Internet Engineering Task Force (IETF)                          A. DeKok
Request for Comments: 8559                                    FreeRADIUS
Updates: 5176, 5580                                          J. Korhonen
Category: Standards Track                                     April 2019
ISSN: 2070-1721

Dynamic Authorization Proxying in the
Remote Authentication Dial-In User Service (RADIUS) Protocol

Abstract

   RFC 5176 defines Change-of-Authorization (CoA) and Disconnect Message
   (DM) behavior for RADIUS.  RFC 5176 also suggests that proxying these
   messages is possible, but it does not provide guidance as to how that
   is done.  This specification updates RFC 5176 to correct that
   omission for scenarios where networks use realm-based proxying as
   defined in RFC 7542.  This specification also updates RFC 5580 to
   allow the Operator-Name attribute in CoA-Request and Disconnect-
   Request packets.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8559.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   1. Introduction ....................................................3
      1.1. Terminology ................................................4
      1.2. Requirements Language ......................................5
   2. Problem Statement ...............................................5
      2.1. Typical RADIUS Proxying ....................................5
      2.2. CoA Processing .............................................6
      2.3. Failure of CoA Proxying ....................................6
   3. How to Perform CoA Proxying .....................................7
      3.1. Changes to Access-Request and Accounting-Request Packets ...8
      3.2. Proxying of CoA-Request and Disconnect-Request Packets .....9
      3.3. Reception of CoA-Request and Disconnect-Request Packets ...10
      3.4. Operator-NAS-Identifier ...................................11
   4. Requirements ...................................................14
      4.1. Requirements on Home Servers ..............................14
      4.2. Requirements on Visited Networks ..........................14
      4.3. Requirements on Proxies ...................................14
           4.3.1. Security Requirements on Proxies ...................15
           4.3.2. Filtering Requirements on Proxies ..................16
   5. Functionality ..................................................17
      5.1. User Login ................................................17
      5.2. CoA Proxying ..............................................17
   6. Security Considerations ........................................18
      6.1. RADIUS Security and Proxies ...............................18
      6.2. Security of the Operator-NAS-Identifier Attribute .........19
   7. IANA Considerations ............................................20
   8. References .....................................................20
      8.1. Normative References ......................................20
      8.2. Informative References ....................................21
   Authors' Addresses ................................................21

1. Introduction

   RFC 5176 [RFC5176] defines Change-of-Authorization (CoA) and
   Disconnect Message (DM) behavior for RADIUS.  Section 3.1 of
   [RFC5176] suggests that proxying these messages is possible, but it
   does not provide guidance as to how that is done.  This omission
   means that in practice, proxying of CoA packets is impossible.

   We partially correct that omission here by explaining how proxying of
   these packets can be done by leveraging an existing RADIUS attribute,
   Operator-Name (Section 4.1 of [RFC5580]).  We then explain how this
   attribute can be used by proxies to route packets "backwards" through
   a RADIUS proxy chain from a home network to a visited network.  We
   then introduce a new attribute: Operator-NAS-Identifier.  This
   attribute permits packets to be routed from the RADIUS server at the
   visited network to the Network Access Server (NAS).

   This correction is limited to the use case of realm-based proxying as
   defined in [RFC7542].  Other forms of proxying are possible but are
   not discussed here.  We note that the recommendations provided in
   this document apply only to those systems that implement proxying of
   CoA packets, and then only to those that implement realm-based CoA
   proxying.  This specification neither requires nor suggests changes
   to any implementation or deployment of any other RADIUS systems.

   We also update the behavior described in [RFC5580] to allow the
   Operator-Name attribute to be used in CoA-Request and Disconnect-
   Request packets, as further described in this document.

   This document is a Standards Track document in order to update the
   behavior described in [RFC5580], as [RFC5580] is also a Standards
   Track document.  This document relies heavily upon and also updates
   some of the behaviors described in RFC 5176, which is an
   Informational document; because the applicability statements in
   Section 1.1 of [RFC5176] do not apply to this document, this document
   does not change the status of [RFC5176].

   We finally conclude with a discussion of the security implications of
   this design and show that they do not decrease the security of the
   network.

1.1. Terminology

   This document frequently uses the following terms:

   CoA

      Change of authorization, e.g., CoA-Request, CoA-ACK, or CoA-NAK,
      as defined in [RFC5176].  [RFC5176] also defines Disconnect-
      Request, Disconnect-ACK, and Disconnect-NAK.  For simplicity,
      where we use "CoA" in this document, we mean a generic
      "CoA-Request or Disconnect-Request" packet.  We use "CoA-Request"
      or "Disconnect-Request" to refer to the specific packet types.

   Network Access Identifier (NAI)

      The user identity submitted by the client during network access
      authentication.  See [RFC7542].  The purpose of the NAI is to
      identify the user as well as assist in the routing of the
      authentication request.  Please note that the NAI may not
      necessarily be the same as the user's email address or the user
      identity submitted in an application-layer authentication.

   Network Access Server (NAS)

      The device that clients connect to in order to get access to the
      network.  In Point-to-Point Tunneling Protocol (PPTP) terminology,
      this is referred to as the PPTP Access Concentrator (PAC), and in
      Layer 2 Tunneling Protocol (L2TP) terminology, it is referred to
      as the L2TP Access Concentrator (LAC).  In IEEE 802.11, it is
      referred to as an Access Point.

   Home Network

      The network that holds the authentication credentials for a user.

   Visited Network

      A network other than the home network, where the user attempts to
      gain network access.  The visited network typically has a
      relationship with the home network, possibly through one or more
      intermediary proxies.

1.2. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2. Problem Statement

   This section describes how RADIUS proxying works, how CoA packets
   work, and why CoA proxying as discussed in [RFC5176] is insufficient
   to create a working system.

2.1. Typical RADIUS Proxying

   When a RADIUS server proxies an Access-Request packet, it typically
   does so based on the contents of the User-Name attribute, which
   contains an NAI [RFC7542].  This specification describes how to use
   the NAI in order to proxy CoA packets across multiple hops.  Other
   methods of proxying CoA packets are possible but are not discussed
   here.

   In order to determine the "next hop" for a packet, the proxying
   server looks up the "realm" portion of the NAI in a logical
   Authentication, Authorization, and Accounting (AAA) routing table, as
   described in Section 3 of [RFC7542].  The entry in that table
   contains information about the next hop to which the packet is sent.
   This information can be IP address, shared secret, certificate, etc.
   The next hop may also be another proxy, or it may be the home server
   for that realm.

   If the next hop is a proxy, that proxy will perform the same realm
   lookup and then proxy the packet as above.  At some point, the
   next hop will be the home server for that realm.

   The home server validates the NAI in the User-Name attribute against
   the list of realms hosted by the home network.  If there is no match,
   then an Access-Reject is returned.  All other packets are processed
   through local site rules, which result in an appropriate response
   packet being sent.  This response packet can be Access-Accept,
   Access-Challenge, or Access-Reject.

   The RADIUS client receiving that response packet will match it to an
   outstanding request.  If the client is part of a proxy, the proxy
   will then send that response packet in turn to the system that
   originated the Access-Request.  This process continues until the
   response packet arrives at the NAS.

   The proxies are typically stateful with respect to ongoing
   request/response packets but are stateless with respect to user
   sessions.  That is, once a response has been sent by the proxy, it
   can discard all information about the request packet, other than what
   is needed for detecting retransmissions as per Section 2.2.2 of
   [RFC5080].

   The same method is used to proxy Accounting-Request packets.
   Proxying both Access-Request and Accounting-Request packets allows
   proxies to connect visited networks to home networks for all AAA
   purposes.

2.2. CoA Processing

   [RFC5176] describes how CoA clients send packets to CoA servers.  We
   note that a system comprising the CoA client is typically co-located
   with, or is the same as, the RADIUS server.  Similarly, the CoA
   server is a system that is either co-located with or the same as the
   RADIUS client.

   In the case of packets sent inside of one network, the source and
   destination of CoA packets are locally determined.  There is thus no
   need for standardization of that process, as networks are free to
   send CoA packets whenever they want, for whatever reason they want.

2.3. Failure of CoA Proxying

   The situation is more complicated when proxies are involved.
   [RFC5176] suggests that CoA proxying is permitted, but [RFC5176] does
   not make any suggestions as to how that proxying should be done.

   If proxies were to track user sessions, it would be possible for a
   proxy to match an incoming CoA packet to a user session and then to
   proxy the CoA packet to the RADIUS client that originated the
   Access-Request for that session.  There are many problems with such a
   scenario.

   The CoA server might not, in fact, be co-located with the RADIUS
   client, in which case it might not have access to user session
   information for performing the reverse path forwarding.

   The CoA server may be down, but there may be a different CoA server
   that could successfully process the packet.  The CoA client should
   then fail over to a different CoA server.  If the reverse path is
   restricted to be the same as the forward path, then such failover is
   not possible.

   In a roaming consortium, the proxies may forward traffic for tens of
   millions of users.  Tracking each user session can be expensive and
   complicated, and doing so does not scale well.  For that reason, most
   proxies do not record user sessions.

   Even if the proxy recorded user sessions, [RFC5176] is silent on the
   topic of what attributes constitute "session identification
   attributes".  That silence means it is impossible for a proxy to
   determine if a CoA packet matches a particular user session.

   The result of all of these issues is that CoA proxying is impossible
   when using the behavior defined in [RFC5176].

3. How to Perform CoA Proxying

   The solution to the above problem is to use realm-based proxying on
   the reverse path, just as with the forward path.  In order for the
   reverse path proxying to work, the proxy decision must be based on an
   attribute other than User-Name.

   The reverse path proxying can be done by using the Operator-Name
   attribute defined in Section 4.1 of [RFC5580].  We repeat a portion
   of that definition here for clarity:

      This attribute carries the operator namespace identifier and the
      operator name.  The operator name is combined with the namespace
      identifier to uniquely identify the owner of an access network.

   ...followed a few paragraphs later by a description of the REALM
   namespace:

      REALM ('1' (0x31)):

         The REALM operator namespace can be used to indicate operator
         names based on any registered domain name.  Such names are
         required to be unique, and the rights to use a given realm name
         are obtained coincident with acquiring the rights to use a
         particular Fully Qualified Domain Name (FQDN). ...

   In short, the Operator-Name attribute contains an ASCII "1", followed
   by the realm of the visited network.  For example, for the
   "example.com" realm, the Operator-Name attribute contains the text
   "1example.com".  This information is precisely what is needed by
   intermediate nodes in order to perform CoA proxying.

   The remainder of this document describes how CoA proxying can be
   performed by using the Operator-Name attribute.  We describe the
   following:

   o  how the forward path has to change in order to allow reverse path
      proxying

   o  how reverse path proxying works

   o  how visited networks and home networks have to behave in order for
      CoA proxying to work

   We note that as a proxied CoA packet is sent to only one destination,
   the Operator-Name attribute MUST NOT occur more than once in a
   packet.  If a packet contains more than one Operator-Name,
   implementations MUST treat the second and subsequent attributes as
   "invalid attributes", as discussed in Section 2.8 of [RFC6929].

3.1. Changes to Access-Request and Accounting-Request Packets

   When a visited network proxies an Access-Request or Accounting-
   Request packet outside of its network, a visited network that wishes
   to support realm-based CoA proxying SHOULD include an Operator-Name
   attribute in the packet, as discussed in Section 4.1 of [RFC5580].
   The contents of the Operator-Name attribute should be "1", followed
   by the realm name of the visited network.  Where the visited network
   has more than one realm name, a "canonical" name SHOULD be chosen and
   used for all packets.

   Visited networks MUST use a consistent value for Operator-Name for
   any one user session.  That is, sending "1example.com" in an
   Access-Request packet and "1example.org" in an Accounting-Request
   packet for that same session is forbidden.  Such behavior would make
   it look like a single user session was active simultaneously in two
   different visited networks, which is impossible.

   Proxies that record user session information SHOULD also record
   Operator-Name.  Proxies that do not record user session information
   do not need to record Operator-Name.

   Home networks SHOULD record Operator-Name along with any other
   information that they record about user sessions.  Home networks that
   expect to send CoA packets to visited networks MUST record
   Operator-Name for each user session that originates from a visited
   network.  Failure to record Operator-Name would mean that the home
   network would not know where to send any CoA packets.

   Networks that host both the RADIUS client and RADIUS server do not
   need to create, record, or track Operator-Name.  That is, if the
   visited network and home network are the same, there is no need to
   use the Operator-Name attribute.

3.2. Proxying of CoA-Request and Disconnect-Request Packets

   When a home network wishes to send a CoA-Request or Disconnect-
   Request packet to a visited network, it MUST include an Operator-Name
   attribute in the CoA packet.  The value of the Operator-Name
   attribute MUST be the value that was recorded earlier for that user
   session.

   The home network MUST look up the realm from the Operator-Name
   attribute in a logical "realm routing table", as discussed in
   Section 3 of [RFC7542].  That logical realm table is defined
   therein as:

      ... a logical AAA routing table, where the "utf8-realm" portion
      acts as a key, and the values stored in the table are one or more
      "next hop" AAA servers.

   In order to support proxying of CoA packets, this table is extended
   to include a mapping between "utf8-realm" and one or more next-hop
   CoA servers.

   When proxying CoA-Request and Disconnect-Request packets, the lookups
   will return data from the "CoA server" field instead of the "AAA
   server" field.

   In practice, this process means that CoA proxying works exactly like
   "normal" RADIUS proxying, except that the proxy decision is made
   using the realm from the Operator-Name attribute instead of using the
   realm from the User-Name attribute.

   Proxies that receive the CoA packet will look up the realm from the
   Operator-Name attribute in a logical "realm routing table", as with
   home servers, above.  The packet is then sent to the proxy for the
   realm that was found in that table.  This process continues with any
   subsequent proxies until the packet reaches a public CoA server at
   the visited network.

   Where the realm is unknown, the proxy MUST return a NAK packet that
   contains an Error-Cause Attribute having value 502 ("Request Not
   Routable").

   Proxies that receive a CoA packet MUST NOT use the NAI from the
   User-Name attribute in order to make proxying decisions.  Doing so
   would result in the CoA packet being forwarded to the home network,
   while the user's session is in the visited network.

   We also update Section 5 of [RFC5580] to permit CoA-Request and
   Disconnect-Request packets to contain zero or one instance of the
   Operator-Name attribute.

3.3. Reception of CoA-Request and Disconnect-Request Packets

   After some proxying, the CoA packet will be received by the CoA
   server in the visited network.  That CoA server MUST validate the NAI
   in the Operator-Name attribute against the list of realms hosted by
   the visited network.  If the realm is not found, then the CoA server
   MUST return a NAK packet that contains an Error-Cause Attribute
   having value 502 ("Request Not Routable").

   Some home networks will not have permission to send CoA packets to
   the visited network.  The CoA server SHOULD therefore also validate
   the NAI contained in the User-Name attribute.  If the home network is
   not permitted to send CoA packets to this visited network, then the
   CoA server MUST return a NAK packet that contains an Error-Cause
   Attribute having value 502 ("Request Not Routable").

   These checks make it more difficult for a malicious home network to
   scan roaming networks in order to determine which visited network
   hosts which realm.  That information should be known to all parties
   in advance and exchanged via methods outside the scope of this
   specification.  Those methods will typically be in the form of
   contractual relationships between parties or membership in a roaming
   consortium.

   The CoA server in the visited network will also ensure that the
   Operator-NAS-Identifier attribute is known, as described below.  If
   the attribute matches a known NAS, then the packet will be sent to
   that NAS.  Otherwise, the CoA server MUST return a NAK packet that
   contains an Error-Cause Attribute having value 403 ("NAS
   Identification Mismatch").

   All other received packets are processed as per local site rules and
   will result in an appropriate response packet being sent.  This
   process mirrors the method used to process Access-Request and
   Accounting-Request packets (described above).

   Processing done by the visited network will normally include sending
   the CoA packet to the NAS, having the NAS process it, and then
   returning any response packets back up the proxy chain to the home
   server.

   The only missing piece here is the procedure by which the visited
   network gets the packet from its public CoA server to the NAS.  The
   visited network could use NAS-Identifier, NAS-IP-Address, or
   NAS-IPv6-Address, but these attributes may have been edited by an
   intermediate proxy or the attributes may be missing entirely.

   These attributes may be incorrect because proxies forwarding
   Access-Request packets often rewrite them for internal policy
   reasons.  These attributes may be missing, because the visited
   network may not want all upstream proxies and home servers to have
   detailed information about the internals of its private network and
   may remove them itself.

   We therefore need a way to identify a NAS in the visited network via
   a method that affords privacy and does not use any existing
   attributes.  Our solution is to define an Operator-NAS-Identifier
   attribute, which identifies an individual NAS in the visited network.

3.4. Operator-NAS-Identifier

   The Operator-NAS-Identifier attribute is an opaque token that
   identifies an individual NAS in a visited network.  It MAY appear in
   the following packets: Access-Request, Accounting-Request,
   CoA-Request, or Disconnect-Request.  Operator-NAS-Identifier MUST NOT
   appear in any other packets.

   Operator-NAS-Identifier MAY occur in a packet if the packet also
   contains an Operator-Name attribute.  Operator-NAS-Identifier
   MUST NOT appear in a packet if there is no Operator-Name in the
   packet.  As each proxied CoA packet is sent to only one NAS, the
   Operator-NAS-Identifier attribute MUST NOT occur more than once in a
   packet.  If a packet contains more than one Operator-NAS-Identifier,
   implementations MUST treat the second and subsequent attributes as
   "invalid attributes", as discussed in Section 2.8 of [RFC6929].

   An Operator-NAS-Identifier attribute SHOULD be added to an
   Access-Request or Accounting-Request packet by a visited network,
   before proxying a packet to an external RADIUS server.  When the
   Operator-NAS-Identifier attribute is added to a packet, the following
   attributes SHOULD be deleted from the packet: NAS-IP-Address,
   NAS-IPv6-Address, and NAS-Identifier.  If these attributes are
   deleted, the proxy MUST then add a new NAS-Identifier attribute,

   in order to satisfy the requirements of Section 4.1 of [RFC2865] and
   Section 4.1 of [RFC2866].  The contents of the new NAS-Identifier
   attribute SHOULD be the realm name of the visited network.

   When a server receives a packet that already contains an Operator-
   NAS-Identifier attribute, no such editing is performed.

   The Operator-NAS-Identifier attribute MUST NOT be added to any packet
   by any other proxy or server in the network.  Only the visited
   network (i.e., the operator) can name a NAS that is inside of the
   visited network.

   The result of these requirements is that for everyone outside of the
   visited network there is only one NAS: the visited network itself.
   Also, the visited network is able to identify its own NASes to its
   own satisfaction.

   This usage of the Operator-NAS-Identifier attribute parallels the
   Operator-Name attribute as defined in Section 4.1 of [RFC5580].

   The Operator-NAS-Identifier attribute is defined as follows.

   Description

      An opaque token describing the NAS a user has logged into.

   Type

241.8 (assigned by IANA from the "short extended space" [RFC6929]
of the "RADIUS Attribute Types" registry).

Length

4 to 35.

      Implementations supporting this attribute MUST be able to handle
      between one (1) and thirty-two (32) octets of data.
      Implementations creating an Operator-NAS-Identifier attribute
      MUST NOT create attributes with more than sixty-four (64) octets
      of data.  A 32-octet string should be more than sufficient for
      future uses.

   Data Type

      The data type of this field is "string".  See Section 3.5 of
      [RFC8044] for a definition.

   Value

      This attribute contains an opaque token that can only be
      interpreted by the visited network.

      This token MUST allow the visited network to direct the packet to
      the NAS for the user's session.  In practice, this requirement
      means that the visited network has two practical methods for
      creating the value.

      The first method is to create an opaque token per NAS and then to
      store that information in a database.  The database can be
      configured to allow querying by NAS IP address in order to find
      the correct Operator-NAS-Identifier.  The database can also be
      configured to allow querying by Operator-NAS-Identifier in order
      to find the correct NAS IP address.

      The second method is to obfuscate the NAS IP address using
      information known locally by the visited network -- for example,
      by XORing it with a locally known secret key.  The output of that
      obfuscation operation is data that can be used as the value of
      Operator-NAS-Identifier.  On reception of a CoA packet, the
      locally known information can be used to unobfuscate the value of
      Operator-NAS-Identifier, in order to determine the actual NAS IP
      address.

      Note that there is no requirement that the value of Operator-NAS-
      Identifier be checked for integrity.  Modification of the value
      can only result in the erroneous transaction being rejected.

      We note that the Access-Request and Accounting-Request packets
      often contain the Media Access Control (MAC) address of the NAS.
      There is therefore no requirement that Operator-NAS-Identifier
      obfuscate or hide in any way the total number of NASes in a
      visited network.  That information is already public knowledge.

4. Requirements

4.1. Requirements on Home Servers

   The Operator-NAS-Identifier attribute MUST be stored by a home server
   along with any user session identification attributes.  When sending
   a CoA packet for a user session, the home server MUST include
   verbatim any Operator-NAS-Identifier it has recorded for that
   session.

   A home server MUST NOT send CoA packets for users of other networks.
   The next few sections describe how other participants in the RADIUS
   ecosystem can help enforce this requirement.

4.2. Requirements on Visited Networks

   A visited network that receives a CoA packet that will be proxied to
   a NAS MUST perform all of the operations required for proxies; see
   Section 4.3.2.  We specify this requirement because we assume that
   the visited network has a proxy between the NAS and any external
   (i.e., third-party) proxy.  Situations where a NAS sends packets
   directly to a third-party RADIUS server are outside the scope of this
   specification.

   The visited network uses the contents of the Operator-NAS-Identifier
   attribute to determine which NAS will receive the packet.

   The visited network MUST remove the Operator-Name and Operator-NAS-
   Identifier attributes from a given CoA packet prior to sending that
   packet to the final CoA server (i.e., NAS).  This step is necessary
   due to the limits specified in Section 2.3 of [RFC5176].

   The visited network MUST also ensure that the CoA packet sent to the
   NAS contains one of the following attributes: NAS-IP-Address,
   NAS-IPv6-Address, or NAS-Identifier.  This step is the inverse of the
   removal suggested above in Section 3.4.

   In general, the NAS should only receive attributes that identify or
   modify a user's session.  It is not appropriate to send to a NAS
   attributes that are used only for inter-proxy signaling.

4.3. Requirements on Proxies

   There are a number of requirements on both CoA proxies and RADIUS
   proxies.  For the purpose of this section, we assume that each RADIUS
   proxy shares a common administration with a corresponding CoA proxy
   and that the two systems can communicate electronically.  There is no
   requirement that these systems be co-located.

4.3.1. Security Requirements on Proxies

   Section 6.1 of [RFC5176] has some security requirements on proxies
   that handle CoA-Request and Disconnect-Request packets:

      ... a proxy MAY perform a "reverse path forwarding" (RPF) check to
      verify that a Disconnect-Request or CoA-Request originates from an
      authorized Dynamic Authorization Client.

   We strengthen that requirement by saying that a proxy MUST perform a
   reverse path forwarding check to verify that a CoA packet originates
   from an authorized Dynamic Authorization Client.  Without this check,
   a proxy may forward packets from misconfigured or malicious parties
   and thus contribute to the problem instead of preventing it.  Where
   the check fails, the proxy MUST return a NAK packet that contains an
   Error-Cause Attribute having value 502 ("Request Not Routable").

   Proxies that record user session information SHOULD verify the
   contents of a received CoA packet against the recorded data for that
   user session.  If the proxy determines that the information in the
   packet does not match the recorded user session, it SHOULD return a
   NAK packet that contains an Error-Cause Attribute having value 503
   ("Session Context Not Found").  These checks cannot be mandated due
   to the fact that [RFC5176] offers no advice on which attributes are
   used to identify a user's session.

   Because a RADIUS proxy will see Access-Request and Accounting-Request
   packets, we recognize that it will have sufficient information to
   forge CoA packets.  The RADIUS proxy will thus have the ability to
   subsequently disconnect any user who was authenticated through
   itself.

   We suggest that the real-world effect of this security problem is
   minimal.  RADIUS proxies can already return Access-Accept or
   Access-Reject for Access-Request packets and can change authorization
   attributes contained in an Access-Accept.  Allowing a proxy to change
   (or disconnect) a user session post-authentication is not
   substantially different from changing (or refusing to connect) a user
   session during the initial process of authentication.

   The biggest problem is that there are no provisions in RADIUS for
   "end-to-end" security.  That is, the visited network and home network
   cannot communicate privately in the presence of proxies.  This
   limitation originates from the design of RADIUS for Access-Request
   and Accounting-Request packets.  That limitation is then carried over
   to CoA-Request and Disconnect-Request packets.

   We therefore cannot prevent proxies or home servers from forging CoA
   packets.  We can only create scenarios where that forgery is hard to
   perform, is likely to be detected, and/or has no effect.

4.3.2. Filtering Requirements on Proxies

   Section 2.3 of [RFC5176] makes the following requirement for CoA
   servers:

      In CoA-Request and Disconnect-Request packets, all attributes MUST
      be treated as mandatory.

   This requirement is too stringent for a CoA proxy.  Only the final
   CoA server (i.e., NAS) can decide which attributes are mandatory and
   which are not.

   Instead, in the case of a CoA proxy, we say that all attributes
   MUST NOT be treated as mandatory.  Proxies implementing this
   specification MUST perform proxying based on Operator-Name.  Other
   schemes are possible but are not discussed here.  Proxies SHOULD
   forward all packets either "as is" or with minimal changes.

   We note that some NAS implementations currently treat signaling
   attributes as mandatory.  For example, some NAS implementations will
   NAK any CoA packet that contains a Proxy-State attribute.  While this
   behavior is based on a straightforward reading of the above text, it
   causes problems in practice.

   We update Section 2.3 of [RFC5176] as follows: in CoA-Request and
   Disconnect-Request packets, the NAS MUST NOT treat as mandatory any
   attribute that is known to not affect the user's session -- for
   example, the Proxy-State attribute.  Proxy-State is an attribute used
   for proxy-to-proxy signaling.  It cannot affect the user's session,
   and therefore Proxy-State (and similar attributes) MUST be ignored by
   the NAS.

   When Operator-Name and/or Operator-NAS-Identifier are received by a
   proxy, the proxy MUST pass those attributes through unchanged.  This
   requirement applies to all proxies, including proxies that forward
   any or all of Access-Request, Accounting-Request, CoA-Request, and
   Disconnect-Request packets.

   All attributes added by a RADIUS proxy when sending packets from the
   visited network to the home network MUST be removed by the
   corresponding CoA proxy from packets traversing the reverse path.
   That is, any editing of attributes that is done on the "forward" path
   MUST be undone on the "reverse" path.

   The result is that a NAS will only ever receive CoA packets that
   either contain (1) attributes sent by the NAS to its local RADIUS
   server or (2) attributes that are sent by the home server in order to
   perform a change of authorization.

   Finally, we extend the above requirement not only to Operator-Name
   and Operator-NAS-Identifier but also to any future attributes that
   are added for proxy-to-proxy signaling.

5. Functionality

This section describes how the two attributes work together to permit
CoA proxying.

5.1. User Login

   In this scenario, we follow a roaming user who is attempting to
   log in to a visited network.  The login attempt is done via a NAS in
   the visited network.  That NAS will send an Access-Request packet to
   the visited RADIUS server.  The visited RADIUS server will see that
   the user is roaming and will add an Operator-Name attribute, with
   value "1" followed by its own realm name, e.g., "1example.com".  The
   visited RADIUS server MAY also add an Operator-NAS-Identifier
   attribute.  The NAS identification attributes are also edited, as
   required by Section 3.4, above.

   The visited server will then proxy the authentication request to an
   upstream server.  That server may be the home server, or it may be a
   proxy.  In the case of a proxy, the proxy will forward the packet
   until the packet reaches the home server.

   The home server will record the Operator-Name and Operator-NAS-
   Identifier attributes, along with other information about the user's
   session, if those attributes are present in a packet.

5.2. CoA Proxying

   At some later point in time, the home server determines that
   (1) a user session should have its authorization changed or
   (2) the user should be disconnected.  The home server looks up the
   Operator-Name and Operator-NAS-Identifier attributes, along with
   other user session identifiers as described in [RFC5176].  The home
   server then looks up the realm from the Operator-Name attribute in
   the logical AAA routing table, in order to find the next-hop CoA
   server for that realm (which may be a proxy).  The CoA-Request is
   then sent to that CoA server.

   The CoA server receives the request and, if it is a proxy, performs a
   lookup similar to the lookup done by the home server.  The packet is
   then proxied repeatedly until it reaches the visited network.

   If the proxy cannot find a destination for the request or if no
   Operator-Name attribute exists in the request, the proxy will return
   a CoA-NAK with Error-Cause 502 ("Request Not Routable").

   The visited network will receive the CoA-Request packet and will use
   the Operator-NAS-Identifier attribute (if available) to determine
   which local CoA server (i.e., NAS) the packet should be sent to.  If
   there is no Operator-NAS-Identifier attribute, the visited network
   may use other means to locate the NAS, such as consulting a local
   database that tracks user sessions.

   The Operator-Name and Operator-NAS-Identifier attributes are then
   removed from the packet; one of NAS-IP-Address, NAS-IPv6-Address, or
   NAS-Identifier is added to the packet; and the packet is then sent to
   the CoA server.

   If no CoA server can be found, the visited network returns a CoA-NAK
   with Error-Cause 403 ("NAS Identification Mismatch").

   Any response from the CoA server (NAS) is returned to the home
   network via the normal method of returning responses to requests.

6. Security Considerations

   This specification incorporates by reference Section 11 of [RFC6929].
   In short, RADIUS has many known issues; those issues are discussed in
   detail in [RFC6929] and do not need to be repeated here.

   This specification adds one new attribute and defines new behavior
   for RADIUS proxying.  As this behavior mirrors existing RADIUS
   proxying, we do not believe that it introduces any new security
   issues.  We note, however, that RADIUS proxying has many inherent
   security issues.

6.1. RADIUS Security and Proxies

   The requirement that packets be signed with a shared secret means
   that a CoA packet can only be received from a trusted party or,
   transitively, received from a third party via a trusted party.  This
   security provision of the base RADIUS protocol makes it impossible
   for untrusted parties to affect the user's session.

   When RADIUS proxying is performed, all packets are signed on a
   hop-by-hop basis.  Any intermediate proxy can therefore forge
   packets, replay packets, or modify the contents of any packet.  Any
   system receiving correctly signed packets must accept them at face
   value and is unable to detect any forgery, replay, or modifications.
   As a result, the secure operation of such a system depends largely on
   trust instead of on technical means.

   CoA packet proxying has all of the same issues as those noted above.
   We note that the proxies that see and can modify CoA packets are
   generally the same proxies that can see or modify Access-Request and
   Accounting-Request packets.  As such, there are few additional
   security implications in allowing CoA proxying.

   The main security implication that remains is that home networks now
   have the ability to disconnect or change the authorization of users
   in a visited network.  As this capability is only enabled when mutual
   agreement is in place, and only for those parties who can already
   control user sessions, there are no new security issues with this
   specification.

6.2. Security of the Operator-NAS-Identifier Attribute

   Nothing in this specification depends on the security of the
   Operator-NAS-Identifier attribute.  The entire process would work
   exactly the same if the Operator-NAS-Identifier attribute simply
   contained the NAS IP address that is hosting the user's session.  The
   only real downside in that situation would be that external parties
   would see some additional private information about the visited
   network.  They would still, however, be unable to leverage that
   information to do anything malicious.

   The main reason to use an opaque token for the Operator-NAS-
   Identifier attribute is that there is no compelling reason to make
   the information public.  We therefore recommend that the value be
   simply an opaque token.  We also state that there is no requirement
   for integrity protection or replay detection of this attribute.  The
   rest of the RADIUS protocol ensures that modification or replay of
   the Operator-NAS-Identifier attribute will either have no effect or
   have the same effect as if the value had not been modified.

   Trusted parties can modify a user's session on the NAS only when they
   have sufficient information to identify that session.  In practice,
   this limitation means that those parties already have access to the
   user's session information.  In other words, those parties are the
   proxies who are already forwarding Access-Request and Accounting-
   Request packets.

   Since those parties already have the ability to see and modify all of
   the information about a user's session, there is no additional
   security issue with allowing them to see and modify CoA packets.

   In short, any security issues with the contents of Operator-NAS-
   Identifier are largely limited by the security of the underlying
   RADIUS protocol.  This limitation means that it does not matter how
   the values of Operator-NAS-Identifier are created, stored, or used.

7. IANA Considerations

   Per Section 3.4 of this document, IANA has allocated one new RADIUS
   attribute (the Operator-NAS-Identifier attribute) from the "short
   extended space" of the "RADIUS Attribute Types" registry as follows:

      Value: 241.8
      Description: Operator-NAS-Identifier
      Data Type: string
      Reference: RFC 8559

8. References

8.1. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, DOI 10.17487/RFC2865, June 2000,
              <https://www.rfc-editor.org/info/rfc2865>.

   [RFC5080]  Nelson, D. and A. DeKok, "Common Remote Authentication
              Dial In User Service (RADIUS) Implementation Issues and
              Suggested Fixes", RFC 5080, DOI 10.17487/RFC5080,
              December 2007, <https://www.rfc-editor.org/info/rfc5080>.

   [RFC5176]  Chiba, M., Dommety, G., Eklund, M., Mitton, D., and
              B. Aboba, "Dynamic Authorization Extensions to Remote
              Authentication Dial In User Service (RADIUS)", RFC 5176,
              DOI 10.17487/RFC5176, January 2008,
              <https://www.rfc-editor.org/info/rfc5176>.

   [RFC5580]  Tschofenig, H., Ed., Adrangi, F., Jones, M., Lior, A., and
              B. Aboba, "Carrying Location Objects in RADIUS and
              Diameter", RFC 5580, DOI 10.17487/RFC5580, August 2009,
              <https://www.rfc-editor.org/info/rfc5580>.

   [RFC6929]  DeKok, A. and A. Lior, "Remote Authentication Dial In User
              Service (RADIUS) Protocol Extensions", RFC 6929,
              DOI 10.17487/RFC6929, April 2013,
              <https://www.rfc-editor.org/info/rfc6929>.

   [RFC7542]  DeKok, A., "The Network Access Identifier", RFC 7542,
              DOI 10.17487/RFC7542, May 2015,
              <https://www.rfc-editor.org/info/rfc7542>.

   [RFC8044]  DeKok, A., "Data Types in RADIUS", RFC 8044,
              DOI 10.17487/RFC8044, January 2017,
              <https://www.rfc-editor.org/info/rfc8044>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,
              <https://www.rfc-editor.org/info/rfc8174>.

8.2. Informative References

   [RFC2866]  Rigney, C., "RADIUS Accounting", RFC 2866,
              DOI 10.17487/RFC2866, June 2000,
              <https://www.rfc-editor.org/info/rfc2866>.

Authors' Addresses

   Alan DeKok
   The FreeRADIUS Server Project

   Email: aland@freeradius.org

   Jouni Korhonen

   Email: jouni.nospam@gmail.com