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Network Working Group                                         Alan DeKok
INTERNET-DRAFT                                                FreeRADIUS
Category: Proposed Standard
<draft-ietf-radext-status-server-03.txt>
Expires: June 16, 2009
16 December 2008


                  Use of Status-Server Packets in the
      Remote Authentication Dial In User Service (RADIUS) Protocol

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Abstract

   RFC 2865 defines a Status-Server code for use in RADIUS, but labels
   it as "Experimental" without further discussion.  This document
   describes a practical use for the Status-Server packet code, which is
   to let clients query the status of a RADIUS server.  These queries,



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   and responses (if any) enable the client to make more informed
   decisions.  The result is a more stable, and more robust RADIUS
   architecture.
















































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Table of Contents

1.  Introduction .............................................    4
   1.1.  Terminology .........................................    4
   1.2.  Requirements Language ...............................    5
2.  Problem Statement ........................................    6
   2.1.  Overloading Access-Request ..........................    6
      2.1.1.  Recommendation against Access-Request ..........    7
   2.2.  Overloading Accounting-Request ......................    7
      2.2.1.  Recommendation against Accounting-Request ......    8
   2.3.  Status-Server as a Solution .........................    8
      2.3.1.  Status-Server to the RADIUS Authentication port.    8
      2.3.2.  Status-Server to the RADIUS Accounting port ....    9
3.  Packet Format ............................................    9
   3.1.  Single definition for Status-Server .................   11
4.  Implementation notes .....................................   11
   4.1.  Client Requirements .................................   12
   4.2.  Server Requirements .................................   14
   4.3.  More Robust Fail-over with Status-Server ............   15
   4.4.  Proxy Server handling of Status-Server ..............   16
   4.5.  Realm Routing .......................................   16
   4.6.  Management Information Base (MIB) Considerations ....   18
      4.6.1.  Interaction with RADIUS Server MIB modules .....   18
      4.6.2.  Interaction with RADIUS Client MIB modules .....   19
5.  Additional considerations ................................   19
   5.1.  Local site testing ..................................   19
   5.2.  RADIUS over reliable transports .....................   21
   5.3.  Other uses for Status-Server ........................   21
6.  Table of Attributes ......................................   21
7.  Examples .................................................   22
   7.1.  Minimal Query to Authentication Port ................   22
   7.2.  Minimal Query to Accounting Port ....................   23
   7.3.  Verbose Query and Response ..........................   24
8.  IANA Considerations ......................................   25
9.  Security Considerations ..................................   25
10.  References ..............................................   25
   10.1.  Normative references ...............................   25
   10.2.  Informative references .............................   25













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1.  Introduction

   The RADIUS Working Group was formed in 1995 to document the protocol
   of the same name, and created a number of standards surrounding the
   protocol.  It also defined experimental commands within the protocol,
   without elaborating further on the potential uses of those commands.
   One of the commands so defined was Status-Server ([RFC2865] Section
   3.).

   This document describes how some current implementations are using
   Status-Server packets as a method for querying the status of a RADIUS
   server.  These queries do not otherwise affect the normal operation
   of a server, and do not result in any side effects other than perhaps
   incrementing an internal packet counter.

   These queries are not intended to implement the application-layer
   watchdog messages described in [RFC3539] Section 3.4.  That document
   describes Authentication, Authorization, and Accounting (AAA)
   protocols that run over reliable transports which handle
   retransmissions internally.  Since RADIUS runs over the User Datagram
   Protocol (UDP) rather than Transport Control Protocol (TCP), the full
   watchdog mechanism is not applicable here.

   The rest of this document is laid out as follows.  Section 2 contains
   the problem statement, and explanations as to why some possible
   solutions can have unwanted side effects.  Section 3 defines the
   Status-Server packet format.  Section 4 contains client and server
   requirements, along with some implementation notes.  Section 5 lists
   additional considerations not covered in the other sections.  The
   remaining text contains a RADIUS table of attributes, and discusses
   security considerations not covered elsewhere in the document.

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 terminology) or RADIUS client.

Home Server
     A RADIUS server that is authoritative for user authorization and
     authentication.

Proxy Server
     A RADIUS server that acts as a Home Server to the NAS, but in turn
     proxies the request to another Proxy Server, or to a Home Server.




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silently discard
     This means the implementation discards the packet without further
     processing.  The implementation MAY provide the capability of
     logging the error, including the contents of the silently discarded
     packet, and SHOULD record the event in a statistics counter.

1.2.  Requirements Language

   In this document, several words are used to signify the requirements
   of the specification.  The key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY",
   and "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].






































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2.  Problem Statement

   It is often useful to know if a RADIUS server is alive and responding
   to requests.  The most accurate way to obtain this information is to
   query the server via application protocol traffic, as other methods
   are either less accurate, or cannot be performed remotely.

   The reasons for wanting to know the status of a server are many.  The
   administrator may simply be curious if the server is responding, and
   may not have access to NAS or traffic data that would give him that
   information.  The queries may also be performed automatically by a
   NAS or proxy server, which is configured to send packets to a RADIUS
   server, and where that server may not be responding.  That is, while
   [RFC2865] Section 2.6 indicates that sending Keep-Alives is harmful,
   it may be useful to send "Are you Alive" queries to a server once it
   has been marked "dead" due to prior unresponsiveness.

   The occasional query to a "dead" server offers little additional load
   on the network or server, and permits clients to more quickly
   discover when the server returns to a responsive state.  Overall,
   status queries can be a useful part of the deployment of a RADIUS
   server.

2.1.  Overloading Access-Request

   One possible solution to the problem of querying server status is for
   a NAS to send specially formed Access-Request packets to a RADIUS
   server's authentication port.  The NAS can then look for a response,
   and use this information to determine if the server is active or
   unresponsive.

   However, the server may see the request as a normal login request for
   a user, and conclude that a real user has logged onto that NAS.  The
   server may then perform actions that are undesirable for a simple
   status query.  The server may alternatively respond with an Access-
   Challenge, indicating that it believes an extended authentication
   conversation is necessary.

   Another possibility is that the server responds with an Access-
   Reject, indicating that the user is not authorized to gain access to
   the network.  As above, the server may also perform local site
   actions, such as warning an administrator of failed login attempts.
   The server may also delay the Access-Reject response, in the
   traditional manner of rate-limiting failed authentication attempts.
   This delay in response means that the querying administrator is
   unsure as to whether or not the server is down, is slow to respond,
   or is intentionally delaying its response to the query.




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   In addition, using Access-Request queries may mean that the server
   may have local users configured whose sole reason for existence is to
   enable these query requests.  Unless the server's policy is designed
   carefully, it may be possible for an attacker to use those
   credentials to gain unauthorized network access.

   We note that some NAS implementations currently use Access-Request
   packets as described above, with a fixed (and non configurable) user
   name and password.  Implementation issues with that equipment means
   that if a RADIUS server does not respond to those queries, it may be
   marked as unresponsive by the NAS.  This marking may happen even if
   the server is actively responding to other Access-Requests from that
   same NAS.  This behavior is confusing to administrators who then need
   to determine why an active server has been marked as "unresponsive".

2.1.1.  Recommendation against Access-Request

   For the reasons outlined above, NAS implementors SHOULD NOT generate
   Access-Request packets solely to see if a server is alive.
   Similarly, site administrators SHOULD NOT configure test users whose
   sole reason for existence is to enable such queries via Access-
   Request packets.

   Note that it still may be useful to configure test users for the
   purpose of performing end-to-end or in-depth testing of a servers
   policy.  While this practice is widespread, we caution administrators
   to use it with care.

2.2.  Overloading Accounting-Request

   A similar solution for the problem of querying server status may be
   for a NAS to send specially formed Accounting-Request packets to a
   RADIUS servers accounting port.  The NAS can then look for a
   response, and use this information to determine if the server is
   active or unresponsive.

   As seen above with Access-Request, the server may then conclude that
   a real user has logged onto a NAS, and perform local site actions
   that are undesirable for a simple status query.

   Another consideration is that some attributes are mandatory to
   include in an Accounting-Request.  This requirement forces the
   administrator to query an accounting server with fake values for
   those attributes in a test packet.  These fake values increase the
   work required to perform a simple query, and may pollute the server's
   accounting database with incorrect data.





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2.2.1.  Recommendation against Accounting-Request

   For the reasons outlined above, NAS implementors SHOULD NOT generate
   Accounting-Request packets solely to see if a server is alive.
   Similarly, site administrators SHOULD NOT configure accounting
   policies whose sole reason for existence is to enable such queries
   via Accounting-Request packets.

   Note that it still may be useful to configure test users for the
   purpose of performing end-to-end or in-depth testing of a servers
   policy.  While this practice is widespread, we caution administrators
   to use it with care.

2.3.  Status-Server as a Solution

   A better solution to the above problems is to use the Status-Server
   packet code.  The name of the code leads us to conclude that it was
   intended for packets that query the status of a server.  Since the
   packet is otherwise undefined, it does not cause interoperability
   issues to create implementation-specific definitions for it.  The
   difficulty until now has been defining an interoperable method of
   performing these queries.

   This document addresses that need.

2.3.1.  Status-Server to the RADIUS Authentication port

   Status-Server SHOULD be used instead of Access-Request to query the
   responsiveness of a server.  In this use case, the protocol exchange
   between client and server is similar to the usual exchange of Access-
   Request and Access-Accept, as shown below.

           NAS                          RADIUS server
           ---                          -------------
           Status-Server/
            Message-Authenticator ->
                                     <- Access-Accept/
                                         Reply-Message

   The Status-Server packet MUST contain a Message-Authenticator
   attribute for security.  The response (if any) to a Status-Server
   packet sent to an authentication port SHOULD be an Access-Accept
   packet.  Other response packet codes are NOT RECOMMENDED.  The list
   of attributes that are permitted in the Access-Accept packet is given
   in the Table of Attributes in Section 6, below.






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2.3.2.  Status-Server to the RADIUS Accounting port

   Status-Server MAY be used instead of Accounting-Request to query the
   responsiveness of a server.  In this use case, the protocol exchange
   between client and server is similar to the usual exchange of
   Accounting-Request and Accounting-Response, as shown below.

           NAS                          RADIUS server
           ---                          -------------
           Status-Server/
            Message-Authenticator ->
                                     <- Accounting-Response

   The Status-Server packet MUST contain a Message-Authenticator
   attribute for security.  The response (if any) to a Status-Server
   packet sent to an accounting port SHOULD be an Accounting-Response
   packet.  Other response packet codes are NOT RECOMMENDED.  The list
   of attributes that are permitted in the Accounting-Response packet is
   given in the Table of Attributes in Section 6, below.

3.  Packet Format

   Status-Server packets reuse the RADIUS packet format, with the fields
   and values for those fields as defined [RFC2865] Section 3.  We do
   not include all of the text or diagrams of that section here, but
   instead explain the differences required to implement Status-Server.

   The Authenticator field of Status-Server packets MUST be generated
   using the same method as that used for the Request Authenticator
   field of Access-Request packets, as given below.

   The role of the Identifier field is the same for Status-Server as for
   other packets.  However, as Status-Server is taking the role of
   Access-Request or Accounting-Request packets, there is the potential
   for Status-Server requests to be in conflict with Access-Request or
   Accounting-Request packets with the same Identifier.  In Section 4.2,
   below, we describe a method for avoiding these problems.  This method
   MUST be used to avoid conflicts between Status-Server and other
   packet types.

      Request Authenticator

         In Status-Server Packets, the Authenticator value is a 16 octet
         random number, called the Request Authenticator.  The value
         SHOULD be unpredictable and unique over the lifetime of a
         secret (the password shared between the client and the RADIUS
         server), since repetition of a request value in conjunction
         with the same secret would permit an attacker to reply with a



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         previously intercepted response.  Since it is expected that the
         same secret MAY be used to authenticate with servers in
         disparate geographic regions, the Request Authenticator field
         SHOULD exhibit global and temporal uniqueness.

         The Request Authenticator value in a Status-Server packet
         SHOULD also be unpredictable, lest an attacker trick a server
         into responding to a predicted future request, and then use the
         response to masquerade as that server to a future Status-Server
         request from a client.

   Similarly, the Response Authenticator field of an Access-Accept
   packet sent in response to Status-Server queries MUST be generated
   using the same method as used for for calculating the Response
   Authenticator of the Access-Accept sent in response to an Access-
   Request, with the Status-Server Request Authenticator taking the
   place of the Access-Request Request Authenticator.

   The Response Authenticator field of an Accounting-Response packet
   sent in response to Status-Server queries MUST be generated using the
   same method as used for for calculating the Response Authenticator of
   the Accounting-Response sent in response to an Accounting-Request,
   with the Status-Server Request Authenticator taking the place of the
   Accounting-Request Request Authenticator.

   Note that when a server responds to a Status-Server request, it MUST
   NOT send more than one response packet.

      Response Authenticator

         The value of the Authenticator field in Access-Accept, or
         Accounting-Response packets is called the Response
         Authenticator, and contains a one-way MD5 hash calculated over
         a stream of octets consisting of: the RADIUS packet, beginning
         with the Code field, including the Identifier, the Length, the
         Request Authenticator field from the Status-Server packet, and
         the response Attributes (if any), followed by the shared
         secret.  That is, ResponseAuth =
         MD5(Code+ID+Length+RequestAuth+Attributes+Secret) where +
         denotes concatenation.

   In addition to the above requirements, all Status-Server packets MUST
   include a Message-Authenticator attribute.  Failure to do so would
   mean that the packets could be trivially spoofed.

   Status-Server packets MAY include NAS-Identifier, and one of NAS-IP-
   Address or NAS-IPv6-Address.  These attributes are not necessary for
   the operation of Status-Server, but may be useful information to a



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   server that receives those packets.

   Other attributes SHOULD NOT be included in a Status-Server packet.
   User authentication credentials such as User-Password, CHAP-Password,
   EAP-Message, etc. MUST NOT appear in a Status-Server packet sent to a
   RADIUS authentication port.  User or NAS accounting attributes such
   as Acct-Session-Id, Acct-Status-Type, Acct-Input-Octets, etc.  MUST
   NOT appear in a Status-Server packet sent to a RADIUS accounting
   port.

   The Access-Accept MAY contain a Reply-Message or Message-
   Authenticator attribute.  It SHOULD NOT contain other attributes.
   The Accounting-Response packets sent in response to a Status-Server
   query SHOULD NOT contain any attributes.  As the intent is to
   implement a simple query instead of user authentication or
   accounting, there is little reason to include other attributes in
   either the query or the corresponding response.

   Examples of Status-Server packet flows are given below in Section 7.

3.1.  Single definition for Status-Server

   When sent to a RADIUS accounting port, contents of the Status-Server
   packets are calculated as described above.  That is, even though the
   packets are being sent to an accounting port, they are not created
   using the same method as for Accounting-Requests.  This difference
   has a number of benefits.

   Having a single definition for Status-Server packets is simpler than
   having different definitions for different destination ports.  In
   addition, if we were to define Status-Server as being similar to
   Accounting-Request but containing no attributes, then those packets
   could be trivially forged.

   We therefore define Status-Server consistently, and vary the response
   packets depending on the port to which the request is sent.  When
   sent to an authentication port, the response to a Status-Server query
   is an Access-Accept packet.  When sent to an accounting port, the
   response to a Status-Server query is an Accounting-Response packet.

4.  Implementation notes

   There are a number of considerations to take into account when
   implementing support for Status-Server.  This section describes
   implementation details and requirements for RADIUS clients and
   servers that support Status-Server.

   The following text applies to the authentication and accounting



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   ports.  We use the generic terms below to simplify the discussion:

      * Request packet

        An Access-Request packet sent to an authentication port, or
        an Accounting-Request packet sent to an accounting port

      * Response packet

        An Access-Accept, Access-Challenge, or Access-Reject packet sent
        from an authentication port, or an Accounting-Response packet
        sent from an accounting port.

   We also refer to "client" as the originator of the Status-Server
   packet, and "server" as the receiver of that packet, and the
   originator of the Response packet.

   Using generic terms to describe the Status-Server conversations is
   simpler than duplicating the text for authentication, and accounting
   packets.

4.1.  Client Requirements

   Clients SHOULD permit administrators to globally enable or disable
   the generation of Status-Server packets.  The default SHOULD be that
   it is disabled.  As it is undesirable to send queries to servers that
   do not support Status-Server, clients SHOULD also have a per-server
   configuration indicating whether or not to enable Status-Server for a
   particular destination.  The default SHOULD be that it is disabled.

   The client SHOULD also have a configurable global timer (Tw) that is
   used when sending periodic Status-Server queries during server fail-
   over.  The default value SHOULD be 30 seconds, and the value MUST NOT
   be permitted to be set below 6 seconds.  If a response has not been
   received within the timeout period, the Status-Server packet is
   deemed to have received no corresponding Response packet, and MUST be
   discarded.

   Clients SHOULD use a jitter of +/- 2 seconds when sending periodic
   Status-Server packets, in order to avoid synchronization.

   When Status-Server packets are sent from a client, they MUST NOT be
   retransmitted.  Instead, the Identity field MUST be changed every
   time a packet is transmitted.  The old packet should be discarded,
   and a new Status-Server packet should be generated and sent, with new
   Identity and Authenticator fields.

   Clients MUST include the Message-Authenticator attribute in all



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   Status-Server packets.  Failure to do so would mean that the packets
   could be trivially spoofed, leading to potential denial of service
   (DoS) attacks.  Other attributes SHOULD NOT appear in a Status-Server
   packet, except as outlined below in Section 6.  As the intent of the
   packet is a simple status query, there is little reason for any
   additional attributes to appear in Status-Server packets.

   The client MAY increment packet counters as a result of sending a
   Status-Server request, or receiving a Response packet.  The client
   MUST NOT perform any other action that is normally performed when it
   receives a Response packet, such as permitting a user to have login
   access to a port.

   Clients MAY send Status-Server requests to the RADIUS destination
   ports from the same source port used to send normal Request packets.
   Other clients MAY choose to send Status-Server requests from a unique
   source port, that is not used to send Request packets.

   The above suggestion for a unique source port for Status-Server
   packets aids in matching responses to requests.  Since the response
   to a Status-Server packet is an Access-Accept or Accounting-Response
   packet, those responses are indistinguishable from other packets sent
   in response to a Request packet.  Therefore, the best way to
   distinguish them from other traffic is to have a unique port.

   A client MAY send a Status-Server packet from a source port also used
   to send Request packets.  In that case, the Identifer field MUST be
   unique across all outstanding Request packets for that source port,
   independent of the value of the RADIUS Code field for those
   outstanding requests.  Once the client has either received a response
   to the Status-Server packet, or has determined that the Status-Server
   packet has timed out, it may reuse that Identifier in another packet.

   Robust implementations SHOULD accept any Response packet as a valid
   response to a Status-Server packet, subject to the validation
   requirements defined above for the Response Authenticator.  The code
   field of the packet matters less than the fact that a valid, signed,
   response has been received.

   That is, prior to accepting the response as valid, the client should
   check that the Response packet Code field is either Access-Accept (2)
   or Accounting-Response (5).  If the code does not match any of these
   values, the packet MUST be silently discarded.  The client MUST then
   validate the Response Authenticator via the algorithm given above in
   Section 3.  If the Response Authenticator is not valid, the packet
   MUST be silently discarded.  If the Response Authenticator is valid,
   then the packet MUST be deemed to be a valid response from the
   server.



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   If the client instead discarded the response because the packet code
   did not match what it expected, then it could erroneously discard
   valid responses from a server, and mark that server as unresponsive.
   This behavior would affect the stability of a RADIUS network, as
   responsive servers would erroneously be marked as unresponsive.  We
   therefore recommend that clients should be liberal in what they
   accept as responses to Status-Server queries.

4.2.  Server Requirements

   Servers SHOULD permit administrators to globally enable or disable
   the acceptance of Status-Server packets.  The default SHOULD be that
   it is enabled.  Servers SHOULD also permit adminstrators to enable or
   disable acceptance of Status-Server packets on a per-client basis.
   The default SHOULD be that it is enabled.

   Status-Server packets originating from clients that are not permitted
   to send the server Request packets MUST be silently discarded.  If a
   server does not support Status-Server packets, or is configured to
   not respond to them, then it MUST silently discard the packet.

   We note that [RFC2865] Section 3 defines a number of RADIUS Codes,
   but does not make statements about which Codes are valid for port
   1812.  In contrast, [RFC2866] Section 3 specifies that only RADIUS
   Accounting packets are to be sent to port 1813.  This specification
   is compatible with [RFC2865], as it uses a known Code for packets to
   port 1812.  This specification is not compatible with [RFC2866], as
   it adds a new code (Status-Server) that is valid for port 1812.
   However, as the category of [RFC2866] is Informational, this conflict
   is acceptable.

   Servers SHOULD silently discard Status-Server packets if they
   determine that a client is sending too many Status-Server requests in
   a particular time period.  The method used by a server to make this
   determination is implementation-specific, and out of scope for this
   specification.

   If a server supports Status-Server packets, and is configured to
   respond to them, and receives a packet from a known client, it MUST
   validate the Message-Authenticator attribute as defined in [RFC3579]
   Section 3.2.  Packets failing that validation MUST be silently
   discarded.

   Servers SHOULD NOT otherwise discard Status-Server packets if they
   have recently sent the client a Response packet.  The query may have
   originated from an administrator who does not have access to the
   Response packet stream, or who is interested in obtaining additional
   information about the server.



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   The server MAY prioritize the handling of Status-Server packets over
   the handling of other requests, subject to the rate limiting
   described above.

   The server MAY decide to not respond to a Status-Server, depending on
   local site policy.  For example, a server that is running but is
   unable to perform its normal activities MAY silently discard Status-
   Server packets.  This situation can happen, for example, when a
   server requires access to a database for normal operation, but the
   connection to that database is down.  Or, it may happen when the
   accepted load on the server is lower than the offered load.

   Some server implementations require that Access-Request packets are
   accepted only on "authentication" ports, (e.g. 1812/udp), and that
   Accounting-Request packets are accepted only on "accounting" ports
   (e.g. 1813/udp).  Those implementations SHOULD reply to Status-Server
   packets sent to an "authentication" port with an Access-Accept
   packet.  Those implementations SHOULD reply to Status-Server packets
   sent to an "accounting" port with an Accounting-Response packet.

   Some server implementations accept both Access-Request and
   Accounting-Request packets on the same port, and do not distinguish
   between "authentication only" ports, and "accounting only" ports.
   Those implementations SHOULD reply to Status-Server packets with an
   Access-Accept packet.

   The server MAY increment packet counters as a result of receiving a
   Status-Server, or sending a Response packet.  The server SHOULD NOT
   perform any other action that is normally performed when it receives
   a Request packet, other than sending a Response packet.

4.3.  More Robust Fail-over with Status-Server

   A common problem in RADIUS client implementations is the
   implementation of a robust fail-over mechanism between servers.  A
   client may have multiple servers configured, with one server marked
   as primary and another marked as secondary.  If the client determines
   that the primary is unresponsive, it can "fail over" to the
   secondary, and send requests to the secondary instead of to the
   primary.

   However, it is difficult in standard RADIUS for a client to know when
   it should start sending requests to the primary again.  Sending test
   Access-Requests or Accounting-Requests to see if the server is alive
   has the issues outlined above in Section 2.  Clients could
   alternately send real traffic to the primary, on the hope that it is
   responsive.  If the server is still unresponsive, however, the result
   may be user login failures.  The Status-Server solution is an ideal



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   way to solve this problem.

   When a client fails over from one server to another because of a lack
   of responsiveness, it SHOULD send periodic Status-Server packets to
   the unresponsive server, using the timer (Tw) defined above.

   Once three time periods have passed where Status-Server packets have
   been sent and responded to, the server should be deemed responsive
   and RADIUS requests may sent to it again.  This determination should
   be made separately for each server that the client has a relationship
   with.  The same algorithm should be used for both authentication and
   accounting ports.  The client MUST treat each destination (ip, port)
   combination as a unique server for the purposes of this
   determination.

   The above behavior is modelled after [RFC3539] Section 3.4.1.  We
   note that if a reliable transport is used for RADIUS, then the
   algorithms specified in [RFC3539] MUST be used in preference to the
   ones given here.

4.4.  Proxy Server handling of Status-Server

   Many RADIUS servers can act as proxy servers, and can forward
   requests to home servers.  Such servers MUST NOT proxy Status-Server
   packets.  The purpose of Status-Server as specified here is to permit
   the client to query the responsiveness of a server that it has a
   direct relationship with.  Proxying Status-Server queries would
   negate any usefulness that may be gained by implementing support for
   them.

   Proxy servers MAY be configured to respond to Status-Server queries
   from clients, and MAY act as clients sending Status-Server queries to
   other servers.  However, those activities MUST be independent of one
   another.

4.5.  Realm Routing

   RADIUS servers are commonly used in an environment where Network
   Access Identifiers (NAIs) are used as routing identifiers [RFC4282].
   In this practice, the User-Name attribute is decorated with realm
   routing information, commonly in the format of "user@realm".  Since a
   particular RADIUS server may act as a proxy for more than one realm,
   the mechanism outlined above may be inadequate.

   The schematic below demonstrates this scenario.

                /-> Proxy Server P -----> Home Server for Realm A
               /                    \ /



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            NAS                      X
               \                    / \
                \-> Proxy Server S -----> Home Server for Realm B

   That is, the NAS has relationships with two Proxy Servers, P and S.
   Each Proxy Server has relationships with Home Servers for both Realm
   A and Realm B.

   In this scenario, the Proxy Servers can determine if one or both of
   the Home Servers are dead or unreachable.  The NAS can determine if
   one or both of the Proxy Servers are dead or unreachable.  There is
   an additional case to consider, however.

   If Proxy Server P cannot reach the Home Server for Realm A, but the
   Proxy Server S can reach that Home Server, then the NAS cannot
   discover this information using the Status-Server queries as outlined
   above.  It would therefore be useful for the NAS to know that Realm A
   is reachable from Proxy Server S, as it can then route all requests
   for Realm A to that Proxy Server.  Without this knowledge, the client
   may route requests to Proxy Server P, where they may be discarded or
   rejected.

   To complicate matters, the behavior of Proxy Servers P and S in this
   situation is not well defined.  Some implementations simply fail to
   respond to the request, and other implementations respond with an
   Access-Reject.  If the implementation fails to respond, then the NAS
   cannot distinguish between the Proxy Server being down, or the next
   server along the proxy chain being unreachable.

   In the worst case, failures in routing for Realm A may affect users
   of Realm B.  For example, if Proxy Server P can reach Realm B but not
   Realm A, and Proxy Server S can reach Realm A but not Realm B, then
   active paths exist to handle all RADIUS requests.  However, depending
   on the NAS and Proxy Server implementation choices, the NAS may not
   be able to determine which server requests may be sent to in order to
   maintain network stability.

   This problem cannot, unfortunately be solved by using Status-Server
   requests.  A robust solution would involve either a RADIUS routing
   table for the NAI realms, or a RADIUS "destination unreachable"
   response to authentication requests.  Either solution would not fit
   into the traditional RADIUS model, and both are therefore outside of
   the scope of this specification.

   The problem is discussed here in order to define how best to use
   Status-Server in this situation, rather than to define a new
   solution.




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   When a server has responded recently to a request from a client, that
   client MUST mark the server as "responsive".  In the above case, a
   Proxy Server may be responding to requests destined for Realm A, but
   not responding to requests destined for Realm B.  The client
   therefore considers the server to be responsive, as it is receiving
   responses from the server.

   The client will then continue to send requests to the Proxy Server
   for destination Realm B, even though the Proxy Server cannot route
   the requests to that destination.  This failure is a known limitation
   of RADIUS, and can be partially addressed through the use of failover
   in the Proxy Servers.

   A more realistic situation than the one outlined above is where each
   Proxy Server also has multiple choices of Home Servers for a realm,
   as outlined below.

                /-> Proxy Server P -----> Home Server P
               /                    \ /
            NAS                      X
               \                    / \
                \-> Proxy Server S -----> Home Server S

   In this situation, if all participants implement Status-Server as
   defined herein, any one link may be broken, and all requests from the
   NAS will still reach a home server.  If two links are broken at
   different places, (i.e. not both links from the NAS), then all
   requests from the NAS will still reach a home server.  In many
   situations where three or more links are broken, then requests from
   the NAS may still reach a home server.

   It is RECOMMENDED, therefore, that implementations desiring the most
   benefit from Status-Server also implement server failover.  The
   combination of these two practices will maximize network reliability
   and stability.

4.6.  Management Information Base (MIB) Considerations


4.6.1.  Interaction with RADIUS Server MIB modules

   Since Status-Server packets are sent to the defined RADIUS ports,
   they can affect the [RFC4669] and [RFC4671] RADIUS server MIB
   modules.  [RFC4669] defines a counter named
   radiusAuthServTotalUnknownTypes, that counts "The number of RADIUS
   packets of unknown type that were received".  [RFC4671] defines a
   similar counter named radiusAcctServTotalUnknownTypes.
   Implementations not supporting Status-Server, or implementations that



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   are configured to not respond to Status-Server packets MUST use these
   counters to track received Status-Server packets.

   If, however, Status-Server is supported and the server is configured
   to respond as described above, then the counters defined in [RFC4669]
   and [RFC4671] MUST NOT be used to track Status-Server requests or
   responses to those requests.  That is, when a server fully implements
   Status-Server, the counters defined in [RFC4669] and [RFC4671] MUST
   be unaffected by the transmission or reception of packets relating to
   Status-Server.

   If a server supports Status-Server and the [RFC4669] or [RFC4671] MIB
   Modules, then it SHOULD also support vendor-specific MIB extensions
   dedicated solely to tracking Status-Server requests and responses.
   Any definition of the server MIB modules for Status-Server is outside
   of the scope of this document.

4.6.2.  Interaction with RADIUS Client MIB modules

   Clients implementing Status-Server MUST NOT increment [RFC4668] or
   [RFC4670] counters upon reception of Response packets to Status-
   Server queries.  That is, when a server fully implements Status-
   Server, the counters defined in [RFC4668] and [RFC4670] MUST be
   unaffected by the transmission or reception of packets relating to
   Status-Server.

   If an implementation supports Status-Server and the [RFC4668] or
   [RFC4670] MIB modules, then it SHOULD also support vendor-specific
   MIB extensions containing similar information as those MIB modules,
   but which are instead dedicated solely to tracking Status-Server
   requests and responses.  Any definition of the client MIB module
   extensions for Status-Server is outside of the scope of this
   document.

5.  Additional considerations

   There are additional topics related to the use of Status-Server that
   may be covered.  As those topics do not fit well into the preceding
   sections, they are covered herein.

5.1.  Local site testing

   There is at least one situation where using Access-Request or
   Accounting-Request packets may be useful, despite the recommendations
   above in Section 2.1.1 and Section 2.2.1.  That situation is local
   site testing, where the RADIUS client, server, and user store are
   under the control of a single administrator or administrative entity.
   In that situation, administrators MAY configure a well-known "test"



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   user to enable local site testing.

   The advantage to creating such a local user is that it is now
   possible for the administrator to send a RADIUS request that performs
   end-to-end testing of the RADIUS server.  As above with Status-
   Server, this testing includes RADIUS server responsiveness.  It may
   also include querying databases of user authentication credentials,
   or storing accounting data to a billing database.  The information
   obtained from performing those queries is that the entire RADIUS
   server infrastructure, including all of its dependencies, is
   functioning as expected.  These queries are most useful in
   deployments where an administrator has internal RADIUS servers that
   proxy to other internal RADIUS servers, such as for load balancing or
   fail over.

   If used, the names utilized for these test users SHOULD be difficult
   to guess by an attacker.  An Access-Request packet for a test user
   otherwise should be treated as follows, depending on its origin:

      o Packets from localhost (127.0.0.1 or ::1):  RADIUS servers
      SHOULD treat the request according to local site policy.

      o Packets from NASes that normally originate Access-Request
      packets (i.e. not proxy servers):  RADIUS servers SHOULD respond
      with an Access-Reject packet, as the use of Status-Server is
      preferred.

      o Packets from other machines controlled by the administrator:
      RADIUS servers SHOULD treat the request according to local site
      policy.

      o Packets originating from machines not controlled by the
      administrator:  RADIUS servers MUST respond with an Access-Reject
      packet.

   If a RADIUS server is configured to support test users for
   Accounting-Request packets, it MAY respond with an Accounting-
   Response packet, independent of the origin of the request.  However,
   any subsequent analysis of the accounting data such as billing or
   usage MUST NOT include the data for the test user.

   If these recommendations are implemented, then it may be possible in
   some situations to safely query a RADIUS server for responsiveness
   using Access-Request or Accounting-Request packets.  However, this
   behavior is still NOT RECOMMENDED.






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5.2.  RADIUS over reliable transports

   Although RADIUS has been assigned two TCP ports (1812/tcp and
   1813/tcp) in addition to the commonly used UDP ports, there has been
   as yet no specification for using TCP as a reliable transport for
   RADIUS.  If such a specification were to be created, then the
   transport issues discussed in [RFC3539] would apply.

   Further, when RADIUS is run over reliable transports, the watchdog
   algorithm described in [RFC3539] Section 3.4 MUST be used rather than
   the algorithm described above.  For the reasons outlined above in
   Section 2, Status-Server packets SHOULD be used as the watchdog
   request, in preference to Access-Request or Accounting-Request
   packets.

   Clients sending Status-Server over reliable transport MUST ensure
   that the Identifier field is unique for all requests on a particular
   connection, independent of the packet code.  That is, if a Status-
   Server with a particular value in the Identifier field is sent to a
   server, the client MUST NOT simultaneously send an Access-Request or
   Accounting-Request packet with that same Identifier value, on that
   connection.  Once the client has either received a response to the
   Status-Server packet, or has determined that the Status-Server packet
   has timed out, it may reuse that Identifier in another packet.

5.3.  Other uses for Status-Server

   While other uses of Status-Server are possible, uses beyond those
   specified here are beyond the scope of this document.  It may be
   tempting to increase the utility of Status-Server by having the
   responses carry additional information, but implementors are warned
   that such uses have not been analyzed for potential security issues
   or network problems.

   Specifically, it may seem useful to leverage a combination of Status-
   Server and CoA ports in order to send realm routing information
   "upstream" from the home servers to the proxy servers, and finally to
   the NAS.  This use of Status-Server is NOT RECOMMENDED, as there has
   been insufficient analysis and deployment experience to know if it is
   useful, or even if it makes the network less reliable.

6.  Table of Attributes

   The following table provides a guide to which attributes may be found
   in Status-Server packets, and in what quantity.  Attributes other
   than the ones listed below SHOULD NOT be found in a Status-Server
   packet.




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   Status-  Access-  Accounting-
   Server   Accept   Response      #    Attribute

   0-1      0        0             4   NAS-IP-Address [Note 1]
   0        0+       0            18   Reply-Message
   0+       0+       0+           26   Vendor-Specific
   0-1      0        0            32   NAS-Identifier [Note 1]
   1        0-1      0-1          80   Message-Authenticator
   0-1      0        0            95   NAS-IPv6-Address [Note 1]

   [Note 1] A Status-Server SHOULD contain one of (NAS-IP-Address or
   NAS-IPv6-Address), or NAS-Identifier, or both NAS-Identifier and one
   of (NAS-IP-Address or NAS-IPv6-Address).

   The following table defines the meaning of the above table entries.

0     This attribute MUST NOT be present in packet.
0+    Zero or more instances of this attribute MAY be present in packet.
0-1   Zero or one instance of this attribute MAY be present in packet.
1     Exactly one instance of this attribute MUST be present in packet.


7.  Examples

   A few examples are presented to illustrate the flow of packets to
   both the authentication and accounting ports.  These examples are not
   intended to be exhaustive, many others are possible.  Hexadecimal
   dumps of the example packets are given in network byte order, using
   the shared secret "xyzzy5461".

7.1.  Minimal Query to Authentication Port

   The NAS sends a Status-Server UDP packet with minimal content to a
   RADIUS server on port 1812.

   The Request Authenticator is a 16 octet random number generated by
   the NAS.  Message-Authenticator is included in order to authenticate
   that the request came from a known client.

      0c da 00 26 8a 54 f4 68 6f b3 94 c5 28 66 e3 02
      18 5d 06 23 50 12 5a 66 5e 2e 1e 84 11 f3 e2 43
      82 20 97 c8 4f a3

       1 Code = Status-Server (12)
       1 ID = 218
       2 Length = 38
      16 Request Authenticator




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      Attributes:
      18 Message-Authenticator (80) = 5a665e2e1e8411f3e243822097c84fa3

   The Response Authenticator is a 16 octet MD5 checksum of the code
   (2), id (218), Length (20), the Request Authenticator from above, and
   the shared secret.

      02 da 00 14 ef 0d 55 2a 4b f2 d6 93 ec 2b 6f e8
      b5 41 1d 66

       1 Code = Access-Accept (2)
       1 ID = 218
       2 Length = 20
      16 Request Authenticator

      Attributes:
         None.


7.2.  Minimal Query to Accounting Port

   The NAS sends a Status-Server UDP packet with minimal content to a
   RADIUS server on port 1813.

   The Request Authenticator is a 16 octet random number generated by
   the NAS.  Message-Authenticator is included in order to authenticate
   that the request came from a known client.

      0c b3 00 26 92 5f 6b 66 dd 5f ed 57 1f cb 1d b7
      ad 38 82 60 80 12 e8 d6 ea bd a9 10 87 5c d9 1f
      da de 26 36 78 58

       1 Code = Status-Server (12)
       1 ID = 179
       2 Length = 38
      16 Request Authenticator

      Attributes:
      18 Message-Authenticator (80) = e8d6eabda910875cd91fdade26367858

   The Response Authenticator is a 16 octet MD5 checksum of the code
   (5), id (179), Length (20), the Request Authenticator from above, and
   the shared secret.

      02 b3 00 1a 0f 6f 92 14 5f 10 7e 2f 50 4e 86 0a
      48 60 66 9c

       1 Code = Accounting-Response (5)



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       1 ID = 179
       2 Length = 20 16 Request Authenticator

      Attributes:
         None.


7.3.  Verbose Query and Response

   The NAS at 192.0.2.16 sends a Status-Server UDP packet to the RADIUS
   server on port 1812.

   The Request Authenticator is a 16 octet random number generated by
   the NAS.

      0c 47 00 2c bf 58 de 56 ae 40 8a d3 b7 0c 85 13
      f9 b0 3f be 04 06 c0 00 02 10 50 12 85 2d 6f ec
      61 e7 ed 74 b8 e3 2d ac 2f 2a 5f b2

       1 Code = Status-Server (12)
       1 ID = 71
       2 Length = 44
      16 Request Authenticator

      Attributes:
       6  NAS-IP-Address (4) = 192.0.2.16
      18 Message-Authenticator (80) = 852d6fec61e7ed74b8e32dac2f2a5fb2

   The Response Authenticator is a 16-octet MD5 checksum of the code
   (2), id (71), Length (52), the Request Authenticator from above, the
   attributes in this reply, and the shared secret.

   The Reply-Message is "RADIUS Server up 2 days, 18:40"

      02 47 00 34 46 f4 3e 62 fd 03 54 42 4c bb eb fd
      6d 21 4e 06 12 20 52 41 44 49 55 53 20 53 65 72
      76 65 72 20 75 70 20 32 20 64 61 79 73 2c 20 31
      38 3a 34 30

       1 Code = Access-Accept (2)
       1 ID = 71
       2 Length = 52
      16 Request Authenticator

      Attributes:
      32 Reply-Message (18)





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8.  IANA Considerations

   This specification does not create any new registries, nor does it
   require assignment of any protocol parameters.

9.  Security Considerations

   This document defines the Status-Server packet as being similar in
   treatment to the Access-Request packet, and is therefore subject to
   the same security considerations as described in [RFC2865], Section
   8.  Status-Server packets also use the Message-Authenticator
   attribute, and are therefore subject to the same security
   considerations as [RFC3579], Section 4.

   We reiterate that Status-Server packets MUST contain a Message-
   Authenticator attribute.  Early implementations supporting Status-
   Server did not enforce this requirement, and may have been vulnerable
   to DoS attacks as a result.

   Where this document differs from [RFC2865] is that it defines a new
   request/response method in RADIUS; the Status-Server request.  As
   this use is based on previously described and implemented standards,
   we know of no additional security considerations that arise from the
   use of Status-Server as defined herein.

10.  References

10.1.  Normative references

[RFC2865]
     Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
     Authentication Dial In User Service (RADIUS)", RFC 2865, June 2000.

[RFC4282]
     Aboba, B., and Beadles, M. at al, "The Network Access Identifier",
     RFC 4282, December 2005.

10.2.  Informative references

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", RFC 2119, March, 1997.

[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

[RFC3539] Aboba, B., Wood, J., "Authentication, Authorization, and
          Accounting (AAA) Transport Profile", RFC 3539, June 2003.





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[RFC3579] Aboba, B., Calhoun, P., "RADIUS (Remote Authentication Dial In
          User Service) Support For Extensible Authentication Protocol
          (EAP)", RFC 3579, September 2003.

[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.

[RFC4670] Nelson, D., "RADIUS Accounting Client MIB for IPv6", RFC 4670,
          August 2006.

[RFC4671] Nelson, D., "RADIUS Accounting Server MIB for IPv6", RFC 4671,
          August 2006.

Acknowledgments

   Parts of the text in Section 3 defining the Request and Response
   Authenticators were taken with minor edits from [RFC2865] Section 3.

   The author would like to thank Mike McCauley of Open Systems
   Consultants for making a Radiator server available for
   interoperability testing.

Authors' Addresses

   Alan DeKok
   The FreeRADIUS Server Project
   http://freeradius.org

   Email: aland@freeradius.org



















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