Internet DRAFT - draft-ietf-radext-coa-proxy
draft-ietf-radext-coa-proxy
.nr HY 0
Network Working Group DeKok, Alan
INTERNET-DRAFT FreeRADIUS
Updates: 5176, 5580 J. Korhonen
Category: Standards Track
<draft-ietf-radext-coa-proxy-10.txt>
22 January 2019
Dynamic Authorization Proxying in
Remote Authorization Dial-In User Service Protocol (RADIUS)
draft-ietf-radext-coa-proxy-10.txt
Abstract
RFC 5176 defines Change of Authorization (CoA) and Disconnect Message
(DM) behavior for RADIUS. That document suggests that proxying these
messages is possible, but gives no guidance as to how it 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 Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 22, 2019.
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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
(http://trustee.ietf.org/license-info/) in effect on the date of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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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. Typical RADIUS Proxying ............................. 6
2.2. CoA Processing ...................................... 7
2.3. Failure of CoA Proxying ............................. 7
3. How to Perform CoA Proxying .............................. 8
3.1. Changes to Access-Request and Accounting-Request pack 9
3.2. Proxying of CoA-Request and Disconnect-Request packet 9
3.3. Reception of CoA-Request and Disconnect-Request packe 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 ............................. 15
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 ......................... 19
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
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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
gives no guidance as to how that is done. This omission means that
in practice, proxying of CoA packets is impossible.
We partially correct that ommission 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 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 of this
document apply only to those systems which 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 of [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 Proposed Standard in order to update the behavior
of [RFC5580], which is also a Proposed Standard. This document
relies heavily upon and also updates some behavior of RFC 5176, which
is an Informational document; though 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, or CoA-ACK, or CoA-NAK,
as defined in [RFC5176]. That specification also defines
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Disconnect-Request, Disconnect-ACK, and Disconnect-NAK. For
simplicity here, where we use "CoA", 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
The Network Access Identifier (NAI) [RFC7542] is the user identity
submitted by the client during network access authentication. The
purpose of the NAI is to identify the user as well as to 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
The Network Access Server (NAS) is 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 which 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.
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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 a Network Access Identifier (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 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 occurs until the
response packet arrives at the NAS.
The proxies are typically stateful with respect to ongoing request /
response packets, but 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. The
combination of the two methods allows proxies to connect Visited
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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 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 is 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 that
specification makes no suggestions for 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 may, in fact, not be co-located with the RADIUS
client. In which case it may 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
which 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 fail-over 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.
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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 [RFC5580], Section 4.1. 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 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 the an ASCII "1",
followed by the Realm of the Visited Network. e.g. 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 how the
forward path has to change in order to allow reverse path proxying.
We then describe how reverse path proxying works. And we describe
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 only to 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].
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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 should be "1", followed by the
realm name of the Visited Network. Where the Visited Network has
more than one realm name, a "canonical" one 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 the Operator-Name would mean that the Home Network
would not know where to send any CoA packet.
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 MUST be
the value which was recorded earlier for that user session.
The Home Network MUST lookup the realm from the Operator-Name in a
logical "realm routing table", as discussed in [RFC7542] Section 3.
That logical realm table is defined there 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.
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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 in a logical "realm routing table", as with Home
Servers, above. The packet is then sent to the proxy for the realm
which 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 which receive a CoA packet MUST NOT use the NAI from the
User-Name 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 instances of the
Operator-Name attribute.
3.3. Reception of CoA-Request and Disconnect-Request packets
After some proxying, the CoA packet will be recieved 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").
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These checks make it more difficult for a malicious Home Network to
scan roaming network 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 of this specification.
Those methods will typically be in the form of contractual
relationships between parties, or as 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.
The processing by Visited Network will normally include sending the
CoA packet to the NAS; having the NAS process it; and then returning
any response packet 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 re-write 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, in
a way which is both private, and which does not use any existing
attribute. 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
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appear in any other packet.
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 only to 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-Identifer 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-Identifer attribute is added to a packet, the following
attributes SHOULD be deleted from the packet: NAS-IP-Address, NAS-
IPv6-Address, NAS-Identifier. If these attributes are deleted, the
proxy MUST then add a NAS-Identifier attribute, in order satisfy the
requirements of Section 4.1 of [RFC2865], and Section 4.1 of
[RFC2866]. The contents of the new NAS-Identifier SHOULD be the
Realm name of the visited network.
When a server receives a packet that already contains an Operator-
NAS-Identifer attribute, no such editing is performed.
The Operator-NAS-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 which 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.
And, 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 which was 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
TBD. To be assigned by IANA from the "short extended space".
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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 MUST NOT
create attributes with more than sixty-four octets of data. A
thirty-two octet string should be more than sufficient for future
uses.
Data Type
string. See [RFC8044] Section 3.6 for a definition.
Value
The contents of this attribute are an opaque token interpretable
only 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 to create
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 un-obfuscate 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 MAC address of the NAS. There is therefore no
requirement that Operator-NAS-Identifier obsfuscate or hide in any
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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 to enforce this requirement.
4.2. Requirements on Visited Networks
A Visited Network which receives a CoA packet that will be proxied to
a NAS MUST perform all of the operations required for proxies by
Section 4.3.2. This requirement is because we assume that the
Visited Network has a proxy in 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 of the scope of this
specification.
The Visited Network uses the content 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 any CoA packet packet prior to sending
that packet to the final CoA server (i.e. NAS). This step is
necessary due to the the limits of 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 which identify or
modify a user's session. It is not appropriate to send a NAS
attributes which are used only for inter-proxy signaling.
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4.3. Requirements on Proxies
There are a number of requirements on proxies, 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 for these systems to 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" (RPF) 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 to identify a user's session.
We recognize that because a RADIUS proxy will see Access-Request and
Accounting-Request packets, 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
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session during the initial process of authentiction.
The largest 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 cannot therefore prevent proxies or Home Servers from forging CoA
packets. We can only create scenarios where that forgery is hard to
perform, and/or 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.
These requirements are too stringent for a CoA proxy. Only the final
CoA server (i.e NAS) can make a decision on which attributes are
mandatory and which are not.
Instead, we say that for a CoA proxy, 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 as-
is, 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] to say that in CoA-Request and
Disconnect-Request packets, the NAS MUST NOT treat as mandatory any
attribute which is known to not affect the users 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 ones that forward any
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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 Network MUST be removed by the
corresponding CoA proxy from packets traversing the reverse path.
That is, any attribute editing 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 attributes sent by the NAS to it's local RADIUS
server, or contain 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 it's own realm name. e.g. "1example.com". The
visited RADIUS server MAY also add an Operator-NAS-Identifier. 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 along with other information about the users session, if
those attributes are present in a packet.
5.2. CoA Proxying
At some later point in time, the Home Server determines that a user
session should have its authorization changed, or be disconnected.
The Home Server looks up the Operator-Name and Operator-NAS-
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Identifer, 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 (that 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
similar lookup as 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 (if available) attribute to determine
which local CoA server (i.e. NAS) the packet should be sent to. If
there is no Opertor-NAS-Identifier attribute, the Visited Network may
use other means to locate the NAS, such as consulting a local
database which tracks user sessions.
The Operator-Name and Operator-NAS-Identifer attributes are then
removed from the packet; one of NAS-IP-Address, or 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 return 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 the Section 11 of
[RFC6929]. In short, RADIUS has many known issues which are
discussed in detail there, and which 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 a series of
inherent security issues.
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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 packets entirely
without detection. 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 noted above. We
note that the proxies which see and can modify CoA packets are
generally the same proxies which 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 left is that Home Networks now have the
capability 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 the users's session, 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 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 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 will either have no effect, or will 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
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have sufficient information to identify that session. In practice,
this limitation means that those parties already have access to the
users's session information. Which is to say, 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
IANA is instructed to allocate one new RADIUS attribute, as per
Section 3.3, above. The Operator-NAS-Identifier attribute is to be
allocated from the RADIUS Attribute Types registry as follows:
Value: [ TBD-at-Registration ]
Description: Operator-NAS-Identifier
Data Type: string
Reference: [ RFC-to-be ]
8. References
8.1. Normative References
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March, 1997, <http://www.rfc-edi-
tor.org/info/rfc2119>.
[RFC2865]
Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote Authen-
tication Dial In User Service (RADIUS)", RFC 2865, June 2000,
<http://www.rfc-editor.org/info/rfc2865>.
[RFC5080]
Nelson, D., and DeKok, A., "Common Remote Authentication Dial In
User Service (RADIUS) Implementation Issues and Suggested Fixes",
RFC 5080, December 2007, <http://www.rfc-editor.org/info/rfc5080>.
[RFC5176]
Chiba, M. et al, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC 5176, January
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2008, <http://www.rfc-editor.org/info/rfc5176>.
[RFC5580]
Tschofenig H., Ed. "Carrying Location Objects in RADIUS and Diame-
ter", RFC 5580, August 2009, <http://www.rfc-edi-
tor.org/info/rfc5580>.
[RFC6929]
DeKok A. and Lior, A., "Remote Authentication Dial-In User Service
(RADIUS) Protocol Extensions", RFC 6929, April 2013,
<http://www.rfc-editor.org/info/rfc6929>.
[RFC7542]
DeKok A., "The Network Access Identifier", RFC 7542, May 2015,
<http://www.rfc-editor.org/info/rfc7542>.
[RFC8044]
DeKok A., "Data Types in the Remote Authentication Dial-In User
Service Protocol (RADIUS)", RFC 8044, January 2017,
<http://www.rfc-editor.org/info/rfc8044>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key
Words", RFC 8174, May 2017, <http://www.rfc-edi-
tor.org/info/rfc8174>.
8.2. Informative References
[RFC2866]
Rigney, C., "RADIUS Accounting", RFC 2866, June 2000,
<http://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
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