Network Working Group D. Mitton
Request for Comments: 2882 Nortel Networks
Category: Informational July 2000
Network Access Servers Requirements:
Extended RADIUS Practices
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 (2000). All Rights Reserved.
Abstract
This document describes current practices implemented in NAS products
that go beyond the scope of the RADIUS RFCs 2138, 2139 [1,2]. The
purpose of this effort is to give examples that show the need for
addressing and standardizing these types of ad-hoc functions. Since
many of these features require a matching server support component,
the ability to deploy and manage interoperable NAS and AAA server
products is severely hindered.
These practices are documented here to show functions that are
obviously desired in developing future AAA protocols for NAS
deployment.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Disclaimers . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Presentation . . . . . . . . . . . . . . . . . . . . . . 3
2. Attribute Usage . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Attribute Conflicts . . . . . . . . . . . . . . . . . . . 4
2.2. Attribute Value Conflicts . . . . . . . . . . . . . . . . 4
2.2.1 Vendor Specific Enumerations Proposal . . . . . . . . . . 4
2.3 Vendor Specific Attribute Usage . . . . . . . . . . . . . 5
2.3.1 VSAs in use by clients: . . . . . . . . . . . . . . . . . 5
2.3.2 Clients that support multiple Vendors: . . . . . . . . . 5
3. Attribute Data Types . . . . . . . . . . . . . . . . . . . 6
4. New Messages . . . . . . . . . . . . . . . . . . . . . . . 7
5. Additional Functions . . . . . . . . . . . . . . . . . . . 7
5.1 Password Change . . . . . . . . . . . . . . . . . . . . . 8
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5.2 Authentication Modes . . . . . . . . . . . . . . . . . . . 8
5.3 Menus . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.4 Pseudo Users . . . . . . . . . . . . . . . . . . . . . . . 9
6. Resource Management . . . . . . . . . . . . . . . . . . . . 9
6.1 Managed Resources . . . . . . . . . . . . . . . . . . . . . 9
6.2 Resource Management Messages . . . . . . . . . . . . . . . 10
6.3 Concurrent Logins . . . . . . . . . . . . . . . . . . . . . 10
6.4 Authorization Changes . . . . . . . . . . . . . . . . . . . 11
7. Policy Services . . . . . . . . . . . . . . . . . . . . . . 11
8. Accounting Extensions . . . . . . . . . . . . . . . . . . . 12
8.1 Auditing/Activity . . . . . . . . . . . . . . . . . . . . . 12
9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . 13
11. Implementation Documents . . . . . . . . . . . . . . . . . 13
11.1. Clients . . . . . . . . . . . . . . . . . . . . . . . . . 13
11.2. Servers . . . . . . . . . . . . . . . . . . . . . . . . . 14
12. References . . . . . . . . . . . . . . . . . . . . . . . . 14
13. Author's Address . . . . . . . . . . . . . . . . . . . . . 15
14. Full Copyright Statement . . . . . . . . . . . . . . . . . 16
1. Introduction
The RADIUS Working Group was formed in 1995 to document the protocol
of the same name, and was chartered to stay within a set of bounds
for dial-in terminal servers. Unfortunately the real world of
Network Access Servers (NASes) hasn't stayed that small and simple,
and continues to evolve at an amazing rate.
This document shows some of the current implementations on the market
have already outstripped the capabilities of the RADIUS protocol. A
quite a few features have been developed completely outside the
protocol. These features use the RADIUS protocol structure and
format, but employ operations and semantics well beyond the RFC
documents.
I learn of the details of these functions from reading industry
manuals and often have to respond to them in competive bid
specifications. As they become deployed in the field, they gather
the force of de-facto standards.
Because they have been done outside scope of the RFCs, they are
vendor specific, and introduce significant problems in offering an
interoperable product.
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1.1. Disclaimers
The data and numbers in this document have been gleaned from public
sources and vendor documents along the way. Actual implementation of
many these features and variation from the documentation has not been
confirmed.
This document is a snapshot of known practices at the time of
writing. It is not intended to standardize these practices here, or
keep this document current, beyond initial publication. While some
detailed information is given, it is not the purpose of this document
to directly or sufficiently describe the functions mentioned to the
level of a complete functional specification.
The author has not transcribed copyrighted material, and is not aware
of whether any of these practices have of intellectual property
restrictions.
Any numeric assignments or functional operations are subject to
change by vendors without notice. I would appreciate any direct
input, preferably first hand, from implementors.
1.2. Presentation
Without any easy organization for the material, information is
arranged in a simple taxonomy from bottom-up complexity:
- Attribute Usage
- Attribute Data Types
- Message Codes
- New Operations
2. Attribute Usage
The RADIUS RFCs define attribute type ranges and specific attribute
definitions.
- There are about 70 defined RADIUS attributes:
- Values 192-223 are reserved for experimental use
- Values 224-240 are reserved for implementation-specific use
- Values 241-255 are reserved and should not be used.
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Attribute 26 was defined to be the Vendor Specific Attribute (VSA)
with further internal structure to allow vendor expansion.
2.1. Attribute conflicts
In practice attributes 92-255 are in use by a vendor. And another
vendor also use attributes in the 90-104 range and conflicts with
this usage.
To deal with these issues, server vendors have added vendor specific
parameters to their client database files. The administrator has to
indicate the vendor type of NAS along with the client IP address and
secret, so that the server can disambiguate the attribute usage.
One server has a single large vendors file to describe the mapping
all attributes to an internal format that retains the vendor id.
Another server implementation uses multiple dictionaries, each
indexed to a NAS and Vendor Model definition list.
2.2. Attribute Value Conflicts
Adding additional attributes may be more trouble than necessary for
simple features. Often existing RADIUS attributes could be extended
with additional values (particularly attributes that are enumerated
choices). But in doing such there is no way to guarantee not
conflicting with other vendor's extensions.
2.2.1. Vendor Specific Enumerations proposal
One proposed solution to this problem was Vendor Specific
Enumerations (VSEs). That is to imbed the vendor's management ID in
the numeric value (ala VSAs) which would to divide up the attribute
value space. This technique has not seen any acceptance by the
working group or other vendors, however, the vendor did accomplish
the goal of not conflicting with working group additions or other
vendor values.
Example dictionary of VSE values:
VALUE Service-Type VSE-Authorize-Only 0x06300001
VALUE Acct-Status-Type VSE-User-Reject 0x06300001
VALUE Acct-Status-Type VSE-Call-Reject 0x06300002
VALUE Acct-Status-Type VSE-IPCP-Start 0x06300003
VALUE Acct-Status-Type VSE-IPXCP-Start 0x06300004
VALUE Acct-Status-Type VSE-ATCP-Start 0x06300005
VALUE Acct-Status-Type VSE-Accounting-Restart 0x06300006
VALUE Acct-Status-Type VSE-Accounting-Shutoff 0x06300007
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VALUE Acct-Status-Type VSE-Tunnel-Start 0x06300008
VALUE Acct-Status-Type VSE-Tunnel-Stop 0x06300009
VALUE Acct-Status-Type VSE-Tunnel-Reject 0x0630000a
VALUE Acct-Status-Type VSE-Tunnel-Link-Start 0x0630000b
VALUE Acct-Status-Type VSE-Tunnel-Link-Stop 0x0630000c
VALUE Acct-Status-Type VSE-MP-Start 0x0630000d
VALUE Acct-Status-Type VSE-MP-Stop 0x0630000e
VALUE Acct-Status-Type VSE-Line-Seizure 0x0630000f
VALUE Acct-Status-Type VSE-Rlogin-Start 0x06300010
VALUE Acct-Status-Type VSE-Rlogin-Stop 0x06300011
2.3. Vendor Specific Attribute Usage
Because attribute 26 Vendor Specific Attributes (VSAs) came late in
the RADIUS working group development, there were some server
implementations that were late to support them. Today, there are
several leading implementations of clients that make extensive use of
VSAs. What's unfortunate is that there is also several different
formats of VSAs implemented. This is because the RFC suggested
format does not support more than 256 attributes.
2.3.1. VSAs in use by some clients:
At the time this document was written, the following had be observed:
- Microsoft: several for MS-CHAP authentication support [9]
- ACC: 42 [10]
- Nortel(Bay): about 60 VSAs and 16 VSEs
- Nortel(Aptis): about 60 VSA: 20 1-byte, ~130 4-byte header.
Aptis VSAs have shifted from a regular format to a 4-byte header
format, due to the large number of attributes implemented.
- 3Com (USR): about 130
USR VSAs are different than the format suggested in RFC 2138.
They have a 4 byte type field and have no internal length.
Some vendors that did not initially use VSAs, have shifted in later
releases VSA usage as a configuration option.
2.3.2. Clients that support Multiple Vendor Attributes
Now that MS-CHAP RADIUS attributes have been published in RFC 2548
[9] as Microsoft VSA attributes, it will become typical that for NAS
clients that support MS-CHAP authentication to process several
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different vendor VSA types. This has implications for RADIUS servers
that filter or "prune" return attributes based on the vendor
make/model of the NAS client.
One NAS implementation can receive up to three different vendor
specific attribute sets, but will only send attributes according to
the "mode" that has been configured by the operator. This allows it
to fit into environments where the customer has become dependent on a
particular set of RADIUS attributes, and allows the NAS to "drop-in"
without server attribute changes.
There is another NAS that supports 3 vendor attributes sets
concurrently. That is, it will normally use a combination of
different vendor VSAs in return profiles from the server. This was
done to support a superset of competing vendor's extensions, as well
as it's own, and include an extensions from a sister product.
3. Attribute Data Types
The base RFCs define only has 4 basic data types:
- integer, 32 bit unsigned
- string, 1-253 bytes, counted.
- ipaddr, 32 bit IPv4
- date, 32 bit Unix format.
Since then, various variations have been added:
The tunnel authentication document [6] adds an optional compound
"tag" byte to tunnel attributes. These are a single byte prepended
to the data field in order to support sets of attributes to be
returned. The byte value must be in the range 01-3F hex or it is
considered to be data.
Note that there is no native support for IPv6 addresses. In fact IPv6
support is missing in some fixed message components too.
There have been special attribute types created within servers. For
packet filters, the format called "abinary" was created. The user
enters an ASCII string filter description in the user profile, but
the server parses it into a binary string before passing it to the
NAS. This lowers the complexity of the NAS parser. Also a
"phonestring" server data type allows additional data type checking
at the entry application.
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4. New Messages
A number of new message types have been introduced by various parties
over time. The base specification has 6, vendors have added 26.
These fall into a number of categories which are described in the
next section below. Some of these messages are actually used between
the RADIUS server and some other resource server, using a RADIUS-like
protocol to implement new functions.
6 Accounting Status
(now Interim Accounting [5])
7 Password Request
8 Password Ack
9 Password Reject
10 Accounting Message
21 Resource Free Request
22 Resource Free Response
23 Resource Query Request
24 Resource Query Response
25 Alternate Resource Reclaim Request
26 NAS Reboot Request
27 NAS Reboot Response
29 Next Passcode
30 New Pin
31 Terminate Session
32 Password Expired
33 Event Request
34 Event Response
40 Disconnect Request
41 Disconnect Ack
42 Disconnect Nak
43 Change Filters Request
44 Change Filters Ack
45 Change Filters Nak
50 IP Address Allocate
51 IP Address Release
5. Additional Functions
These are operations performed using RADIUS extensions and additional
messages types.
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5.1. Password Change
Remotely requested password change operations were described and
proposed, but rejected by the working group. None the less, the
feature is still deployed in a number of products.
Message types:
- Password Request
- Password Ack or Reject
5.2. Authentication Modes
Additional message types have been added to negotiate passcode
changes for token card servers.
- Next Passcode
- New PIN
- Password Expired
They allow the NAS or RADIUS server negotiate passcode changes with
an external security server.
5.3. Menus
At least two vendors have built menuing interaction systems for use
with terminal dial-ins.
One implementation uses the Reply-Message string as the menu text to
be displayed, and the State attribute to keep track of the place in
the menu. The menu is displayed using the Access-Challenge message.
The response is encoded in the User-Password field like an ordinary
challenge sequence would.
Some RADIUS clients have problems with this because they cannot
handle long or multiple Reply-Message attributes that may have
embedded carriage returns and line-feeds. The new Echo attribute
should also control echo behavior on the menu response. Use of the
State attribute to keep track of a Challenge sequence is also
standard behavior.
Another implementation uses two vendor attributes (VSA-Menu-Item, and
VSA-Menu-Selector as well as VSA-Third-Prompt) to convey this
information. This implementation is vendor specific.
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5.4. Pseudo Users
One client implementation takes advantage of your vanilla RADIUS
server's ability to be used as a remote database server. By using
some well-known, implementation specific, strings for Username and
Password attributes, the NAS can request information from the server,
such as: Static IP routes, Static IPX routes, or the Message of the
Day.
These are called pseudo-user requests, because they use a user entry
with this manufactured name, for purposes other than authentication.
Another client also uses a pseudo-user technique for resolving
unknown Filter-ID(11) values. An Access-Request message is sent to
the RADIUS server with the Filter-ID as the Username value, the
password is a known string, and the Service-Type is VSE-
Authorization-Only. The response must also be of the same Service-
Type, or the response will be ignored. The responding profile should
contain the IP-Filter VSA attributes that will define the desired
filter.
It should be noticed that pseudo-user profiles could be a security
problem if a specific or operationally invalid Service-Type is not
attached to the profile. The client should test for this returned
value, to prevent normal dial-in users from gaining access via this
profile.
6. Resource Management
Authorized sessions may need to be allocated additional dynamic
resources in order to perform their services. The most typical is IP
addresses. The allocation may want to be delayed until needed or
coordinated on a scale independent of the RADIUS server. Additional
messages may be used to allocate and free these resources. The
RADIUS server may proxy these requests to another server.
Examples: Certain servers can allocate addresses local to the NAS or
use an outboard address server. Other servers have an internal
address pool capability, which will fill in the Framed-IP-Address
attribute with an assigned value based on pool selected.
6.1. Managed Resources:
Resources managed include: IP Addresses, Concurrent Logins, Dial-in
Port allocation policies, Tunnel limits and load distribution.
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There are several different types of implementation techniques:
- Explicit request/free resource requests
- Monitor usage with deamons watching the state
- Explicit messages to a state deamon
- Monitor Accounting messages for state changes
6.2. Resource Management Messages
Messages used for resource management
- IP Address Allocate
- IP Address Release
- Resource Request
- Resource Response
- Resource Free Request
- Resource Free Response
- Resource Reclaim Request
- NAS Reboot Request/Response
These messages are used to allocate and free resources for a NAS from
a centralized server. These mechanisms allows the service provider
better administrative control than some automated LAN services, which
don't have policy inputs or controls.
6.3. Concurrent Logins
The RADIUS protocol was designed to allow stateless servers. That
is, servers that don't know the status of the active sessions.
However, it is very important for many service providers to keep
track of how many sessions a given user may have open, and
accordingly disallow access.
There are several different techniques used to implement login limits
on a RADIUS environment. Some vendors have build NAS monitoring
tools either into their RADIUS servers, either directly or as
auxiliary deamons, that can check the session status of the
controlled NASes by SNMP or proprietary methods.
Other vendors monitor the RADIUS accesses and accounting messages and
derive state information from the requests. This monitoring is not
as reliable as directly auditing the NAS, but it is also less vendor
specific, and can work with any RADIUS NAS, provided it sends both
streams to the same server.
Some of the approaches used:
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- SNMP commands
- Telnet monitor deamon
- Accounting monitor
6.4. Authorization Changes:
To implement an active changes to a running session, such as filter
changes or timeout and disconnect, at least one vendor has added a
RADIUS "server" to his NAS. This server accepts messages sent from an
application in the network, and upon matching some session
information, will perform such operations.
Messages sent from Server to NAS
- Change Filter Request
- Change Filter Ack / Nak
- Disconnect Request
- Disconnect Response
Filters are used to limit the access the user has to the network by
restricting the systems and protocols he can send packets to. Upon
fulfilling some registration with an authorization server, the
service provider may wish to remove those restrictions, or disconnect
the user.
7. Policy Services
Some vendors have implemented policy servers using RADIUS as the
control protocol. Two prominent Policy Managers act as RADIUS proxy
filters and use RADIUS messages to deny access to new sessions that
exceed active policy limits.
One implementation behaves like a RADIUS proxy server, but with a
policy process governing it's forward decisions. Typically a pre-
authentication message (like the new RADIUS draft Service-Type =
CallCheck) is emitted by the NAS upon call arrival. This message will
contain only the Dialed-Number information in the Username field.
The server receives the Access-Request messages and processes them
against it's knowledge of the network state and the provisioned
policies.
An Access-Accept will be returned to the system to accept the call,
and many also contain dynamic policy information and Virtual POP
specific default parameters. When the real PPP authentication is
engaged, the proxy will forwards the Access-Request to a RADIUS
server, if this session was approved at pre-auth. It can also
process Access-Requests that were not preceded by a pre-auth
exchange, and use the Username field for information about the
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desired realm, in it's policy evaluation.
The other implementation performs a similar operations. It uses VSAs
in the Access-Request to distinguish pre-authentication message
types.
8. Accounting Extensions
Traditional Accounting only records session starts and stops which is
pretty boring. Additional session information reporting can be added
easily which gives a better picture of operation in use as they
happen. Some event types are listed below.
8.1. Auditing/Activity
- Call or Modem Starts, Stops
- Tunnel Starts, Stops
- Tunnel Link Starts & Stops
- Admin changes
These events if monitored by a stateful server can be used to gather
information about the usage of the network on a user/session basis.
Information about when a particular user entered the network is more
relevant to network service management than attempting track
backwards from low level IP address flows. Useful information about
port usage across a range of NASes allows service provider to quickly
find problem areas or users.
Information about call failures, successes, and quality are also
deemed important many service providers.
Extending RADIUS accounting is easy, it's surprising that more
implementations have not been made in this area.
9. Conclusions
In real life RADIUS Servers are becoming rather complex software
implementations. They are often brokering authentication and
authorization to other authorities or repositories. Variants of
RADIUS protocol is often used as glue protocol for these type of
solutions.
Some of the solutions are kludges that could be cleaned up by better
underlying services.
What this means to the implementor is that RADIUS as the RFCs
describe it is becoming less relevant. Many additional features
require matching client and server processing message processing.
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Without standardization of these functions we don't have much
interoperability in the field and much effort is spent in reverse
engineering and reaction to unknown areas.
This memo is not a complete survey by any means. It is a
representative summary of practices that I am aware of at the time of
writing. I still appreciate input from vendors or users on practices
and details known, and particularly any reference material you can
pass me.
10. Security Considerations
This document documents known practices, and does not propose any
particular new protocols. Extensions to RADIUS protocols create new
security implications because of their functions go beyond those
considered in the RFCs. Some of these include:
- The ability to change passwords via a RADIUS exchange was
deliberately left out of the protocol by the working group,
because of security concerns.
- The Pseudo-User profiles and the Call-Check operation may allow
unintended access if static and well-know account names and
passwords are allowed to be used by regular interactive accounts.
- Resource Management operations must be protected from denial of
service attacks.
- Client side authorization change exchanges need to be secured from
attacks that could disconnect or restrict user services.
11. Implementation Documents
Information about the following implementations can be obtained from
the respective owners. Most listed are available from the
manufacturer's web site.
11.1. Clients:
- 3Com(USR) Total Control Hub
- Ericsson(ACC) Tigris
draft-ilgun-radius-accvsa-01.txt, Dec 1998
- Lucent(Ascend) MAX TNT
- Lucent(Livingston) Portmaster
- Nortel(Aptis) CVX 1800
- Nortel(Bay Networks) Versalar 5399/8000 Remote Access Controller
- Intel(Shiva)
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11.2. Servers:
- Ericsson(ACC) Virtual Port Server Manager
- Funk Steel-Belted RADIUS
- Intel(Shiva) Access Manager
- Lucent(Ascend) Access Control
- Lucent(Ascend) NavisAccess
- Lucent(Ascend) Modified Livingston 1.16
- Lucent(Livingston) V2.01
- Lucent(Livingston) ABM
- Lucent Port Authority
- MERIT AAA Servers
- Nortel(Bay Networks) BaySecure Access Control
- Nortel Preside Radius
- Nortel CVX Policy Manager
12. References
[1] Rigney, C., Rubens, A., Simpson, W. and S. Willens, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2138, April
1997.
[2] Rigney, C., "RADIUS Accounting", RFC 2139, April 1997.
[3] Rigney, C., Willens, S., Ruebens, A. and W. Simpson, "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[4] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[5] Rigney, C., Willats, W. and P. Calhoun, "RADIUS Extensions", RFC
2869, June 2000.
[6] Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M. and
I. Goyret, "RADIUS Attributes for Tunnel Protocol Support", RFC
2868, June 2000.
[7] Zorn, G., Aboba, B. and D. Mitton, "RADIUS Accounting
Modifications for Tunnel Protocol Support", RFC 2867, June 2000.
[8] Aboba, B. and G. Zorn, "Implementation of L2TP Compulsory
Tunneling via RADIUS", RFC 2809, April 2000.
[9] Zorn, G., "Microsoft Vendor-specific RADIUS Attributes", RFC
2548, March 1999.
[10] Ilgun, K., "RADIUS Vendor Specific Attributes for ACC/Ericsson
Datacom Access", Work in Progress.
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13. Author's Address
David Mitton
Nortel Networks
880 Technology Park Drive
Billerica, MA 01821
Phone: 978-288-4570
EMail: dmitton@nortelnetworks.com
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14. Full Copyright Statement
Copyright (C) The Internet Society (2000). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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