Reverse CoA in RADIUS

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Table of Contents

1. Introduction

[RFC5176] defines the ability to change a users authorization, or disconnect the user via what are generally called "Change of Authorization" or "CoA" packets. This term refers to either of the RADIUS packet types CoA-Request or Disconnect-Request. The initial transport protocol for all RADIUS was the User Datagram Protocol (UDP).¶

[RFC6614] updated previous specifications to allow packets to be sent over the Transport Layer Security (TLS) protocol. Section 2.5 of that document explicitly allows all packets (including CoA) to be sent over a TLS connection:¶

Due to the use of one single TCP port for all packet types, it is required that a RADIUS/TLS server signal which types of packets are supported on a server to a connecting peer. See also Section 3.4 for a discussion of signaling. ¶

These specifications assume that a RADIUS client can directly contact a RADIUS server, which is the normal "forward" path for packets between a client and server. However, it is not always possible for the RADIUS server to send CoA packets to the RADIUS client. If a RADIUS server wishes to act as a CoA client, and send CoA packets to the NAS (CoA server), the "reverse" path can be blocked by a firewall, NAT gateway, etc. That is, a RADIUS server has to be reachable by a NAS, but there is usually no requirement that the NAS is reachable from a public system. To the contrary, there is usually a requirement that the NAS is not publicly accessible.¶

This scenario is most evident in a roaming / federated environment such as Eduroam or OpenRoaming. It is in general impossible for a home server to signal the NAS to disconnect a user. There is no direct reverse path from the home server to the NAS, as the NAS is not publicly addressible. Even if there was a public reverse path, it would generally be unknowable, as intermediate proxies can (and do) attribute rewriting to hide NAS identies.¶

These limitations can result in business losses and security problems, such as the inability to disconnect an online user when their account has been terminated.¶

As the reverse path is usally blocked, it means that it is in general possible only to send CoA packets to a NAS when the NAS and RADIUS server share the same private network (private IP space or IPSec). Even though [RFC8559] defines CoA proxying, that specification does not address the issue of NAS reachability.¶

This specification solves that problem. The solution is to simply allow CoA packets to go in "reverse" down an existing RADIUS/TLS connection. That is, when a NAS connects to a RADIUS server it normally sends request packets (Access-Request, etc.) and expects to receive response packets (Access-Accept, etc.). This specification extends RADIUS/TLS by permitting a RADIUS server to re-use an existing TLS connection to send CoA packets to the NAS, and permitting the NAS to send CoA response packets to the RADIUS server over that same connection.¶

We note that while this document specifically mentions RADIUS/TLS, it should be possible to use the same mechanisms on RADIUS/DTLS [RFC7360]. However at the time of writing this specification, no implementations exist for "reverse CoA" over RADIUS/DTLS. As such, when we refer to "TLS" here, or "RADIUS/TLS", we implicitly include RADIUS/DTLS in that description.¶

We also note that while this same mechanism could theoretically be used for RADIUS/UDP and RADIUS/TCP, there is no value in defining "reverse CoA" for those transports. Therefore for practial purposes, "reverse CoA" means RADIUS/TLS and RADIUS/DTLS.¶

There are additional considerations for proxies. While [RFC8559] describes CoA proxying, there are still issues which need to be addressed for the "reverse CoA" use-case. This specification describes how a proxy can implement "reverse CoA" proxying, including signalling necessary to negotiate this functionality.¶

2. Terminology

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.¶

• CoA¶

• Change of Authorization packets. For brevity, when this document refers to "CoA" packets, it means either or both of CoA-Request and Disconnect-Request packets.¶

• ACK¶

• Change of Authorization "positive acknowlegement" packets. For brevity, when this document refers to "ACK" packets, it means either or both of CoA-ACK and Disconnect-ACK packets.¶

• NAK¶

• Change of Authorization "negative acknowlegement" packets. For brevity, when this document refers to "ACK" packets, it means either or both of CoA-NAK and Disconnect-NAK packets.¶

• RADIUS/TLS¶

• RADIUS over the Transport Layer Security protocol [RFC6614]¶

• RADIUS/DTLS¶

• RADIUS over the Datagram Transport Layer Security protocol [RFC7360]¶

• TLS¶

• Either RADIUS/TLS or RADIUS/DTLS.¶

• reverse CoA¶

• CoA, ACK, or NAK packets sent over a RADIUS/TLS or RADIUS/DTLS connection which was made from a RADIUS client to a RADIUS server.¶

3. Concepts

The reverse CoA functionality is based on two additions to RADIUS. The first addition is a configuration and signalling, to indicate that a RADIUS client is capable of accepting reverse CoA packets. The second addition is an extension to the "reverse" routing table for CoA packets which was first described in Section 2.1 of [RFC8559].¶

4. Capability Configuration and Signalling

In order for a RADIUS server to send reverse CoA packets to a client, it must first know that the client is capable of accepting these packets.¶

This functionality can be enabled in one of two ways. The first is a simple static configuration between client and server, where both are configured to allow reverse CoA. The second method is via per-connection signalling between client and server.¶

The server manages this functionality with two boolean flags, one per-client, and one per-connection. The per-client flag can be statically configured, and if not present MUST be treated as having a "false" value. The per-connection flag MUST be initialized from the per-client flag, and then can be dynamically negotiated after that.¶

5. Reverse Routing

The "reverse" routing table for CoA packets was first described in Section 2.1 of [RFC8559]. We extend that table here.¶

In our extension, the table does not map realms to home servers. Instead, it maps keys to connections. The keys will be defined in more detail below. For now, we say that keys can be derived from a RADIUS client to server connection, and from the contents of a CoA packet which needs to be routed.¶

When the server recieves a TLS connection from a client, it derives a key for that connection, and associates the connection with that key. A server MUST support associating one particular key value with multiple connections. A server MUST support associating multiple keys for one connection. That is, the "key to connection" mapping is N to M. It is not one-to-one, or 1-N, or M-1.¶

When the server recieves a CoA packet, it derives a key from that packet, and determines if there is a connection or connections which maps to that key. Where there is no available connection, the server MUST return a NAK packet that contains an Error-Cause Attribute having value 502 ("Request Not Routable").¶

As with normal proxying, a particular packet can sometimes have the choice more than one connection which can be used to reach a destination. In that case, issues of load-balancing, fail-over, etc. are implementation-defined, and are not discussed here. The server simply chooses one connection, and sends the reverse CoA packet down that connection.¶

The server then waits for a reply, doing retransmission if necessary. For all issues other than the connection being used, reverse CoA packets are handled as defined in [RFC5176] and in [RFC8559].¶

That is, when the NAS and server are known to each other, [RFC5176] is followed when sending CoA packets to the NAS. The difference is that instead of originating connections to the NAS, the server simply re-uses inbound TLS connections from the NAS. The NAS is identified by attributes such as NAS-Identifier, NAS-IP-Address, and NAS-IPv6-Address.¶

When a server is proxying to another server, [RFC8559] is following when proxying CoA packets. The "next hop" is identified either by Operator-Name for proxy-to-proxy connections. When the CoA packet reaches a visited network, that network identifies the NAS by examining the Operator-NAS-Identifier attribute.¶

6. Implementation Status

FreeRADIUS supports CoA proxying using Vendor-Specific attributes. It also permits RADIUS clients to send Status-Server packets over a RADIUS/TLS connection which contain Operator-Name. This information is used to determne which realms are accessible via reverse CoA over which RADIUS/TLS connection.¶

Cisco supports reverse CoA as of Cisco IOS XE Bengaluru 17.6.1 via Vendor-Specific attributes. https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/17-6/configuration_guide/sec/b_176_sec_9300_cg/configuring_radsec.pdf¶

Aruba documentation states that "Instant supports dynamic CoA (RFC 3576) over RadSec and the RADIUS server uses an existing TLS connection opened by the Instant AP to send the request." https://www.arubanetworks.com/techdocs/Instant_83_WebHelp/Content/Instant_UG/Authentication/ConfiguringRadSec.htm¶

7. Privacy Considerations

This document does not change or add any privacy considerations over previous RADIUS specifications.¶

8. Security Considerations

This document increases network security by removing the requirement for non-standard "reverse" paths for CoA-Request and Disconnect-Request packets.¶

9. IANA Considerations

TBD - new RADIUS attribute - Capability¶

User Operator Namespace Identifier namespace.¶

+,Realm Add,(this document) -,Realm Delete,(this document) ¶

10. Acknowledgements

Thanks to Heikki Vatiainen for testing a preliminary implementation in Radiator, and for verifying interoperability with NAS equipment.¶

11. Changelog

Authors' Addresses