarly, a server that is
proxying requests should preferentially process Access-Accept,
Access-Challenge, or Access-Reject packets from home servers before
processing new requests from a NAS.
These methods will allow some users to gain access to the network,
reducing the load created by ongoing access attempts.
2.3. Accounting Issues
2.3.1. Attributes Allowed in an Interim Update
[RFC2866] indicates that Acct-Input-Octets, Acct-Output-Octets,
Acct-Session-Time, Acct-Input-Packets, Acct-Output-Packets and Acct-
Terminate-Cause attributes "can only be present in Accounting-Request
records where the Acct-Status-Type is set to Stop".
However [RFC2869] Section 2.1 states:
It is envisioned that an Interim Accounting record (with Acct-
Status-Type = Interim-Update (3)) would contain all of the
attributes normally found in an Accounting Stop message with the
exception of the Acct-Term-Cause attribute.
Although [RFC2869] does not indicate that it updates [RFC2866], this
is an oversight, and the above attributes are allowable in an Interim
Accounting record.
2.3.2. Acct-Session-Id and Acct-Multi-Session-Id
[RFC2866] Section 5.5 describes Acct-Session-Id as Text within the
figure summarizing the attribute format, but then goes on to state
that "The String field SHOULD be a string of UTF-8 encoded 10646
characters".
[RFC2865] defines the Text type as "containing UTF-8 encoded 10646
characters", which is compatible with the description of Acct-
Session-Id. Since other attributes are consistently described as
"Text" within both the figure summarizing the attribute format, and
the following attribute definition, it appears that this is a
typographical error, and that Acct-Session-Id is of type Text, and
not of type String.
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The definition of the Acct-Multi-Session-Id attribute also has
typographical errors. It says:
A summary of the Acct-Session-Id attribute format ...
This text should read:
A summary of the Acct-Multi-Session-Id attribute format ...
The Acct-Multi-Session-Id attribute is also defined as being of type
String. However, the language in the text strongly recommends that
implementors consider the attribute as being of type Text. It is
unclear why the type String was chosen for this attribute when the
type Text would be sufficient. This attribute SHOULD be treated as
Text.
2.3.3. Request Authenticator
[RFC2866] Section 4.1 states:
The Request Authenticator of an Accounting-Request contains a 16-
octet MD5 hash value calculated according to the method described
in "Request Authenticator" above.
However, the text does not indicate any action to take when an
Accounting-Request packet contains an invalid Request Authenticator.
The following text should be considered to be part of the above
description:
The Request Authenticator field MUST contain the correct data, as
given by the above calculation. Invalid packets are silently
discarded. Note that some early implementations always set the
Request Authenticator to all zeros. New implementations of RADIUS
clients MUST use the above algorithm to calculate the Request
Authenticator field. New RADIUS server implementations MUST
silently discard invalid packets.
2.3.4. Interim-Accounting-Interval
[RFC2869] Section 2.1 states:
It is also possible to statically configure an interim value on
the NAS itself. Note that a locally configured value on the NAS
MUST override the value found in an Access-Accept.
This requirement may be phrased too strongly. It is conceivable that
a NAS implementation has a setting for a "minimum" value of Interim-
Accounting-Interval, based on resource constraints in the NAS, and
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network loading in the local environment of the NAS. In such cases,
the value administratively provisioned in the NAS should not be
over-ridden by a smaller value from an Access-Accept message. The
NAS's value could be over-ridden by a larger one, however. The
intent is that the NAS sends accounting information at fixed
intervals that are short enough so that the potential loss of
billable revenue is limited, but also that the accounting updates are
infrequent enough so that the NAS, network, and RADIUS server are not
overloaded.
2.3.5. Counter Values in the RADIUS Management Information Base (MIB)
The RADIUS Authentication and Authorization Client MIB module
([RFC2618] [RFC4668]) includes counters of packet statistics. In the
descriptive text of the MIB module, formulas are provided for certain
counter objects. Implementors have noted apparent inconsistencies in
the formulas that could result in negative values.
Since the original MIB module specified in [RFC2618] had been widely
implemented, the RADEXT WG chose not to change the object definitions
or to create new ones within the revised MIB module [RFC4668].
However, this section explains the issues and provides guidance for
implementors regarding the interpretation of the textual description
and comments for certain MIB objects.
The issues raised can be summarized as follows:
Issue (1):
-- TotalIncomingPackets = Accepts + Rejects + Challenges +
UnknownTypes
--
-- TotalIncomingPackets - MalformedResponses - BadAuthenticators -
-- UnknownTypes - PacketsDropped = Successfully received
--
-- AccessRequests + PendingRequests + ClientTimeouts =
-- Successfully Received
It appears that the value of "Successfully Received" could be
negative, since various counters are subtracted from
TotalIncomingPackets that are not included in the calculation of
TotalIncomingPackets.
It also appears that "AccessRequests + PendingRequests +
ClientTimeouts = Successfully Received" should read "AccessRequests +
PendingRequests + ClientTimeouts = Successfully Transmitted".
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"TotalIncomingPackets" and "Successfully Received" are temporary
variables, i.e., not objects within the MIB module. The comment text
in the MIB modules is intended, therefore, to aid in understanding.
What's of consequence is the consistency of values of the objects in
the MIB module, and that does not appear to be impacted by the
inconsistencies noted above. It does appear, however, that the
"Successfully Received" variable should be labeled "Successfully
Transmitted".
In addition, the definition of Accept, Reject or Challenge counters
indicates that they MUST be incremented before the message is
validated. If the message is invalid, one of MalformedResponses,
BadAuthenticators, or PacketsDropped counters will be additionally
incremented. In that case, the first two equations are consistent,
i.e., "Successfully Received" could not be negative.
Issue (2):
It appears that the radiusAuthClientPendingRequests counter is
decremented upon retransmission. That would mean a retransmitted
packet is not considered as being pending, although such
retransmissions can still be considered as being pending requests.
The definition of this MIB object in [RFC2618] is as follows:
The number of RADIUS Access-Request packets destined for this
server that have not yet timed out or received a response. This
variable is incremented when an Access-Request is sent and
decremented due to receipt of an Access-Accept, Access-Reject or
Access-Challenge, a timeout or retransmission.
This object purports to count the number of pending request packets.
It is open to interpretation whether or not retransmissions of a
request are to be counted as additional pending packets. In either
event, it seems appropriate to treat retransmissions consistently
with respect to incrementing and decrementing this counter.
2.4. Multiple Filter-ID Attributes
[RFC2865] Section 5.11 states:
Zero or more Filter-Id attributes MAY be sent in an Access-Accept
packet.
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In practice, the behavior of a RADIUS client receiving multiple
Filter-ID attributes is implementation dependent. For example, some
implementations treat multiple instances of the Filter-ID attribute
as alternative filters; the first Filter-ID attribute having a name
matching a locally defined filter is used, and the remaining ones are
discarded. Other implementations may combine matching filters.
As a result, the interpretation of multiple Filter-ID attributes is
undefined within RADIUS. The sending of multiple Filter-ID
attributes within an Access-Accept SHOULD be avoided within
heterogeneous deployments and roaming scenarios, where it is likely
to produce unpredictable results.
2.5. Mandatory and Optional Attributes
RADIUS attributes do not explicitly state whether they are optional
or mandatory. Nevertheless, there are instances where RADIUS
attributes need to be treated as mandatory.
[RFC2865] Section 1.1 states:
A NAS that does not implement a given service MUST NOT implement
the RADIUS attributes for that service. For example, a NAS that
is unable to offer Apple Remote Access Protocol (ARAP) service
MUST NOT implement the RADIUS attributes for ARAP. A NAS MUST
treat a RADIUS access-accept authorizing an unavailable service as
an access-reject instead.
With respect to the Service-Type attribute, [RFC2865] Section 5.6
says:
This Attribute indicates the type of service the user has
requested, or the type of service to be provided. It MAY be used
in both Access-Request and Access-Accept packets. A NAS is not
required to implement all of these service types, and MUST treat
unknown or unsupported Service-Types as though an Access-Reject
had been received instead.
[RFC2865] Section 5 states:
A RADIUS server MAY ignore Attributes with an unknown Type.
A RADIUS client MAY ignore Attributes with an unknown Type.
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With respect to Vendor-Specific Attributes (VSAs), [RFC2865] Section
5.26 states:
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.
It is possible for either a standard attribute or a VSA to represent
a request for an unavailable service. However, where the Type,
Vendor-ID, or Vendor-Type is unknown, a RADIUS client will not know
whether or not the attribute defines a service.
In general, it is best for a RADIUS client to err on the side of
caution. On receiving an Access-Accept including an attribute of
known Type for an unimplemented service, a RADIUS client MUST treat
it as an Access-Reject, as directed in [RFC2865] Section 1.1. On
receiving an Access-Accept including an attribute of unknown Type, a
RADIUS client SHOULD assume that it is a potential service
definition, and treat it as an Access-Reject. Unknown VSAs SHOULD be
ignored by RADIUS clients.
In order to avoid introducing changes in default behavior, existing
implementations that do not obey this recommendation should make the
behavior configurable, with the legacy behavior being enabled by
default. A configuration flag such as "treat unknown attributes as
reject" can be exposed to the system administrator. If the flag is
set to true, then Access-Accepts containing unknown attributes are
treated as Access-Rejects. If the flag is set to false, then unknown
attributes in Access-Accepts are silently ignored.
On receiving a packet including an attribute of unknown Type, RADIUS
authentication server implementations SHOULD ignore such attributes.
However, RADIUS accounting server implementations typically do not
need to understand attributes in order to write them to stable
storage or pass them to the billing engine. Therefore, accounting
server implementations SHOULD be equipped to handle unknown
attributes.
To avoid misinterpretation of service requests encoded within VSAs,
RADIUS servers SHOULD NOT send VSAs containing service requests to
RADIUS clients that are not known to understand them. For example, a
RADIUS server should not send a VSA encoding a filter without
knowledge that the RADIUS client supports the VSA.
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2.6. Interpretation of Access-Reject
2.6.1. Improper Use of Access-Reject
The intent of an Access-Reject is to deny access to the requested
service. [RFC2865] Section 2 states:
If any condition is not met, the RADIUS server sends an "Access-
Reject" response indicating that this user request is invalid. If
desired, the server MAY include a text message in the Access-
Reject which MAY be displayed by the client to the user. No other
Attributes (except Proxy-State) are permitted in an Access-Reject.
This text makes it clear that RADIUS does not allow the provisioning
of services within an Access-Reject. If the desire is to allow
limited access, then an Access-Accept can be sent with attributes
provisioning limited access. Attributes within an Access-Reject are
restricted to those necessary to route the message (e.g., Proxy-
State), attributes providing the user with an indication that access
has been denied (e.g., an EAP-Message attribute containing an EAP-
Failure), or attributes conveying an error message (e.g., a Reply-
Message or Error-Cause attribute).
Unfortunately, there are examples where this requirement has been
misunderstood. [RFC2869] Section 2.2 states:
If that authentication fails, the RADIUS server should return an
Access-Reject packet to the NAS, with optional Password-Retry and
Reply-Messages attributes. The presence of Password-Retry
indicates the ARAP NAS MAY choose to initiate another challenge-
response cycle...
This paragraph is problematic from two perspectives. Firstly, a
Password-Retry attribute is being returned in an Access-Reject; this
attribute does not fit into the categories established in [RFC2865].
Secondly, an Access-Reject packet is being sent in the context of a
continuing authentication conversation; [RFC2865] requires use of an
Access-Challenge for this. [RFC2869] uses the phrase "challenge-
response" to describe this use of Access-Reject, indicating that the
semantics of Access-Challenge are being used.
[RFC2865] Section 4.4 addresses the semantics of Access-Challenge
being equivalent to Access-Reject in some cases:
If the NAS does not support challenge/response, it MUST treat an
Access-Challenge as though it had received an Access-Reject
instead.
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While it is difficult to correct existing deployments of [RFC2869],
we make the following recommendations:
[1] New RADIUS specifications and implementations MUST NOT use
Access-Reject where the semantics of Access-Challenge are
intended.
[2] Access-Reject MUST mean denial of access to the requested
service. In response to an Access-Reject, the NAS MUST NOT
send any additional Access-Request packets for that user
session.
[3] New deployments of ARAP [RFC2869] SHOULD use Access-
Challenge instead of Access-Reject packets in the
conversations described in [RFC2869] Section 2.2.
We also note that the table of attributes in [RFC2869] Section 5.19
has an error for the Password-Retry attribute. It says:
Request Accept Reject Challenge # Attribute
0 0 0-1 0 75 Password-Retry
However, the text in [RFC2869], Section 2.3.2 says that Password-
Retry can be included within an Access-Challenge packet for EAP
authentication sessions. We recommend a correction to the table that
removes the "0-1" from the Reject column, and moves it to the
Challenge column. We also add a "Note 2" to follow the existing
"Note 1" in the document to clarify the use of this attribute.
Request Accept Reject Challenge # Attribute
0 0 0 0-1 75 Password-Retry [Note 2]
[Note 2] As per RFC 3579, the use of the Password-Retry in EAP
authentications is deprecated. The Password-Retry attribute can be
used only for ARAP authentication.
2.6.2. Service Request Denial
RADIUS has been deployed for purposes outside network access
authentication, authorization, and accounting. For example, RADIUS
has been deployed as a "back-end" for authenticating Voice Over IP
(VOIP) connections, Hypertext Transfer Protocol (HTTP) sessions
(e.g., Apache), File Transfer Protocol (FTP) sessions (e.g.,
proftpd), and machine logins for multiple operating systems (e.g.,
bsdi, pam, and gina). In those contexts, an Access-Reject sent to
the RADIUS client MUST be interpreted as a rejection of the request
for service, and the RADIUS client MUST NOT offer that service to the
user.
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For example, when an authentication failure occurs in the context of
an FTP session, the normal semantics for rejecting FTP services
apply. The rejection does not necessarily cause the FTP server to
terminate the underlying TCP connection, but the FTP server MUST NOT
offer any services protected by user authentication.
Users may request multiple services from the NAS. Where those
services are independent, the deployment MUST treat the RADIUS
sessions as being independent.
For example, a NAS may offer multi-link services where a user may
have multiple simultaneous network connections. In that case, an
Access-Reject for a later multi-link connection request does not
necessarily mean that earlier multi-link connections are torn down.
Similarly, if a NAS offers both dialup and VOIP services, the
rejection of a VOIP attempt does not mean that the dialup session is
torn down.
2.7. Addressing
2.7.1. Link-Local Addresses
Since Link-Local addresses are unique only on the local link, if the
NAS and RADIUS server are not on the same link, then an IPv6 Link-
Local address [RFC4862] or an IPv4 Link-Local Address [RFC3927]
cannot be used to uniquely identify the NAS. A NAS SHOULD NOT
utilize a link-scope address within a NAS-IPv6-Address or NAS-IP-
Address attribute. A RADIUS server receiving a NAS-IPv6-Address or
NAS-IP-Address attribute containing a Link-Local address SHOULD NOT
count such an attribute toward satisfying the requirements of
[RFC3162] Section 2.1:
NAS-IPv6-Address and/or NAS-IP-Address MAY be present in an
Access-Request packet; however, if neither attribute is present
then NAS-Identifier MUST be present.
2.7.2. Multiple Addresses
There are situations in which a RADIUS client or server may have
multiple addresses. For example, a dual stack host can have both
IPv4 and IPv6 addresses; a host that is a member of multiple VLANs
could have IPv4 and/or IPv6 addresses on each VLAN; a host can have
multiple IPv4 or IPv6 addresses on a single interface. However,
[RFC2865] Section 5.44 only permits zero or one NAS-IP-Address
attributes within an Access-Request, and [RFC3162] Section 3 only
permits zero or one NAS-IPv6-Address attributes within an Access-
Request. When a NAS has more than one global address and no ability
to determine which is used for identification in a particular
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request, it is RECOMMENDED that the NAS include the NAS-Identifier
attribute in an Access-Request in order to identify itself to the
RADIUS server.
[RFC2865] Section 3 states:
A RADIUS server MUST use the source IP address of the RADIUS UDP
packet to decide which shared secret to use, so that RADIUS
requests can be proxied.
Therefore, if a RADIUS client sends packets from more than one source
address, a shared secret will need to be configured on both the
client and server for each source address.
2.8. Idle-Timeout
With respect to the Idle-Timeout attribute, [RFC2865] Section 5.28
states:
This Attribute sets the maximum number of consecutive seconds of
idle connection allowed to the user before termination of the
session or prompt. This Attribute is available to be sent by the
server to the client in an Access-Accept or Access-Challenge.
[RFC3580] Section 3.12 states:
The Idle-Timeout attribute is described in [RFC2865]. For IEEE
802 media other than 802.11 the media are always on. As a result
the Idle-Timeout attribute is typically only used with wireless
media such as IEEE 802.11. It is possible for a wireless device
to wander out of range of all Access Points. In this case, the
Idle-Timeout attribute indicates the maximum time that a wireless
device may remain idle.
In the above paragraphs "idle" may not necessarily mean "no traffic";
the NAS may support filters defining what traffic is included in the
idle time determination. As a result, an "idle connection" is
defined by local policy in the absence of other attributes.
2.9. Unknown Identity
[RFC3748] Section 5.1 states:
If the Identity is unknown, the Identity Response field should be
zero bytes in length.
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However, [RFC2865] Section 5.1 describes the User-Name attribute as
follows:
The String field is one or more octets.
How should the RADIUS client behave if it receives an EAP-
Response/Identity that is zero octets in length?
[RFC2865] Section 5.1 states:
This Attribute indicates the name of the user to be authenticated.
It MUST be sent in Access-Request packets if available.
This suggests that the User-Name attribute may be omitted if it is
unavailable.
However, [RFC3579] Section 2.1 states:
In order to permit non-EAP aware RADIUS proxies to forward the
Access-Request packet, if the NAS initially sends an EAP-
Request/Identity message to the peer, the NAS MUST copy the
contents of the Type-Data field of the EAP-Response/Identity
received from the peer into the User-Name attribute and MUST
include the Type-Data field of the EAP-Response/Identity in the
User-Name attribute in every subsequent Access-Request.
This suggests that the User-Name attribute should contain the
contents of the Type-Data field of the EAP-Response/Identity, even if
it is zero octets in length.
Note that [RFC4282] does not permit a Network Access Identifier (NAI)
of zero octets, so that an EAP-Response/Identity with a Type-Data
field of zero octets MUST NOT be construed as a request for privacy
(e.g., anonymous NAI).
When a NAS receives an EAP-Response/Identity with a Type-Data field
that is zero octets in length, it is RECOMMENDED that it either omit
the User-Name attribute in the Access-Request or include the
Calling-Station-Id in the User-Name attribute, along with a Calling-
Station-Id attribute.
2.10. Responses After Retransmissions
Some implementations do not correctly handle the receipt of RADIUS
responses after retransmissions. [RFC2865] Section 2.5 states:
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If the NAS is retransmitting a RADIUS request to the same server
as before, and the attributes haven't changed, you MUST use the
same Request Authenticator, ID, and source port. If any
attributes have changed, you MUST use a new Request Authenticator
and ID.
Note that changing the Request ID for a retransmission may have
undesirable side effects. Since RADIUS does not have a clear
definition of a "session", it is perfectly valid for a RADIUS server
to treat a retransmission as a new session request, and to reject it
due to, for example, the enforcement of restrictions on multiple
simultaneous logins.
In that situation, the NAS may receive a belated Access-Accept for
the first request, and an Access-Reject for the retransmitted
request, both of which apply to the same "session".
We suggest that the contents of Access-Request packets SHOULD NOT be
changed during retransmissions. If they must be changed due to the
inclusion of an Event-Timestamp attribute, for example, then
responses to earlier transmissions MUST be silently discarded. Any
response to the current request MUST be treated as the definitive
response, even if as noted above, it disagrees with earlier
responses.
This problem can be made worse by implementations that use a fixed
retransmission timeout (30 seconds is common). We reiterate the
suggestions in Section 2.1 about using congestive backoff. In that
case, responses to earlier transmissions MAY be used as data points
for congestive backoff, even if their contents are discarded.
2.11. Framed-IPv6-Prefix
[RFC3162] Section 2.3 says:
This Attribute indicates an IPv6 prefix (and corresponding route)
to be configured for the user. It MAY be used in Access-Accept
packets, and can appear multiple times. It MAY be used in an
Access-Request packet as a hint by the NAS to the server that it
would prefer these prefix(es), but the server is not required to
honor the hint. Since it is assumed that the NAS will plumb a
route corresponding to the prefix, it is not necessary for the
server to also send a Framed-IPv6-Route attribute for the same
prefix.
An Internet Service Provider (ISP) may desire to support Prefix
Delegation [RFC4818] at the same time that it would like to assign a
prefix for the link between the NAS and the user. The intent of the
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paragraph was to enable the NAS to advertise the prefix (such as via
a Router Advertisement). If the Framed-Routing attribute is used, it
is also possible that the prefix would be advertised in a routing
protocol such as Routing Information Protocol Next Generation
(RIPNG). RFC 2865 Section 5.10 describes the purpose of Framed-
Routing:
This Attribute indicates the routing method for the user, when the
user is a router to a network. It is only used in Access-Accept
packets.
The description of the Prefix-Length field in RFC 3162 indicates
excessively wide latitude:
The length of the prefix, in bits. At least 0 and no larger than
128.
This length appears too broad, because it is not clear what a NAS
should do with a prefix of greater granularity than /64. For
example, the Framed-IPv6-Prefix may contain a /128. This does not
imply that the NAS should assign an IPv6 address to the end user,
because RFC 3162 already defines a Framed-IPv6-Identifier attribute
to handle the Identifier portion.
It appears that the Framed-IPv6-Prefix is used for the link between
the NAS and Customer Premises Equipment (CPE) only if a /64 prefix is
assigned. When a /64 or larger prefix is sent, the intent is for the
NAS to send a routing advertisement containing the information
present in the Framed-IPv6-Prefix attribute.
The CPE may also require a delegated prefix for its own use, if it is
decrementing the Hop Limit field of IP headers. In that case, it
should be delegated a prefix by the NAS via the Delegated-IPv6-Prefix
attribute [RFC4818]. If the CPE is not decrementing Hop Limit, it
does not require a delegated prefix.
3. Security Considerations
The contents of the State attribute are available to both the RADIUS
client and observers of the RADIUS protocol. RADIUS server
implementations should ensure that the State attribute does not
disclose sensitive information to a RADIUS client or third parties
observing the RADIUS protocol.
The cache mechanism described in Section 2.2.2 is vulnerable to
attacks when Access-Request packets do not contain a Message-
Authenticator attribute. If the server accepts requests without a
Message-Authenticator, then RADIUS packets can be trivially forged by
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an attacker. Cache entries can then be forcibly expired, negating
the utility of the cache. This attack can be mitigated by following
the suggestions in [RFC3579] Section 4, or by requiring the presence
of Message-Authenticator, as described in Sections 2.1.1 and 2.2.2.
Since this document describes the use of RADIUS for purposes of
authentication, authorization, and accounting in a wide variety of
networks, applications using these specifications are vulnerable to
all of the threats that are present in other RADIUS applications.
For a discussion of these threats, see [RFC2865], [RFC2607],
[RFC3162], [RFC3579], and [RFC3580].
4. References
4.1. 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.
[RFC4818] Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix
Attribute", RFC 4818, April 2007.
4.2. Informative References
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC2618] Aboba, B. and G. Zorn, "RADIUS Authentication Client
MIB", RFC 2618, June 1999.
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[RFC2869] Rigney, C., Willats, W., and P. Calhoun, "RADIUS
Extensions", RFC 2869, June 2000.
[RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",
RFC 3162, August 2001.
[RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
C., and M. Carney, "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC 3315, July 2003.
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[RFC3576] Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
Aboba, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 3576,
July 2003.
[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., and J.
Roese, "IEEE 802.1X Remote Authentication Dial In User
Service (RADIUS) Usage Guidelines", RFC 3580, September
2003.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, June 2004.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
May 2005.
[RFC4282] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005.
[RFC4668] Nelson, D., "RADIUS Authentication Client MIB for IPv6",
RFC 4668, August 2006.
[RFC4669] Nelson, D., "RADIUS Authentication Server MIB for IPv6",
RFC 4669, August 2006.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
[PANA] Forsberg, D., Ohba, Y.,Ed., Patil, B., Tschofenig, H.,
and A. Yegin, "Protocol for Carrying Authentication for
Network Access (PANA)", Work in Progress.
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Acknowledgments
The authors would like to acknowledge Glen Zorn and Bernard Aboba for
contributions to this document.
The alternate algorithm to [RFC3579] Section 2.6.1 that is described
in Section 2.1.2 of this document was designed by Raghu Dendukuri.
The text discussing retransmissions in Section 2.2.1 is taken with
minor edits from Section 9 of" Protocol for Carrying Authentication
for Network Access (PANA)" [PANA].
Alan DeKok wishes to acknowledge the support of Quiconnect Inc.,
where he was employed during much of the work on this document.
David Nelson wishes to acknowledge the support of Enterasys Networks,
where he was employed during much of the work on this document.
Authors' Addresses
David B. Nelson
Elbrys Networks, Inc.
75 Rochester Ave., Unit 3
Portsmouth, N.H. 03801 USA
Phone: +1.603.570.2636
EMail: dnelson@elbrysnetworks.com
Alan DeKok
The FreeRADIUS Server Project
http://freeradius.org/
EMail: aland@freeradius.org
Nelson & DeKok Standards Track [Page 27]
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Nelson & DeKok Standards Track [Page 28]