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Introduction In many environments, users need to connect to a captive portal device and agree to an Acceptable Use Policy (AUP) and/or provide billing information before they can access the Internet. Regardless of how that mechanism operates, this document provides functionality to allow the client to know when it is behind a captive portal and how to contact it. In order to present users with the payment or AUP pages, a captive portal enforcement device presently has to intercept the user's connections and redirect the user to a captive portal server, using methods that are very similar to man-in-the-middle (MITM) attacks. As increasing focus is placed on security, and end nodes adopt a more secure stance, these interception techniques will become less effective and/or more intrusive. This document describes a DHCPv4 and DHCPv6 option (Captive-Portal) and an IPv6 Router Advertisement (RA) option that informs clients that they are behind a captive portal enforcement device and the API endpoint that the host can contact for more information. This document replaces RFC 7710 , which used DHCP code point 160. Due to a conflict, this document specifies 114. Consequently, this document also updates . Requirements Notation 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 when, and only when, they appear in all capitals, as shown here. The Captive-Portal Option The Captive-Portal DHCP/RA Option informs the client that it may be behind a captive portal and provides the URI to access an API as defined by . This is primarily intended to improve the user experience by showing the user the captive portal information faster and more reliably. Note that, for the foreseeable future, captive portals will still need to implement interception techniques to serve legacy clients, and clients will need to perform probing to detect captive portals; nonetheless, the mechanism provided by this document provides a more reliable and performant way to do so, and is therefore the preferred mechanism for captive portal detection. Clients that support the Captive Portal DHCP option SHOULD include the option in the Parameter Request List in DHCPREQUEST messages. DHCP servers MAY send the Captive Portal option without any explicit request. In order to support multiple "classes" of clients (e.g., IPv4 only, IPv6 only with DHCPv6 ( ), and IPv6 only with RA), the captive network can provision the client with the URI via multiple methods (IPv4 DHCP, IPv6 DHCP, and IPv6 RA). The captive portal operator SHOULD ensure that the URIs provisioned by each method are identical to reduce the chance of operational problems. As the maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, URIs longer than this SHOULD NOT be provisioned by any of the IPv6 options described in this document. In IPv6-only environments, this restriction can be relaxed. In all variants of this option, the URI MUST be that of the captive portal API endpoint ( ). A captive portal MAY do content negotiation ( ) and attempt to redirect clients querying without an explicit indication of support for the captive portal API content type (i.e., without application/capport+json listed explicitly anywhere within an Accept header field as described in ). In so doing, the captive portal SHOULD redirect the client to the value associated with the "user-portal-url" API key. When performing such content negotiation ( ), implementors of captive portals need to keep in mind that such responses might be cached, and therefore SHOULD include an appropriate Vary header field ( ) or set the Cache-Control header field in any responses to "private" or a more restrictive value such as "no-store" ( ). The URI SHOULD NOT contain an IP address literal. Exceptions to this might include networks with only one operational IP address family where DNS is either not available or not fully functional until the captive portal has been satisfied. Use of IP Address certificates ( ) adds considerations that are out of scope for this document. Networks with no captive portals may explicitly indicate this condition by using this option with the IANA-assigned URI for this purpose. Clients observing the URI value "urn:ietf:params:capport:unrestricted" may forego time-consuming forms of captive portal detection. IPv4 DHCP Option The format of the IPv4 Captive-Portal DHCP option is shown below. Captive-Portal DHCPv4 Option Format 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Code | Len | URI (variable length) ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . ...URI continued... . | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ See for more on the format of IPv4 DHCP options. Note that the URI parameter is not null terminated. IPv6 DHCP Option The format of the IPv6 Captive-Portal DHCP option is shown below. Captive-Portal DHCPv6 Option Format 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | option-code | option-len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . URI (variable length) . | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ See for more examples of DHCP Options with URIs. See for more on the format of IPv6 DHCP options. Note that the URI parameter is not null terminated. As the maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, URIs longer than this SHOULD NOT be provisioned via IPv6 DHCP options. The Captive-Portal IPv6 RA Option This section describes the Captive-Portal Router Advertisement option. Captive-Portal RA Option Format 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 | URI . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Note that the URI parameter is not guaranteed to be null terminated. As the maximum length of the URI that can be carried in IPv4 DHCP is 255 bytes, URIs longer than this SHOULD NOT be provisioned via IPv6 RA options. Precedence of API URIs A device may learn about Captive Portal API URIs through more than one of (or indeed all of) the above options. Implementations can select their own precedence order (e.g., prefer one of the IPv6 options before the DHCPv4 option, or vice versa, et cetera). If the URIs learned via more than one option described in are not all identical, this condition should be logged for the device owner or administrator; it is a network configuration error if the learned URIs are not all identical. IANA Considerations IANA has registered a new IETF URN protocol parameter ( ). IANA has also reallocated two DHCPv4 option codes (see for background) and updated the references for previously registered DHCPv6 and IPv6 ND options. Captive Portal Unrestricted Identifier IANA has registered a new entry in the "IETF URN Sub-namespace for Registered Protocol Parameter Identifiers" registry defined in :
Registered Parameter Identifier:
capport:unrestricted
Reference:
RFC 8910
IANA Registry Reference:
RFC 8910
Only one value is defined (see URN above). No hierarchy is defined and, therefore, no sub-namespace registrations are possible. BOOTP Vendor Extensions and DHCP Options Code Change IANA has updated the "BOOTP Vendor Extensions and DHCP Options" registry ( ) as follows.
Tag:
114
Name:
DHCP Captive-Portal
Data Length:
N
Meaning:
DHCP Captive-Portal
Reference:
RFC 8910
Tag:
160
Name:
Unassigned
Data Length:
Meaning:
Previously assigned by ; known to also be used by Polycom.
Reference:
RFC 8910
Update DHCPv6 and IPv6 ND Options Registries IANA has updated the DHCPv6 (103 - DHCP Captive-Portal) and IPv6 ND (37 - DHCP Captive-Portal) options previously registered in to reference this document. Security Considerations By removing or reducing the need for captive portals to perform MITM hijacking, this mechanism improves security by making the portal and its actions visible, rather than hidden, and reduces the likelihood that users will disable useful security safeguards like DNSSEC validation, VPNs, etc. in order to interact with the captive portal. In addition, because the system knows that it is behind a captive portal, it can know not to send cookies, credentials, etc. By handing out a URI that is protected with TLS, the captive portal operator can attempt to reassure the user that the captive portal is not malicious. Clients processing these options SHOULD validate that the option's contents conform to the validation requirements for URIs, including those described in . Each of the options described in this document is presented to a node using the same protocols used to provision other information critical to the node's successful configuration on a network. The security considerations applicable to each of these provisioning mechanisms also apply when the node is attempting to learn the information conveyed in these options. In the absence of security measures like RA-Guard ( , ) or DHCPv6-Shield , an attacker could inject, modify, or block DHCP messages or RAs. An attacker with the ability to inject DHCP messages or RAs could include an option from this document to force users to contact an address of the attacker's choosing. An attacker with this capability could simply list themselves as the default gateway (and so intercept all the victim's traffic); this does not provide them with significantly more capabilities, but because this document removes the need for interception, the attacker may have an easier time performing the attack. However, as the operating systems and application(s) that make use of this information know that they are connecting to a captive portal device (as opposed to intercepted connections where the OS/application may not know that they are connecting to a captive portal or hostile device), they can render the page in a sandboxed environment and take other precautions such as clearly labeling the page as untrusted. The means of sandboxing and a user interface presenting this information is not covered in this document; by its nature, it is implementation specific and best left to the application and user interface designers. Devices and systems that automatically connect to an open network could potentially be tracked using the techniques described in this document (forcing the user to continually resatisfy the Captive Portal conditions or exposing their browser fingerprint). However, similar tracking can already be performed with the presently common captive portal mechanisms, so this technique does not give the attackers more capabilities. Captive portals are increasingly hijacking TLS connections to force browsers to talk to the portal. Providing the portal's URI via a DHCP or RA option is a cleaner technique, and reduces user expectations of being hijacked; this may improve security by making users more reluctant to accept TLS hijacking, which can be performed from beyond the network associated with the captive portal. References Normative References Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Dynamic Host Configuration Protocol The Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCPIP network. DHCP is based on the Bootstrap Protocol (BOOTP), adding the capability of automatic allocation of reusable network addresses and additional configuration options. [STANDARDS-TRACK] DHCP Options and BOOTP Vendor Extensions This document specifies the current set of DHCP options. Future options will be specified in separate RFCs. The current list of valid options is also available in ftp://ftp.isi.edu/in-notes/iana/assignments. [STANDARDS-TRACK] An IETF URN Sub-namespace for Registered Protocol Parameters This document describes a new sub-delegation for the 'ietf' URN namespace for registered protocol items. The 'ietf' URN namespace is defined in RFC 2648 as a root for persistent URIs that refer to IETF- defined resources. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Uniform Resource Identifier (URI): Generic Syntax A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. This specification defines the generic URI syntax and a process for resolving URI references that might be in relative form, along with guidelines and security considerations for the use of URIs on the Internet. The URI syntax defines a grammar that is a superset of all valid URIs, allowing an implementation to parse the common components of a URI reference without knowing the scheme-specific requirements of every possible identifier. This specification does not define a generative grammar for URIs; that task is performed by the individual specifications of each URI scheme. [STANDARDS-TRACK] Neighbor Discovery for IP version 6 (IPv6) This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence, to determine each other's link-layer addresses, to find routers, and to maintain reachability information about the paths to active neighbors. [STANDARDS-TRACK] Guidelines for Creating New DHCPv6 Options This document provides guidance to prospective DHCPv6 option developers to help them create option formats that are easily adoptable by existing DHCPv6 software. It also provides guidelines for expert reviewers to evaluate new registrations. This document updates RFC 3315. Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation. Hypertext Transfer Protocol (HTTP/1.1): Caching The Hypertext Transfer Protocol (HTTP) is a stateless \%application- level protocol for distributed, collaborative, hypertext information systems. This document defines HTTP caches and the associated header fields that control cache behavior or indicate cacheable response messages. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Dynamic Host Configuration Protocol for IPv6 (DHCPv6) This document describes the Dynamic Host Configuration Protocol for IPv6 (DHCPv6): an extensible mechanism for configuring nodes with network configuration parameters, IP addresses, and prefixes. Parameters can be provided statelessly, or in combination with stateful assignment of one or more IPv6 addresses and/or IPv6 prefixes. DHCPv6 can operate either in place of or in addition to stateless address autoconfiguration (SLAAC). This document updates the text from RFC 3315 (the original DHCPv6 specification) and incorporates prefix delegation (RFC 3633), stateless DHCPv6 (RFC 3736), an option to specify an upper bound for how long a client should wait before refreshing information (RFC 4242), a mechanism for throttling DHCPv6 clients when DHCPv6 service is not available (RFC 7083), and relay agent handling of unknown messages (RFC 7283). In addition, this document clarifies the interactions between models of operation (RFC 7550). As such, this document obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083, RFC 7283, and RFC 7550. Informative References Unused Dynamic Host Configuration Protocol (DHCP) Option Codes Prior to the publication of RFC 2489 (which was updated by RFC 2939), several option codes were assigned to proposed Dynamic Host Configuration Protocol (DHCP) options that were subsequently never used. This document lists those unused option codes and directs IANA to make these option codes available for assignment to other DHCP options in the future. The document also lists several option codes that are not currently documented in an RFC but should not be made available for reassignment to future DHCP options. X.509 Extensions for IP Addresses and AS Identifiers This document defines two X.509 v3 certificate extensions. The first binds a list of IP address blocks, or prefixes, to the subject of a certificate. The second binds a list of autonomous system identifiers to the subject of a certificate. These extensions may be used to convey the authorization of the subject to use the IP addresses and autonomous system identifiers contained in the extensions. [STANDARDS-TRACK] IPv6 Router Advertisement Guard Routed protocols are often susceptible to spoof attacks. The canonical solution for IPv6 is Secure Neighbor Discovery (SEND), a solution that is non-trivial to deploy. This document proposes a light-weight alternative and complement to SEND based on filtering in the layer-2 network fabric, using a variety of filtering criteria, including, for example, SEND status. This document is not an Internet Standards Track specification; it is published for informational purposes. Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard) The IPv6 Router Advertisement Guard (RA-Guard) mechanism is commonly employed to mitigate attack vectors based on forged ICMPv6 Router Advertisement messages. Many existing IPv6 deployments rely on RA-Guard as the first line of defense against the aforementioned attack vectors. However, some implementations of RA-Guard have been found to be prone to circumvention by employing IPv6 Extension Headers. This document describes the evasion techniques that affect the aforementioned implementations and formally updates RFC 6105, such that the aforementioned RA-Guard evasion vectors are eliminated. DHCPv6-Shield: Protecting against Rogue DHCPv6 Servers This document specifies a mechanism for protecting hosts connected to a switched network against rogue DHCPv6 servers. It is based on DHCPv6 packet filtering at the layer 2 device at which the packets are received. A similar mechanism has been widely deployed in IPv4 networks ('DHCP snooping'); hence, it is desirable that similar functionality be provided for IPv6 networks. This document specifies a Best Current Practice for the implementation of DHCPv6-Shield. Captive-Portal Identification Using DHCP or Router Advertisements (RAs) In many environments offering short-term or temporary Internet access (such as coffee shops), it is common to start new connections in a captive-portal mode. This highly restricts what the customer can do until the customer has authenticated. This document describes a DHCP option (and a Router Advertisement (RA) extension) to inform clients that they are behind some sort of captive-portal device and that they will need to authenticate to get Internet access. It is not a full solution to address all of the issues that clients may have with captive portals; it is designed to be used in larger solutions. The method of authenticating to and interacting with the captive portal is out of scope for this document. Captive Portal API Changes from RFC 7710 This document incorporates the following changes from .
  1. Clarified that IP string literals are NOT RECOMMENDED .
  2. Clarified that the option URI MUST be that of the captive portal API endpoint.
  3. Clarified that captive portals MAY do content negotiation.
  4. Added text about Captive Portal API URI precedence in the event of a network configuration error.
  5. Added urn:ietf:params:capport:unrestricted URN.
  6. Noted that the DHCPv4 Option Code changed from 160 to 114.
Observations from IETF 106 Network Experiment During IETF 106 in Singapore, an experiment enabling clients compatible with the Captive Portal API to discover a venue-info-url (see experiment description for more detail) revealed that some Polycom devices on the same network made use of DHCPv4 option code 160 for other purposes . The presence of DHCPv4 Option code 160 holding a value indicating the Captive Portal API URL caused these devices to not function as desired. For this reason, IANA has deprecated option code 160 and allocated a different value to be used for the Captive Portal API URL. Acknowledgements This document is a -bis of RFC 7710. Thanks to all of the original authors ( , , , and ) and original contributors. Also thanks to the CAPPORT WG for all of the discussion and improvements, including contributions and review from , , , , , , , , , , , and . Authors' Addresses Google 1600 Amphitheatre Parkway Mountain View CA 94043 United States of America warren@kumari.net Loon 1600 Amphitheatre Parkway Mountain View CA 94043 United States of America ek@loon.com

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