RFC 4849

RADIUS Filter Rule Attribute

📅 published April 2007, from ietf

RADIUS Filter Rule Attribute 🎀

Part 1 of 2 🦋

okay bestie, this is RFC 4849 📜 and it's giving main character spec energy ✨ It came out in April 2007 from the Network Working Group. The authors are P. Congdon and M. Sanchez (both at ProCurve Networking by HP) and B. Aboba (at Microsoft Corporation). It's a Standards Track document, and it doesn't list any RFCs it obsoletes or updates 💅

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and asks for discussion and suggestions for improvements. For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited 🌸

Copyright Notice

Abstract

okay so here's the tea 🍓 RFC 2865 defines the Filter-Id attribute, but it needs the Network Access Server (NAS) to be pre-populated with the filters you want. If the server operator doesn't know which filters are already pre-populated, it's super useful to spell the filter rules out explicitly. This document defines the NAS-Filter-Rule attribute inside the Remote Authentication Dial In User Service (RADIUS) 💖 It's based on the Diameter NAS-Filter-Rule Attribute Value Pair (AVP) from RFC 4005, and on the IPFilterRule syntax from RFC 3588 ✨

   1. Introduction ....................................................2
      1.1. Terminology ................................................2
      1.2. Requirements Language ......................................3
      1.3. Attribute Interpretation ...................................3
   2. NAS-Filter-Rule Attribute .......................................3
   3. Table of Attributes .............................................5
   4. Diameter Considerations .........................................5
   5. IANA Considerations .............................................6
   6. Security Considerations .........................................6
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................7
   8. Acknowledgments .................................................7

1. Introduction 🌈

this doc defines the NAS-Filter-Rule attribute inside RADIUS 🧚 It has the same functionality as the Diameter NAS-Filter-Rule AVP (400) from [RFC4005], Section 6.6, and the same syntax as an IPFilterRule from [RFC3588], Section 4.3. This attribute may prove useful for provisioning filter rules 🫶

Like, [RFC2865], Section 5.11, defines the Filter-Id attribute (11), but it needs the Network Access Server (NAS) to be pre-populated with the desired filters. When the server operator doesn't know which filters have been pre-populated, it's useful to specify filter rules explicitly 💅

1.1. Terminology

this document uses these terms, bestie 📖

Network Access Server (NAS)

A device that provides an access service for a user to a network. ✨

RADIUS server

A RADIUS authentication server is an entity that provides an authentication service to a NAS. 🦋

RADIUS proxy

A RADIUS proxy acts as an authentication server to the NAS, and a RADIUS client to the RADIUS server. 🌸

1.2. Requirements Language

okay this part is the key-word rulebook 🔑 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 🎀

1.3. Attribute Interpretation

okay this section is where it gets serious 😭 If a NAS that follows this spec gets an Access-Accept packet with a NAS-Filter-Rule attribute it can't apply, it MUST act as though it had received an Access-Reject 💀

[RFC3576] says that a NAS receiving a Change of Authorization Request (CoA-Request) has to reply with a CoA-NAK if the Request contains an unsupported attribute. It is RECOMMENDED that an Error-Cause attribute with value set to "Unsupported Attribute" (401) be included in the CoA-NAK 🙈

As noted in [RFC3576], authorization changes are atomic. So this situation does not end the session, and the pre-existing configuration stays unchanged 🫶 Because of that, no accounting packets should be generated because of the CoA-Request 💜

2. NAS-Filter-Rule Attribute 🧚

Description

this attribute carries the filter rules to apply for this user 💖 Zero or more NAS-Filter-Rule attributes MAY be sent in Access-Accept, CoA-Request, or Accounting-Request packets ✨

The NAS-Filter-Rule attribute isn't meant to be used at the same time as any other filter rule attribute, including Filter-Id (11) and NAS-Traffic-Rule [Traffic] attributes 🙅‍♀️ NAS-Filter-Rule and NAS-Traffic-Rule attributes MUST NOT appear in the same RADIUS packet. If a NAS-Traffic-Rule attribute is present, a NAS implementing this specification MUST silently discard any NAS-Filter-Rule attributes that are present 😭 Filter-Id and NAS-Filter-Rule attributes SHOULD NOT appear in the same RADIUS packet.

Here's the messy part 🙃 RFC 4005 has no well-defined precedence rules for combining Filter-Id and NAS-Filter-Rule attributes into a single rule set. So the behavior of NASes that receive both attributes is undefined, and a RADIUS server implementation therefore cannot assume a consistent behavior 💀

When a RADIUS packet includes multiple NAS-Filter-Rule attributes, the String fields of those attributes get concatenated to form one set of filter rules 🌈 As noted in [RFC2865], Section 2.3, "the forwarding server MUST NOT change the order of any attributes of the same type", so RADIUS proxies won't reorder NAS-Filter-Rule attributes 🦋

Here's the format summary. The fields are sent from left to right 👑

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |      String...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

92 🍓

Length

=3 🌸

String

The String field is one or more octets. It holds filter rules in the IPFilterRule syntax from [RFC3588], Section 4.3, with individual filter rules separated by a NUL (0x00) 🧁 A NAS-Filter-Rule attribute may contain a partial rule, one rule, or more than one rule. Filter rules may continue across attribute boundaries, so implementations cannot assume that individual filter rules begin or end on attribute boundaries 🙈

The set of NAS-Filter-Rule attributes SHOULD be created by concatenating the individual filter rules, separated by a NUL (0x00) octet. The resulting data should be split on 253-octet boundaries to get a set of NAS-Filter-Rule attributes ✨ On reception, the individual filter rules are determined by concatenating the contents of all NAS-Filter-Rule attributes, then splitting the individual filter rules on the NUL octet (0x00) as a delimiter 💖

Part 2 is coming up next, covering Section 3 onward 🦋

6. Security Considerations 🔐

Okay so bestie, this section is SERIOUS but also kind of iconic. 💅 RADIUS is used for authentication, authorization and accounting, and the threats and security issues for that are described in [RFC3579] and [RFC3580]. Roaming security stuff lives in [RFC2607]. Go read those if you want the full tea. 🍵

This spec adds a new attribute that can ride along in existing RADIUS packets, and those packets are protected the way [RFC3579] and [RFC3576] describe. ✨ Those docs go way deeper, so check them out, periodt. 🦋

Here's the big-brain part 🧠: RADIUS and Diameter security is all about stopping an attacker from spoofing packets or messing with them in transit. 🛡️ BUT it does NOT stop an authorized RADIUS/Diameter server or proxy from modifying, inserting, or removing attributes on purpose. 😭 Like, a proxy could strip out filter attributes, and then a user might get network access without the right filters. Yikes 💀. And if a proxy also tampered with accounting packets, that change wouldn't show up in the accounting server logs. Not me being dramatic, it's literally a real risk. 🙈

Another lowkey scary thing: RADIUS doesn't support capability negotiation yet, so a RADIUS server can't automatically figure out whether a NAS supports the NAS-Filter-Rule attribute. 🤷‍♀️ A legacy NAS that doesn't follow this spec might silently throw the NAS-Filter-Rule attribute away while still letting the user onto the network. Result? Users can end up getting unfiltered access they shouldn't have. 😱 So the bottom line is that the NAS-Filter-Rule attribute SHOULD only be sent to a NAS that is known to support it. 👑 That's the rule, bestie. 🫶

7. References 📚

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March, 1997.

[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
          "Remote Authentication Dial In User Service (RADIUS)", RFC
          2865, June 2000.

[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
          Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

[RFC4005] Calhoun, P., Zorn, G., Spence, D., and D. Mitton, "Diameter
          Network Access Server Application", RFC 4005, August 2005.

7.2. Informative References

[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
          Implementation in Roaming", RFC 2607, June 1999.

[RFC3576] Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
          Aboba, "Dynamic Authorization Extensions to Remote
          Authentication Dial In User Service (RADIUS)", RFC 3576,
          July 2003.

[RFC3579] Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
          Dial In User Service) Support For Extensible Authentication
          Protocol (EAP)", RFC 3579, September 2003.

[RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese,
          "IEEE 802.1X Remote Authentication Dial In User Service
          (RADIUS) Usage Guidelines", RFC 3580, September 2003.

[Traffic] Congdon, P., Sanchez, M., Lior, A., Adrangi, F., and B.
          Aboba, "RADIUS Attributes for Filtering and Redirection",
          Work in Progress, March 2007.

8. Acknowledgments 🫶

The authors want to give a shoutout to Emile Bergen, Alan DeKok, Greg Weber, Glen Zorn, Pasi Eronen, David Mitton, and David Nelson for helping out with this document. 💖 Thank you, queens. 👑

Authors' Addresses 📮

Paul Congdon
Hewlett Packard Company
ProCurve Networking by HP
8000 Foothills Blvd, M/S 5662
Roseville, CA  95747

EMail: paul.congdon@hp.com
Phone: +1 916 785 5753
Fax:   +1 916 785 8478


Mauricio Sanchez
Hewlett Packard Company
ProCurve Networking by HP
8000 Foothills Blvd, M/S 5559
Roseville, CA  95747

EMail: mauricio.sanchez@hp.com
Phone: +1 916 785 1910
Fax:   +1 916 785 1815


Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052

EMail: bernarda@microsoft.com
Phone: +1 425 706 6605
Fax:   +1 425 936 7329

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