Network Working Group B. Aboba
Request for Comments: 2607 Microsoft Corporation
Category: Informational J. Vollbrecht
Merit Networks, Inc.
June 1999
Proxy Chaining and Policy Implementation in Roaming
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
1. Abstract
This document describes how proxy chaining and policy implementation
can be supported in roaming systems. The mechanisms described in this
document are in current use.
However, as noted in the security considerations section, the
techniques outlined in this document are vulnerable to attack from
external parties as well as susceptible to fraud perpetrated by the
roaming partners themselves. As a result, such methods are not
suitable for wide-scale deployment on the Internet.
2. Terminology
This document frequently uses the following terms:
Network Access Server
The Network Access Server (NAS) is the device that clients contact
in order to get access to the network.
RADIUS server
This is a server which provides for authentication/authorization
via the protocol described in [3], and for accounting as described
in [4].
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RADIUS proxy
In order to provide for the routing of RADIUS authentication and
accounting requests, a RADIUS proxy can be employed. To the NAS,
the RADIUS proxy appears to act as a RADIUS server, and to the
RADIUS server, the proxy appears to act as a RADIUS client.
Network Access Identifier
In order to provide for the routing of RADIUS authentication and
accounting requests, the userID field used in PPP (known as the
Network Access Identifier or NAI) and in the subsequent RADIUS
authentication and accounting requests, can contain structure.
This structure provides a means by which the RADIUS proxy will
locate the RADIUS server that is to receive the request. The NAI
is defined in [6].
Roaming relationships
Roaming relationships include relationships between companies and
ISPs, relationships among peer ISPs within a roaming association,
and relationships between an ISP and a roaming consortia.
Together, the set of relationships forming a path between a local
ISP's authentication proxy and the home authentication server is
known as the roaming relationship path.
3. Requirements language
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in [5].
4. Introduction
Today, as described in [1], proxy chaining is widely deployed for the
purposes of providing roaming services. In such systems,
authentication/authorization and accounting packets are routed
between a NAS device and a home server through a series of proxies.
Consultation of the home server is required for password-based
authentication, since the home server maintains the password database
and thus it is necessary for the NAS to communicate with the home
authentication server in order to verify the user's identity.
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4.1. Advantages of proxy chaining
Proxies serve a number of functions in roaming, including:
Scalability improvement
Authentication forwarding
Capabilities adjustment
Policy implementation
Accounting reliability improvement
Atomic operation
Scalability improvement
In large scale roaming systems, it is necessary to provide for
scalable management of keys used for integrity protection and
authentication.
Proxy chaining enables implementation of hierarchical
forwarding within roaming systems, which improves scalability
in roaming consortia based on authentication protocols without
automated key management. Since RADIUS as described in [3]
requires a shared secret for each client-server pair, a
consortium of 100 roaming partners would require 4950 shared
secrets if each partner were to contact each other directly,
one for each partner pair. However, were the partners to
route authentication requests through a central proxy, only
100 shared secrets would be needed, one for each partner. The
reduction in the number of partner pairs also brings with it
other benefits, such as a reduction in the number of bilateral
agreements and accounting and auditing overhead. Thus,
hierarchical routing might be desirable even if an
authentiation protocol supporting automated key exchange were
available.
Capabilities adjustment
As part of the authentication exchange with the home server,
the NAS receives authorization parameters describing the
service to be provided to the roaming user. Since RADIUS,
described in [3], does not support capabilities negotiation,
it is possible that the authorization parameters sent by the
home server will not match those required by the NAS. For
example, a static IP address could be specified that would not
be routable by the NAS. As a result, capbilities adjustment is
performed by proxies in order to enable communication between
NASes and home servers with very different feature sets.
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As part of capabilities adjustment, proxies can edit
attributes within the Access-Accept in order to ensure
compatibility with the NAS. Such editing may include
addition, deletion, or modification of attributes. In
addition, in some cases it may be desirable for a proxy to
edit attributes within an Access-Request in order to clean up
or even hide information destined for the home server. Note
that if the proxy edits attributes within the Access-Accept,
then it is possible that the service provided to the user may
not be the same as that requested by the home server. This
creates the possibility of disputes arising from inappropriate
capabilities adjustment.
Note that were roaming to be implemented based on an
authentication/authorization protocol with built-in capability
negotiation, proxy-based capabilities adjustment would
probably not be necessary.
Authentication forwarding
Since roaming associations frequently implement hierarchical
forwarding in order to improve scalability, in order for a NAS
and home server to communicate, authentication and accounting
packets are forwarded by one or more proxies. The path
travelled by these packets, known as the roaming relationship
path, is determined from the Network Access Identifier (NAI),
described in [6]. Since most NAS devices do not implement
forwarding logic, a proxy is needed to enable forwarding of
authentication and accounting packets. For reasons that are
described in the security section, in proxy systems it is
desirable for accounting and authentication packets to follow
the same path.
Note: The way a proxy learns the mapping between NAI and the
home server is beyond the scope of this document. This
mapping can be accomplished by static configuration in the
proxy, or by some currently undefined protocol that provides
for dynamic mapping. For the purposes of this document, it is
assumed that such a mapping capability exists in the proxy.
Policy implementation
In roaming systems it is often desirable to be able to
implement policy. For example, a given partner may only be
entitled to use of a given NAS during certain times of the
day. In order to implement such policies, proxies may be
implemented at the interface between administrative domains
and programmed to modify authentication/authorization packets
forwarded between the NAS and the home server. As a result,
from a security point of view, a proxy implementing policy
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operates as a "man in the middle."
Accounting reliability improvement
In roaming systems based on proxy chaining, it is necessary
for accounting information to be forwarded between the NAS and
the home server. Thus roaming is inherently an interdomain
application.
This represents a problem since the RADIUS accounting
protocol, described in [4] is not designed for use on an
Internet scale. Given that in roaming accounting packets
travel between administrative domains, packets will often pass
through network access points (NAPs) where packet loss may be
substantial. This can result in unacceptable rates of
accounting data loss.
For example, in a proxy chaining system involving four
systems, a one percent failure rate on each hop can result in
loss of 3.9 percent of all accounting transactions. Placement
of an accounting proxy near the NAS may improve reliability by
enabling enabling persistent storage of accounting records and
long duration retry.
Atomic operation
In order to ensure consistency among all parties required to
process accounting data, it can be desirable to assure that
transmission of accounting data is handled as an atomic
operation. This implies that all parties on the roaming
relationship path will receive and acknowledge the receipt of
the accounting data for the operation to complete. Proxies can
be used to ensure atomic delivery of accounting data by
arranging for delivery of the accounting data in a serial
fashion, as discussed in section 5.2.
5. Proxy chaining
An example of a proxy chaining system is shown below.
(request) (request) (request)
NAS ----------> Proxy1 ----------> Proxy2 ----------> Home
(reply) (reply) (reply) Server
<--------- <--------- <---------
In the above diagram, the NAS generates a request and sends it to
Proxy1. Proxy1 forwards the request to Proxy2 and Proxy2 forwards
the request to the Home Server. The Home Server generates a reply
and sends it to Proxy2. Proxy2 receives the reply, matches it with
the request it had sent, and forwards a reply to Proxy1. Proxy1
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matches the reply with the request it sent earlier and forwards a
reply to the NAS. This model applies to all requests, including
Access Requests and Accounting Requests.
Except for the two cases described below, a proxy server such as
Proxy2 in the diagram above SHOULD NOT send a Reply packet to Proxy1
without first having received a Reply packet initiated by the Home
Server. The two exceptions are when the proxy is enforcing policy as
described in section 5.1 and when the proxy is acting as an
accounting store (as in store and forward), as described in section
5.2.
The RADIUS protocol described in [3] does not provide for end-to-end
security services, including integrity or replay protection,
authentication or confidentiality. As noted in the security
considerations section, this omission results in several security
problems within proxy chaining systems.
5.1. Policy implementation
Proxies are frequently used to implement policy in roaming
situations. Proxies implementing policy MAY reply directly to
Access-Requests without forwarding the request. When replying
directly to an Access-Request, the proxy MUST reply either with an
Access-Reject or an Access-Challenge packet. A proxy MUST NOT reply
directly with an Access-Accept. An example of this would be when the
proxy refuses all connections from a particular realm during prime
time. In this case the home server will never receive th Access-
Request. This situation is shown below:
(request) (request)
NAS ----------> Proxy1 ----------> Proxy2 Home
(reply) (reply) Server
<--------- <---------
A proxy MAY also decide to Reject a Request that has been accepted by
the home server. This could be based on the set of attributes
returned by the home server. In this case the Proxy SHOULD send an
Access-Reject to the NAS and an Accounting-Request with Acct-Status-
Type=Proxy-Stop (6) to the home server. This lets the home server
know that the session it approved has been denied downstream by the
proxy. However, a proxy MUST NOT send an Access-Accept after
receiving an Access-Reject from a proxy or from the home server.
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(Access-Req) (Access-Req) (Access-Req)
NAS ----------> Proxy1 ----------> Proxy2 ----------> Home
(Access-Reject) (Access-Accept) (Access-Accept) Server
<--------- <--------- <---------
(AcctPxStop) (AcctPxStop)
----------> ---------->
5.2. Accounting behavior
As described above, a proxy MUST NOT reply directly with an Access-
Accept, and MUST NOT reply with an Access-Accept when it has received
an Access-Reject from another proxy or Home Server. As a result, in
all cases where an accounting record is to be generated (accepted
sessions), no direct replies have occurred, and the Access-Request
and Access-Accept have passed through the same set of systems.
In order to allow proxies to match incoming Accounting-Requests with
previously handled Access-Requests and Access-Accepts, a proxy SHOULD
route the Accounting-Request along the same realm path travelled in
authentication/authorization. Note that this does not imply that
accounting packets will necessarily travel the identical path,
machine by machine, as did authentication/authorization packets.
This is because it is conceivable that a proxy may have gone down,
and as a result the Accounting-request may need to be forwarded to an
alternate server. It is also conceivable that
authentication/authorization and accounting may be handled by
different servers within a realm.
The Class attribute can be used to match Accounting Requests with
prior Access Requests. It can also be used to match session log
records between the home Server, proxies, and NAS. This matching can
be accomplished either in real-time (in the case that authentication
and accounting packets follow the same path, machine by machine), or
after the fact.
Home servers SHOULD insert a unique session identifier in the Class
attribute in an Access-Accept and Access-Challenge. Proxies and
NASes MUST forward the unmodified Class attribute. The NAS MUST
include the Class attribute in subsequent requests, in particular for
Accounting-Requests. The sequence of events is shown below:
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Authentication/Authorization
--------> --------> --------->
NAS Proxy1 Proxy2 Home (add class)
<-class-- <-class- <-class--
Accounting
(Accounting-req) (Accounting-req) (Accounting-req)
w/class w/class w/class
NAS ----------> Proxy1 ----------> Proxy2 ----------> Home
(Accounting-reply) (Accounting-reply)(Accounting-reply) Server
<--------- <--------- <---------
Since there is no need to implement policy in accounting, a proxy
MUST forward all Accounting Requests to the next server on the path.
The proxy MUST guarantee that the Accounting Request is received by
the End Server and all intermediate servers. The proxy may do this
either by: 1) forwarding the Accounting Request and not sending a
Reply until it receives the matching Reply from the upstream server,
or 2) acting as a store point which takes responsibility for
reforwarding the Accounting Request until it receives a Reply.
Note that when the proxy does not send a reply until it receives a
matching reply, this ensures that Accounting Start and Stop messages
are received and can be logged by all servers along the roaming
relationship path. If one of the servers is not available, then the
operation will fail. As a result the entire accounting transaction
will either succeed or fail as a unit, and thus can be said to be
atomic.
Where store and forward is implemented, it is possible that one or
more servers along the roaming relationship path will not receive the
accounting data while others will. The accounting operation will not
succeed or fail as a unit, and is therefore not atomic. As a result,
it may not be possible for the roaming partners to reconcile their
audit logs, opening new opportunities for fraud. Where store and
forward is implemented, forwarding of Accounting Requests SHOULD be
done as they are received so the downstream servers will receive them
in a timely way.
Note that there are cases where a proxy will need to forward an
Accounting packet to more than one system. For example, in order to
allow for proper accounting in the case of a NAS that is shutting
down, the proxy can send an Accounting-Request with Acct-Status-
Type=Accounting-Off (8) to all realms that it forwards to. In turn,
these proxies will also flood the packet to their connected realms.
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6. References
[1] Aboba, B., Lu J., Alsop J., Ding J. and W. Wang, "Review of
Roaming Implementations", RFC 2194, September 1997.
[2] Aboba, B. and G. Zorn, "Criteria for Evaluating Roaming
Protocols", RFC 2477, January 1999.
[3] Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2138, April
1997.
[4] Rigney, C., "RADIUS Accounting", RFC 2139, April 1997.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999.
7. Security Considerations
The RADIUS protocol described in [3] was designed for intra-domain
use, where the NAS, proxy, and home server exist within a single
administrative domain, and proxies may be considered a trusted
component. However, in roaming the NAS, proxies, and home server will
typically be managed by different administrative entities. As a
result, roaming is inherently an inter-domain application, and
proxies cannot necessarily be trusted. This results in a number of
security threats, including:
Message editing
Attribute editing
Theft of passwords
Theft and modification of accounting data
Replay attacks
Connection hijacking
Fraudulent accounting
7.1. Message editing
Through the use of shared secrets it is possible for proxies
operating in different domains to establish a trust relationship.
However, if only hop-by-hop security is available then untrusted
proxies are capable of perpetrating a number of man-in-the-middle
attacks. These include modification of messages.
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For example, an Access-Accept could be substituted for an Access-
Reject, and without end-to-end integrity protection, there is no way
for the NAS to detect this. On the home server, this will result in
an accounting log entry for a session that was not authorized.
However, if the proxy does not forward accounting packets or session
records to the home server, then the home server will not be able to
detect the discrepancy until a bill is received and audited.
Note that a proxy can also send an Access-Reject to the NAS after
receiving an Access-Accept from the home server. This will result in
an authentication log entry without a corresponding accounting log
entry. Without the proxy sending an Accounting-Request with Acct-
Status-Type=Proxy-Stop (6) to the home server, then there will be no
way for the home server to determine whether the discrepancy is due
to policy implementation or loss of accounting packets. Thus the use
of Acct-Status-Type=Proxy-Stop can be of value in debugging roaming
systems.
It should be noted that even if end-to-end security were to be
available, a number of sticky questions would remain. While the end-
points would be able to detect that the message from the home server
had been modified by an intermediary, the question arises as to what
action should be taken. While the modified packet could be silently
discarded, this could affect the ability of the home server to .
accept an Acct-Status-Type=Proxy-Stop message from an intermediate
proxy. Since this message would not be signed by the NAS, it may need
to be dropped by the home server.
This is similar to the problem that IPSEC-capable systems face in
making use of ICMP messages from systems with whom they do not have a
security association. The problem is more difficult here, since in
RADIUS retransmission is driven by the NAS. Therefore the home
server does not receive acknowledgement for Access-Accepts and thus
would have no way of knowing that its response has not been honored.
7.2. Attribute editing
RADIUS as defined in [3] does not provide for end-to-end security or
capabilities negotiation. As a result there is no way for a home
server to securely negotiate a mutually acceptable configuration with
the NAS or proxies. As a result, a number of attribute editing
attacks are possible.
For example, EAP attributes might be removed or modified so as to
cause a client to authenticate with EAP MD5 or PAP, instead of a
stronger authentication method. Alternatively, tunnel attributes
might be removed or modified so as to remove encryption, redirect the
tunnel to a rogue tunnel server, or otherwise lessen the security
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provided to the client. The mismatch between requested and received
services may only be detectable after the fact by comparing the
Access-Accept attributes against the attributes included in the
Accounting-Request. However, without end-to-end security services, it
is possible for a rogue proxy to cover its tracks.
Due to the complexity of proxy configuration, such attacks need not
involve malice, but can occur due to mis-configuration or
implementation deficiencies. Today several proxy implementations
remove attributes that they do not understand, or can be set up to
replace attribute sets sent in the Access-Accept with sets of
attributes appropriate for a particular NAS.
In practice, it is not possible to define a set of guidelines for
attribute editing, since the requirements are very often
implementation-specific. At the same time, protection against
inappropriate attribute editing is necessary to guard against attacks
and provide assurance that users are provisioned as directed by the
home server.
Since it is not possible to determine beforehand whether a given
attribute is editable or not, a mechanism needs to be provided to
allow senders to indicate which attributes are editable and which are
not, and for the receivers to detect modifications of "non-editable"
attributes. Through implementation of end-to-end security it may be
possible to detect unauthorized addition, deletion, or modification
of integrity-protected attributes. However, it would still possible
for a rogue proxy to add, delete or modify attributes that are not
integrity-protected. If such attributes influence subsequent charges,
then the possibility of fraud would remain.
7.3. Theft of passwords
RADIUS as defined in [3] does not provide for end-to-end
confidentiality. As a result, where clients authenticate using PAP,
each proxy along the path between the local NAS and the home server
will have access to the cleartext password. In many circumstances,
this represents an unacceptable security risk.
7.4. Theft and modification of accounting data
Typically in roaming systems, accounting packets are provided to all
the participants along the roaming relationship path, in order to
allow them to audit subsequent invoices. RADIUS as described in [3]
does not provide for end-to-end security services, including
integrity protection or confidentiality. Without end-to-end integrity
protection, it is possible for proxies to modify accounting packets
or session records. Without end-to-end confidentiality, accounting
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data will be accessible to proxies. However, if the objective is
merely to prevent snooping of accounting data on the wire, then IPSEC
ESP can be used.
7.5. Replay attacks
In this attack, a man in the middle or rogue proxy collects CHAP-
Challenge and CHAP-Response attributes, and later replays them. If
this attack is performed in collaboration with an unscrupulous ISP,
it can be used to subsequently submit fraudulent accounting records
for payment. The system performing the replay need not necessarily
be the one that initially captured the CHAP Challenge/Response pair.
While RADIUS as described in [3] is vulnerable to replay attacks,
without roaming the threat is restricted to proxies operating in the
home server's domain. With roaming, such an attack can be mounted by
any proxy capable of reaching the home server.
7.6. Connection hijacking
In this form of attack, the attacker attempts to inject packets into
the conversation between the NAS and the home server. RADIUS as
described in [3] is vulnerable to such attacks since only Access-
Reply and Access-Challenge packets are authenticated.
7.7. Fraudulent accounting
In this form of attack, a local proxy transmits fraudulent accounting
packets or session records in an effort to collect fees to which they
are not entitled. This includes submission of packets or session
records for non-existent sessions. Since in RADIUS as described in
[3], there is no end-to-end security, a rogue proxy may insert or
edit packets without fear of detection.
In order to detect submissions of accounting packets or session
records for non-existent sessions, parties receiving accounting
packets or session records would be prudent to reconcile them with
the authentication logs. Such reconciliation is only typically
possible when the party acts as an authentication proxy for all
sessions for which an accounting record will subsequently be
submitted.
In order to make reconciliation easier, home servers involved in
roaming include a Class attribute in the Access-Accept. The Class
attribute uniquely identifies a session, so as to allow an
authentication log entry to be matched with a corresponding
accounting packet or session record.
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If reconciliation is put in place and all accounting log entries
without a corresponding authentication are rejected, then the
attacker will need to have obtained a valid user password prior to
submitting accounting packets or session records on non-existent
sessions. While use of end-to-end security can defeat unauthorized
injection or editing of accounting or authentication packets by
intermediate proxies, other attacks remain feasible. For example,
unless replay protection is put in place, it is still feasible for an
intermediate proxy to resubmit authentication or accounting packets
or session records. In addition, end-to-end security does not provide
protection against attacks by the local proxy, since this is
typically where end-to-end security will be initiated. To detect such
attacks, other measures need to be put in place, such as systems for
detecting unusual activity of ISP or user accounts, or for
determining whether a user or ISP account is within their credit
limit.
Note that implementation of the store and forward approach to proxy
accounting makes it possible for some systems in the roaming
relationship path to receive accounting records that other systems do
not get. This can result in audit discrepancies. About the best that
is achievable in such cases is to verify that the accounting data is
missing by checking against the authentication logs.
8. Acknowledgments
Thanks to Pat Calhoun of Sun Microsystems, Mark Beadles of
CompuServe, Aydin Edguer of Morningstar, Bill Bulley of Merit, and
Steven P. Crain of Shore.Net for useful discussions of this problem
space.
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9. Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: 425-936-6605
EMail: bernarda@microsoft.com
John R. Vollbrecht
Merit Network, Inc.
4251 Plymouth Rd.
Ann Arbor, MI 48105-2785
Phone: 313-763-1206
EMail: jrv@merit.edu
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10. Full Copyright Statement
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