|RFC 9445||RADIUS DHCP Options||August 2023|
|Boucadair, et al.||Standards Track||[Page]|
RADIUS Extensions for DHCP-Configured Services
This document specifies two new Remote Authentication Dial-In User Service (RADIUS) attributes that carry DHCP options. The specification is generic and can be applicable to any service that relies upon DHCP. Both DHCPv4- and DHCPv6-configured services are covered.¶
Also, this document updates RFC 4014 by relaxing a constraint on permitted RADIUS attributes in the RADIUS Attributes DHCP suboption.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
In the context of broadband services, Internet Service Providers (ISPs) usually provide DNS resolvers to their customers. To that aim, ISPs deploy dedicated mechanisms (e.g., DHCP [RFC2132] [RFC8415] and IPv6 Router Advertisement [RFC4861]) to advertise a list of DNS recursive servers to their customers. Typically, the information used to populate DHCP messages and/or IPv6 Router Advertisements relies upon specific Remote Authentication Dial-In User Service (RADIUS) [RFC2865] attributes, such as the DNS-Server-IPv6-Address Attribute specified in [RFC6911].¶
With the advent of encrypted DNS (e.g., DNS over HTTPS (DoH) [RFC8484], DNS over TLS (DoT) [RFC7858], or DNS over QUIC (DoQ) [RFC9250]), additional means are required to provision hosts with network-designated encrypted DNS. To fill that void, [DNR] leverages existing protocols such as DHCP to provide hosts with the required information to connect to an encrypted DNS resolver. However, there are no RADIUS attributes that can be used to populate the discovery messages discussed in [DNR]. The same concern is likely to be encountered for future services that are configured using DHCP.¶
This document specifies two new RADIUS attributes: DHCPv6-Options (Section 3.1) and DHCPv4-Options (Section 3.2). These attributes can include DHCP options that are listed in the "DHCPv6 Options Permitted in the RADIUS DHCPv6-Options Attribute" registry (Section 8.4.1) and the "DHCP Options Permitted in the RADIUS DHCPv4-Options Attribute" registry (Section 8.4.2). These two attributes are specified in order to accommodate both IPv4 and IPv6 deployment contexts while taking into account the constraints in Section 3.4 of [RFC6158].¶
The mechanism specified in this document is a generic mechanism and might be employed in network scenarios where the DHCP server and the RADIUS client are located in the same device. The new attributes can also be used in deployments that rely upon the mechanisms defined in [RFC4014] or [RFC7037], which allow a DHCP relay agent that is collocated with a RADIUS client to pass attributes obtained from a RADIUS server to a DHCP server. However, an update to [RFC4014] is required so that a DHCP relay agent can pass the DHCPv4-Options Attribute obtained from a RADIUS server to a DHCP server (Section 4).¶
DHCP options that are included in the new RADIUS attributes can be controlled by a deployment-specific policy. Discussing such a policy is out of scope.¶
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.¶
- refers to both DHCPv4 [RFC2132] and DHCPv6 [RFC8415].¶
- Encrypted DNS:
- refers to a scheme where DNS exchanges are transported over an encrypted channel. Examples of encrypted DNS are DoT, DoH, and DoQ.¶
- Encrypted DNS resolver:
- refers to a resolver (Section 6 of [RFC8499]) that supports encrypted DNS.¶
- refers to the DHCPv4-Options and DHCPv6-Options Attributes (Section 3).¶
This section specifies two new RADIUS attributes for RADIUS clients and servers to exchange DHCP-encoded data. This data is then used to feed the DHCP procedure between a DHCP client and a DHCP server.¶
These attributes use the "Long Extended Type" format in order to permit the transport of attributes encapsulating more than 253 octets of data. DHCP options that can be included in the RADIUS DHCP*-Options Attributes are limited by the maximum packet size of 4096 bytes (Section 3 of [RFC2865]). In order to accommodate deployments with large DHCP options, RADIUS implementations are RECOMMENDED to support a packet size up to 65535 bytes. Such a recommendation can be met if RADIUS implementations support a mechanism that relaxes the limit of 4096 bytes (e.g., the mechanisms described in [RFC7499] or [RFC7930]).¶
RADIUS implementations may support a configuration parameter to control the DHCP options that can be included in a RADIUS DHCP*-Options Attribute. Likewise, DHCP server implementations may support a configuration parameter to control the permitted DHCP options in a RADIUS DHCP*-Options Attribute. Absent explicit configuration, RADIUS implementations and DHCP server implementations SHOULD ignore non-permitted DHCP options received in a RADIUS DHCP*-Options Attribute.¶
RADIUS-supplied data is specific configuration data that is returned as a function of authentication and authorization checks. As such, absent any explicit configuration on the DHCP server, RADIUS-supplied data by means of the DHCP*-Options Attributes take precedence over any local configuration.¶
These attributes are defined with globally unique names. The naming of the attributes follows the guidelines in Section 2.7.1 of [RFC6929]. Invalid attributes are handled as per Section 2.8 of [RFC6929].¶
The DHCPv6-Options Attribute MAY appear in a RADIUS Access-Accept packet. It MAY also appear in a RADIUS Access-Request packet as a hint to the RADIUS server to indicate a preference. However, the server is not required to honor such a preference.¶
The DHCPv6-Options Attribute MAY appear in a RADIUS CoA-Request packet.¶
The DHCPv6-Options Attribute MAY appear in a RADIUS Accounting-Request packet.¶
The DHCPv6-Options Attribute MUST NOT appear in any other RADIUS packet.¶
The DHCPv6-Options Attribute is structured as follows:¶
- This field indicates the total length, in octets, of all fields of this attribute, including the Type, Length, Extended-Type, and Value fields.¶
- 3 (see Section 8.1)¶
This field contains a list of DHCPv6 options (Section 21 of [RFC8415]). Multiple instances of the same DHCPv6 option MAY be included. If an option appears multiple times, each instance is considered separate, and the data areas of the options MUST NOT be concatenated or otherwise combined. Consistent with Section 17 of [RFC7227], this document does not impose any option order when multiple options are present.¶
The DHCPv6-Options Attribute is associated with the following identifier: 245.3.¶
The DHCPv4-Options Attribute MAY appear in a RADIUS Access-Accept packet. It MAY also appear in a RADIUS Access-Request packet as a hint to the RADIUS server to indicate a preference. However, the server is not required to honor such a preference.¶
The DHCPv4-Options Attribute MAY appear in a RADIUS CoA-Request packet.¶
The DHCPv4-Options Attribute MAY appear in a RADIUS Accounting-Request packet.¶
The DHCPv4-Options Attribute MUST NOT appear in any other RADIUS packet.¶
The DHCPv4-Options Attribute is structured as follows:¶
- This field indicates the total length, in octets, of all fields of this attribute, including the Type, Length, Extended-Type, and Value fields.¶
- 4 (see Section 8.1)¶
This field contains a list of DHCPv4 options. Multiple instances of the same DHCPv4 option MAY be included, especially for concatenation-requiring options that exceed the maximum DHCPv4 option size of 255 octets. The mechanism specified in [RFC3396] MUST be used for splitting and concatenating the instances of a concatenation-requiring option.¶
The DHCPv4-Options Attribute is associated with the following identifier: 245.4.¶
The RADIUS Attributes DHCP suboption [RFC4014] enables a DHCPv4 relay agent to pass identification and authorization attributes received during RADIUS authentication to a DHCPv4 server. However, [RFC4014] defines a frozen set of RADIUS attributes that can be included in such a suboption. This limitation is suboptimal in contexts where new services are deployed (e.g., support of encrypted DNS [DNR]).¶
Section 4.2 updates [RFC4014] by relaxing that constraint and allowing additional RADIUS attributes to be tagged as permitted in the RADIUS Attributes DHCP suboption. The permitted attributes are registered in the new "RADIUS Attributes Permitted in RADIUS Attributes DHCP Suboption" registry (Section 8.3).¶
To avoid dependencies between the address allocation and other state information between the RADIUS server and the DHCP server, the DHCP relay agent SHOULD include only the attributes in the table below in an instance of the RADIUS Attributes suboption. The table, based on the analysis in RFC 3580 , lists attributes that MAY be included:¶# Attribute --- --------- 1 User-Name (RFC 2865 ) 6 Service-Type (RFC 2865) 26 Vendor-Specific (RFC 2865) 27 Session-Timeout (RFC 2865) 88 Framed-Pool (RFC 2869) 100 Framed-IPv6-Pool (RFC 3162 )¶
To avoid dependencies between the address allocation and other state information between the RADIUS server and the DHCP server, the DHCP relay agent SHOULD only include the attributes in the "RADIUS Attributes Permitted in RADIUS Attributes DHCP Suboption" registry (Section 8.3 of [RFC9445]) in an instance of the RADIUS Attributes DHCP suboption. The DHCP relay agent may support a configuration parameter to control the attributes in a RADIUS Attributes DHCP suboption.¶
If the relay agent relays RADIUS attributes not included in the table in Section 4, the DHCP server SHOULD ignore them.¶
If the relay agent relays RADIUS attributes not included in the "RADIUS Attributes Permitted in RADIUS Attributes DHCP Suboption" registry (Section 8.3 of [RFC9445]) and explicit configuration is absent, the DHCP server SHOULD ignore them.¶
Typical deployment scenarios are similar to those described, for instance, in Section 2 of [RFC6911]. For illustration purposes, Figure 1 shows an example where a Customer Premises Equipment (CPE) is provided with an encrypted DNS resolver. This example assumes that the Network Access Server (NAS) embeds both RADIUS client and DHCPv6 server capabilities.¶
Upon receipt of the DHCPv6 Solicit message from a CPE, the NAS sends a RADIUS Access-Request message to the Authentication, Authorization, and Accounting (AAA) server. Once the AAA server receives the request, it replies with an Access-Accept message (possibly after having sent a RADIUS Access-Challenge message and assuming the CPE is entitled to connect to the network) that carries a list of parameters to be used for this session, which includes the encrypted DNS information. Such information is encoded as OPTION_V6_DNR (144) instances [DNR] in the RADIUS DHCPv6-Options Attribute. These instances are then used by the NAS to complete the DHCPv6 procedure that the CPE initiated to retrieve information about the encrypted DNS service to use. The Discovery of Network-designated Resolvers (DNR) procedure defined in [DNR] is then followed between the DHCPv6 client and the DHCPv6 server.¶
Should any encrypted DNS-related information (e.g., Authentication Domain Name (ADN) and IPv6 address) change, the RADIUS server sends a RADIUS Change-of-Authorization (CoA) message [RFC5176] that carries the DHCPv6-Options Attribute with the updated OPTION_V6_DNR information to the NAS. Once that message is received and validated by the NAS, it replies with a RADIUS CoA ACK message. The NAS replaces the old encrypted DNS resolver information with the new one and sends a DHCPv6 Reconfigure message, which leads the DHCPv6 client to initiate a Renew/Reply message exchange with the DHCPv6 server.¶
In deployments where the NAS behaves as a DHCPv6 relay agent, the procedure discussed in Section 3 of [RFC7037] can be followed. To that aim, the "RADIUS Attributes Permitted in DHCPv6 RADIUS Option" registry has been updated (Section 8.2). CoA-Requests can be used following the procedure specified in [RFC6977].¶
Other deployment scenarios can be envisaged, such as returning customized service parameters (e.g., different DoH URI Templates) as a function of the service, policies, and preferences that are set by a network administrator. How an administrator indicates its service, policies, and preferences to a AAA server is out of scope.¶
DHCPv6-related security issues are discussed in Section 22 of [RFC8415], while DHCPv4-related security issues are discussed in Section 7 of [RFC2131]. Security considerations specific to the DHCP options that are carried in RADIUS are discussed in relevant documents that specify these options. For example, security considerations (including traffic theft) are discussed in Section 7 of [DNR].¶
RADIUS servers have conventionally tolerated the input of arbitrary data via the "string" data type (Section 3.5 of [RFC8044]). This practice allows RADIUS servers to support newer standards without software upgrades, by allowing administrators to manually create complex attribute content and then pass that content to a RADIUS server as opaque strings. While this practice is useful, it is RECOMMENDED that RADIUS servers that implement the present specification are updated to understand the format and encoding of DHCP options. Administrators can thus enter the DHCP options as options instead of manually encoded opaque strings. This recommendation increases security and interoperability by ensuring that the options are encoded correctly. It also increases usability for administrators.¶
The considerations discussed in Section 7 of [RFC4014] and Section 8 of [RFC7037] should be taken into account in deployments where DHCP relay agents pass the DHCP*-Options Attributes to DHCP servers. Additional considerations specific to the use of Reconfigure messages are discussed in Section 9 of [RFC6977].¶
The following table provides a guide as to what type of RADIUS packets may contain these attributes and in what quantity.¶
IANA has created a new subregistry entitled "RADIUS Attributes Permitted in RADIUS Attributes DHCP Suboption" in the "Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) Parameters" registry [BOOTP].¶
The allocation policy of this new subregistry is "Expert Review" (Section 4.5 of [RFC8126]). Designated experts should carefully consider the security implications of allowing a relay agent to include new RADIUS attributes in this subregistry. Additional considerations are provided in Section 8.4.3.¶
The initial content of this subregistry is listed in Table 5. The Value and Description fields echo those in the "Option Codes" subregistry of [DHCPv6]. The Reference field includes the document that registers or specifies the option.¶
IANA has created a new subregistry entitled "DHCP Options Permitted in the RADIUS DHCPv4-Options Attribute" in the "Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) Parameters" registry [BOOTP].¶
The initial content of this subregistry is listed in Table 6. The Tag and Name fields echo those in the "BOOTP Vendor Extensions and DHCP Options" subregistry of [BOOTP]. The Reference field includes the document that registers or specifies the option.¶
It is suggested that multiple designated experts be appointed for registry change requests.¶
Criteria that should be applied by the designated experts include determining whether the proposed registration duplicates existing entries and whether the registration description is clear and fits the purpose of this registry.¶
Registration requests are to be sent to <email@example.com> and are evaluated within a three-week review period on the advice of one or more designated experts. Within the review period, the designated experts will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful.¶
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
- Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)", RFC 2865, DOI 10.17487/RFC2865, , <https://www.rfc-editor.org/info/rfc2865>.
- Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396, DOI 10.17487/RFC3396, , <https://www.rfc-editor.org/info/rfc3396>.
- Droms, R. and J. Schnizlein, "Remote Authentication Dial-In User Service (RADIUS) Attributes Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Information Option", RFC 4014, DOI 10.17487/RFC4014, , <https://www.rfc-editor.org/info/rfc4014>.
- DeKok, A., Ed. and G. Weber, "RADIUS Design Guidelines", BCP 158, RFC 6158, DOI 10.17487/RFC6158, , <https://www.rfc-editor.org/info/rfc6158>.
- DeKok, A. and A. Lior, "Remote Authentication Dial In User Service (RADIUS) Protocol Extensions", RFC 6929, DOI 10.17487/RFC6929, , <https://www.rfc-editor.org/info/rfc6929>.
- DeKok, A., "Data Types in RADIUS", RFC 8044, DOI 10.17487/RFC8044, , <https://www.rfc-editor.org/info/rfc8044>.
- Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <https://www.rfc-editor.org/info/rfc8126>.
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
- Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., Richardson, M., Jiang, S., Lemon, T., and T. Winters, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 8415, DOI 10.17487/RFC8415, , <https://www.rfc-editor.org/info/rfc8415>.
- IANA, "Dynamic Host Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP) Parameters", <https://www.iana.org/assignments/bootp-dhcp-parameters>.
- IANA, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", <https://www.iana.org/assignments/dhcpv6-parameters>.
- Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N., and T. Jensen, "DHCP and Router Advertisement Options for the Discovery of Network-designated Resolvers (DNR)", Work in Progress, Internet-Draft, draft-ietf-add-dnr-16, , <https://datatracker.ietf.org/doc/html/draft-ietf-add-dnr-16>.
- IANA, "RADIUS Types", <http://www.iana.org/assignments/radius-types>.
- Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, DOI 10.17487/RFC2131, , <https://www.rfc-editor.org/info/rfc2131>.
- Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, DOI 10.17487/RFC2132, , <https://www.rfc-editor.org/info/rfc2132>.
- Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege, M., and I. Goyret, "RADIUS Attributes for Tunnel Protocol Support", RFC 2868, DOI 10.17487/RFC2868, , <https://www.rfc-editor.org/info/rfc2868>.
- Rigney, C., Willats, W., and P. Calhoun, "RADIUS Extensions", RFC 2869, DOI 10.17487/RFC2869, , <https://www.rfc-editor.org/info/rfc2869>.
- Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6", RFC 3162, DOI 10.17487/RFC3162, , <https://www.rfc-editor.org/info/rfc3162>.
- Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, , <https://www.rfc-editor.org/info/rfc4861>.
- Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B. Aboba, "Dynamic Authorization Extensions to Remote Authentication Dial In User Service (RADIUS)", RFC 5176, DOI 10.17487/RFC5176, , <https://www.rfc-editor.org/info/rfc5176>.
- Dec, W., Ed., Sarikaya, B., Zorn, G., Ed., Miles, D., and B. Lourdelet, "RADIUS Attributes for IPv6 Access Networks", RFC 6911, DOI 10.17487/RFC6911, , <https://www.rfc-editor.org/info/rfc6911>.
- Boucadair, M. and X. Pougnard, "Triggering DHCPv6 Reconfiguration from Relay Agents", RFC 6977, DOI 10.17487/RFC6977, , <https://www.rfc-editor.org/info/rfc6977>.
- Yeh, L. and M. Boucadair, "RADIUS Option for the DHCPv6 Relay Agent", RFC 7037, DOI 10.17487/RFC7037, , <https://www.rfc-editor.org/info/rfc7037>.
- Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and S. Krishnan, "Guidelines for Creating New DHCPv6 Options", BCP 187, RFC 7227, DOI 10.17487/RFC7227, , <https://www.rfc-editor.org/info/rfc7227>.
- Perez-Mendez, A., Ed., Marin-Lopez, R., Pereniguez-Garcia, F., Lopez-Millan, G., Lopez, D., and A. DeKok, "Support of Fragmentation of RADIUS Packets", RFC 7499, DOI 10.17487/RFC7499, , <https://www.rfc-editor.org/info/rfc7499>.
- Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., and P. Hoffman, "Specification for DNS over Transport Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, , <https://www.rfc-editor.org/info/rfc7858>.
- Hartman, S., "Larger Packets for RADIUS over TCP", RFC 7930, DOI 10.17487/RFC7930, , <https://www.rfc-editor.org/info/rfc7930>.
- Hoffman, P. and P. McManus, "DNS Queries over HTTPS (DoH)", RFC 8484, DOI 10.17487/RFC8484, , <https://www.rfc-editor.org/info/rfc8484>.
- Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, , <https://www.rfc-editor.org/info/rfc8499>.
- Huitema, C., Dickinson, S., and A. Mankin, "DNS over Dedicated QUIC Connections", RFC 9250, DOI 10.17487/RFC9250, , <https://www.rfc-editor.org/info/rfc9250>.
Thanks to Christian Jacquenet, Neil Cook, Joe Clarke, Qin Wu, Dirk von-Hugo, Tom Petch, and Chongfeng Xie for the review and suggestions.¶
Thanks to Ben Schwartz and Bernie Volz for the comments.¶
Thanks to Rob Wilton for the careful AD review.¶
Thanks to Ralf Weber for the dnsdir reviews, Robert Sparks for the genart review, and Tatuya Jinmei for the intdir review.¶
Thanks to Éric Vyncke, Paul Wouters, and Warren Kumari for the IESG review.¶