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rfc:rfc8559

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.

DeKok & Korhonen Standards Track [Page 1] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

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.

Table of Contents

 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

DeKok & Korhonen Standards Track [Page 2] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

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.

DeKok & Korhonen Standards Track [Page 3] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

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.

DeKok & Korhonen Standards Track [Page 4] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

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.

DeKok & Korhonen Standards Track [Page 5] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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.

DeKok & Korhonen Standards Track [Page 6] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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.

DeKok & Korhonen Standards Track [Page 7] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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.

DeKok & Korhonen Standards Track [Page 8] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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").

DeKok & Korhonen Standards Track [Page 9] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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).

DeKok & Korhonen Standards Track [Page 10] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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,

DeKok & Korhonen Standards Track [Page 11] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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.

DeKok & Korhonen Standards Track [Page 12] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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.

DeKok & Korhonen Standards Track [Page 13] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

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.

DeKok & Korhonen Standards Track [Page 14] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

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.

DeKok & Korhonen Standards Track [Page 15] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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.

DeKok & Korhonen Standards Track [Page 16] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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.

DeKok & Korhonen Standards Track [Page 17] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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.

DeKok & Korhonen Standards Track [Page 18] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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.

DeKok & Korhonen Standards Track [Page 19] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 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>.

DeKok & Korhonen Standards Track [Page 20] RFC 8559 Dynamic Authorization Proxying in RADIUS April 2019

 [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

DeKok & Korhonen Standards Track [Page 21]

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