ROAMOPS Working Group Bernard Aboba
INTERNET-DRAFT Microsoft Corporation
Category: Standards Track John R. Vollbrecht
<draft-ietf-roamops-auth-08.txt> Merit Networks, Inc.
13 October 1998
Proxy Chaining and Policy Implementation in Roaming
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working docu-
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The distribution of this memo is unlimited. It is filed as <draft-
ietf-roamops-auth-08.txt>, and expires May 1, 1999. Please send com-
ments to the authors.
2. Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
3. Abstract
This document describes the use of proxy chaining in roaming and how
policy may be implemented concurrently with end-to-end security.
4. 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
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authentication/authorization via the protocol described in
[3], and for accounting as described in [4].
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 subse-
quent 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 compa-
nies and ISPs, relationships among peer ISPs within a roam-
ing association, and relationships between an ISP and a
roaming consortia. Together, the set of relationships form-
ing a path between a local ISP's authentication proxy and
the home authentication server is known as the roaming rela-
tionship path.
5. 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].
6. Introduction
Today, as described in [1], proxy chaining is widely deployed for the
purposes of providing roaming services. In such systems, authentica-
tion and accounting packets are routed between a NAS device and a home
server through a series of proxies.
Proxies serve a number of functions in roaming, including:
Scalability improvement
Authentication forwarding
Capabilities adjustment
Policy implementation
Accounting reliability improvement
Atomic operation
It should be noted that while a number of these functions can be pro-
vided within a new protocol, thus reducing the need to use proxies to
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perform these functions, the policy implementation function is funda-
mental and therefore is likely to remain, regardless of the protocol
chosen.
Scalability improvement
Proxy chaining enables implementation of hierarchical for-
warding within roaming systems, which significantly improves
scalability. Since RADIUS requires a shared secret for each
communicating pair of systems, 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.
Authentication forwarding
Since roaming partners typically do not communicate directly
due to scalability concerns, 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 proper routing
of authentication and accounting packets.
Note: The way a proxy learns the mapping between NAI and
end servers is beyond the scope of this document. This
mapping can be done by static configuration in the proxy, or
by some currently undefined pro- tocol that get the mapping
information dynamically. The assumption is that such a map-
ping exists in the proxy.
Capabilities adjustment
Since RADIUS does not support capabilities negotiation, it
is possible that network parameters sent back from the home
server will not match those required by the NAS. Proxies
can edit attributes within the Access-Accept in order to
ensure compatibility. 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. Note that if the proxy
edits attributes within the Access-Accept, then it is possi-
ble that the service provided to the user may not be the
same as that requested by the home server.
Policy implementation
RADIUS proxies can be used to implement policy. For example,
a given partner may only be entitled to use of a given NAS
during certain times of the day.
Accounting reliability improvement
The RADIUS accounting protocol, described in [4] is not
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designed for use on an Internet scale. This is a significant
issue in roaming, which is inherently an interdomain appli-
cation. 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 sys-
tem 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.
7. 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 agram, 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 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 7.1 and when the proxy is acting as an
accounting store (as in store and forward), as described in section
7.2.
While the RADIUS protocol described in [3] does not provide for end-
to-end security services, this is made possible using the attributes
described in [7]. The Security-Parameter-Index and End-to-End-
Signature attributes SHOULD be included in packets sent between admin-
istrative domains, including Access-Request, Access-Challenge,
Access-Accept, and Access-Reject packets. The Hidden attribute MAY be
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included, as necessary, in order to prevent disclosure of passwords or
keys to untrusted proxies.
7.1. 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 the 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 receiv-
ing an Access-Reject from a proxy or from the home server.
(Access-Req) (Access-Req) (Access-Req)
NAS ----------> Proxy1 ----------> Proxy2 ----------> Home
(Access-Reject) (Access-Accept) (Access-Accept) Server
<--------- <--------- <---------
(AcctPxStop) (AcctPxStop)
----------> ---------->
7.2. 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 ses-
sions), 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
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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:
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) forarding 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 in the
former arrangement, atmoic operation can be supported, while in the
latter case it typically cannot.
This ensures that Accounting Start and Stop messages are received, and
can be logged by all servers along the authentication/authorization
path. 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 may 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
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down, the proxy may need to 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.
8. Attribute editing
One of the biggest obstacles to interoperation of proxies today
results from editing behavior. 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 implementa-
tion-specific. However, using the end-to-end security attributes
defined in [7], it is possible to provide for both "protected" and
"unprotected" attributes. Protected attributes preceed an End-to-End-
Signature attribute within the packet, and as a result, these
attributes are integrity-protected end-to-end. Protected attributes
MUST NOT be added, deleted, or modified by a proxy.
Unprotected attributes follow the End-to-End-Signature attribute, and
are not covered by the message integrity check. As a result, these
attributes MAY be added, deleted, or modified by a proxy.
The choice of which attributes are protected or unprotected is left up
to the sender of the packet. For example, if the home server wishes to
guarantee that the client will be tunneled to a given destination,
then it will integrity protect tunnel attributes by placing them prior
to the End-to-End-Signature attribute. In general, home servers SHOULD
protect attributes whose modification would compromise security,
including tunnel attributes, and EAP-Message attributes.
If a proxy is unable to accept a protected attribute within an Access-
Request, then it MUST reply to the NAS with an Access-Reject packet.
If a proxy is unable to accept a protected attribute within an Access-
Accept or Access-Challenge packet, then it SHOULD send an Access-
Reject to the NAS, as well as well as an Accounting-Request with Acct-
Status-Type=Proxy-Stop (6) to the home server.
9. References
[1] Aboba, B., Lu J., Alsop J.,Ding J., and W. Wang, "Review of Roam-
ing Implementations", RFC 2194, September 1997.
[2] Aboba, B., and G. Zorn, "Roaming Requirements", Internet draft
(work in progress), draft-ietf-roamops-roamreq-10.txt, May 1998.
[3] Rigney C., Rubens A., Simpson W., and S. Willens, "Remote Authen-
tication Dial In User Service (RADIUS)", RFC 2138, April 1997.
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[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",
Internet draft (work in progress), draft-ietf-roamops-nai-10.txt, May
1998.
[7] Calhoun, P., and B. Aboba, "End-to-End Security in Roaming",
Internet draft (work in progress), draft-ietf-roamops-roamsec-01.txt,
July 1998.
10. Security Considerations
The following security threats have been identified in roaming sys-
tems:
Rogue proxies
Theft of passwords
Theft of accounting data
Replay attacks
Connection hijacking
Fraudulent accounting
10.1. Rogue proxies
In conventional ISP application, the NAS, proxy, and home server exist
within a single administrative entity. As a result, the proxy may be
considered a trusted component.
However, in a roaming system implemented with proxy chaining, the NAS,
proxies, and home server may be managed by different administrative
entities. Through the use of shared secrets it is possible for proxies
operating in different domains to establish a trust relationship. How-
ever, if packets are only authenticated on a hop-by-hop basis, then
untrusted proxies are capable of perpetrating a number of man-in-the-
middle attacks.
These attacks typically involve the editing of attributes, or the mod-
ification or insertion of messages, such as the substitution of an
Access-Accept for an Access-Reject. For example, a proxy may modify
an Access-Accept sent by the home server so as to lessen the security
obtained by the client. 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, tun-
nel attributes might be removed or modified so as to remove encryp-
tion, redirect the tunnel to a rogue tunnel server, or otherwise
lessen the security provided to the client.
Through implementation of the End-to-End-Signature attribute, it is
possible to detect unauthorized addition, deletion, or modification of
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protected attributes. Note that it is still possible for a rogue proxy
to add, delete or modify unprotected attributes.
While a proxy MUST NOT send an Access-Accept to the NAS after receiv-
ing an Access-Reject from the home server, a proxy MAY send an Access-
Reject to the NAS after receiving an Access-Accept from the home
server. Note that in the latter case, a Security-Parameter-Index
attribute should be used denoting the security association between the
proxy and the NAS, rather than that between the home server and the
NAS, since the proxy has originated the packet. This will allow the
NAS to verify the End-to-End-Signature attribute within the packet,
and decide whether to silently discard the packet. As noted earlier,
an Access-Accept originated by a proxy MUST be silently discarded by
the NAS, even if the End-to-End-Signature attribute can be verified.
The determination of whether end-to-end security is to be used in a
conversation is made using out-of-band mechanisms. Typically this is
based either on static configuration or on the outcome of a key
exchange conversation between the two endpoints. However, once it is
determined that the end systems wish to use end-to-end security, all
packets sent MUST include an End-to-End-Signature attribute and pack-
ets received without an End-to-End-Signature attribute MUST be
silently discarded. Note that policy determination using an out-of-
band mechanism rather than a proxied conversation limits the ability
of a rogue proxy to interfere with the security negotiation between
the two end systems.
10.2. Theft of passwords or keys
Unless the Hidden attribute is used, where clients authenticate using
PAP, or where the Tunnel-Password attribute is included with the
Access-Accept, each proxy along the path between the local NAS and the
home server will have access to the cleartext password or key. In many
circumstances, this represents an unacceptable security risk. As a
result, the Hidden attribute SHOULD be used to provide end-to-end con-
fidentiality for User-Password or Tunnel-Password attributes.
10.3. Integrity and confidentiality 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. In order to prevent modifica-
tion of accounting packets by untrusted proxies, the End-to-End-Signa-
ture attribute MAY be used. If it is desired that accounting data be
kept confidential from a proxy, the Hidden attribute MAY be used. If
the objective is to prevent snooping of accounting data on the wire,
then IPSEC ESP MAY be used.
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10.4. Replay attacks
In this attack, a man in the middle or rogue proxy collects CHAP-Chal-
lenge 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 pay-
ment. 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.
In order to protect against replay attacks, CHAP-Challenge and CHAP-
Response attributes MAY be protected using the Hidden attribute. CHAP
replay attacks can also be defeated by means of an end-to-end chal-
lenge-response exchange. For example, if the home server returns an
Access-Challenge packet containing a CHAP-Challenge attribute and
maintains state with respect to outstanding challenges, replay attacks
cannot succeed.
However, it should also be noted that end-to-end challenges (as prac-
ticed within the EAP MD5 authentication method, or in the CHAP-Chal-
lenge method described above) are also subject to attacks by rogue
proxies. In such an attack a proxy substitutes a static challenge for
the challenge sent by the home server, and on receiving the response,
checks it against a databases of hashes applied against a dictionary.
This attack may be prevented through use of the End-to-End-Signature
attribute.
10.5. 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. This attack may be
defeated via use of an End-to-End-Signature attribute as described in
[7].
10.6. 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..
In order to detect submissions of accounting packets or session
records for non-existent sessions, parties receiving accounting pack-
ets or session records will typically wish 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
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which an accounting record will subsequently be submitted.
In order to make reconciliation easier, home servers involved in roam-
ing SHOULD include a Class attribute in the Access-Accept. The Class
attribute SHOULD uniquely identify a session, so as to allow an
authentication log entry to be matched with a corresponding accounting
packet or session record.
Note that in order to prevent submission of accounting packets or ses-
sion records for non-existent sessions, it is necessary to prevent
replay.
11. 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.
12. 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
13. Full Copyright Statement
Copyright (C) The Internet Society (1997). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this docu-
ment itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other Inter-
net organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights defined
in the Internet Standards process must be followed, or as required to
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INTERNET-DRAFT Proxy Chaining and Policy in Roaming 13 October 1998
translate it into languages other than English. The limited permis-
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Internet Society or its successors or assigns. This document and the
information contained herein is provided on an "AS IS" basis and THE
INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL
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14. Expiration Date
This memo is filed as <draft-ietf-roamops-auth-08>, and expires May
1, 1999.
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