Internet Engineering Task Force (IETF) W. Kumari
Request for Comments: 8910 Google
Obsoletes: 7710 E. Kline
Updates: 3679 Loon
Category: Standards Track September 2020
ISSN: 2070-1721
Captive-Portal Identification in DHCP and Router Advertisements (RAs)
Abstract
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 user can do
until the user has satisfied the captive portal conditions.
This document describes a DHCPv4 and DHCPv6 option and a Router
Advertisement (RA) option to inform clients that they are behind some
sort of captive portal enforcement device, and that they will need to
satisfy the Captive Portal conditions 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 one component of a
standardized approach for hosts to interact with such portals. While
this document defines how the network operator may convey the captive
portal API endpoint to hosts, the specific methods of satisfying and
interacting with the captive portal are out of scope of this
document.
This document replaces RFC 7710, which used DHCP code point 160. Due
to a conflict, this document specifies 114. Consequently, this
document also updates RFC 3679.
Status of This Memo
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.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8910.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Requirements Notation
2. The Captive-Portal Option
2.1. IPv4 DHCP Option
2.2. IPv6 DHCP Option
2.3. The Captive-Portal IPv6 RA Option
3. Precedence of API URIs
4. IANA Considerations
4.1. Captive Portal Unrestricted Identifier
4.2. BOOTP Vendor Extensions and DHCP Options Code Change
4.3. Update DHCPv6 and IPv6 ND Options Registries
5. Security Considerations
6. References
6.1. Normative References
6.2. Informative References
Appendix A. Changes from RFC 7710
Appendix B. Observations from IETF 106 Network Experiment
Acknowledgements
Authors' Addresses
1. 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 [RFC2131] and DHCPv6 [RFC8415]
option (Captive-Portal) and an IPv6 Router Advertisement (RA)
[RFC4861] 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 [RFC7710], which used DHCP code point
160. Due to a conflict, this document specifies 114. Consequently,
this document also updates [RFC3679].
1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. 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 [RFC8908]. 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 ([RFC8415]), 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 ([RFC8908]).
A captive portal MAY do content negotiation (Section 3.4 of
[RFC7231]) 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 Section 5.3 of
[RFC7231]). 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 (Section 3.4 of [RFC7231]),
implementors of captive portals need to keep in mind that such
responses might be cached, and therefore SHOULD include an
appropriate Vary header field (Section 7.1.4 of [RFC7231]) or set the
Cache-Control header field in any responses to "private" or a more
restrictive value such as "no-store" (Section 5.2.2.3 of [RFC7234]).
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
([RFC3779]) 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.
2.1. IPv4 DHCP Option
The format of the IPv4 Captive-Portal DHCP option is shown below.
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... .
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Captive-Portal DHCPv4 Option Format
Code: The Captive-Portal DHCPv4 Option (114) (one octet).
Len: The length (one octet), in octets, of the URI.
URI: The URI for the captive portal API endpoint to which the
user should connect (encoded following the rules in [RFC3986]).
See Section 2 of [RFC2132] for more on the format of IPv4 DHCP
options.
Note that the URI parameter is not null terminated.
2.2. IPv6 DHCP Option
The format of the IPv6 Captive-Portal DHCP option is shown below.
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) .
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Captive-Portal DHCPv6 Option Format
option-code: The Captive-Portal DHCPv6 Option (103) (two octets).
option-len: The unsigned 16-bit length, in octets, of the URI.
URI: The URI for the captive portal API endpoint to which the
user should connect (encoded following the rules in [RFC3986]).
See Section 5.7 of [RFC7227] for more examples of DHCP Options with
URIs. See Section 21.1 of [RFC8415] 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.
2.3. The Captive-Portal IPv6 RA Option
This section describes the Captive-Portal Router Advertisement
option.
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 .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .
. .
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Captive-Portal RA Option Format
Type: 37
Length: 8-bit unsigned integer. The length of the option
(including the Type and Length fields) in units of 8 bytes.
URI: The URI for the captive portal API endpoint to which the
user should connect. This MUST be padded with NUL (0x00) to
make the total option length (including the Type and Length
fields) a multiple of 8 bytes.
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.
3. 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 Section 2
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.
4. IANA Considerations
IANA has registered a new IETF URN protocol parameter ([RFC3553]).
IANA has also reallocated two DHCPv4 option codes (see Appendix B for
background) and updated the references for previously registered
DHCPv6 and IPv6 ND options.
4.1. Captive Portal Unrestricted Identifier
IANA has registered a new entry in the "IETF URN Sub-namespace for
Registered Protocol Parameter Identifiers" registry defined in
[RFC3553]:
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.
4.2. BOOTP Vendor Extensions and DHCP Options Code Change
IANA has updated the "BOOTP Vendor Extensions and DHCP Options"
registry (https://www.iana.org/assignments/bootp-dhcp-parameters) 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 [RFC7710]; known to also be used by
Polycom.
Reference: [RFC7710] RFC 8910
4.3. 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 [RFC7710]
to reference this document.
5. 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 [RFC3986].
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 ([RFC6105], [RFC7113]) or DHCPv6-Shield [RFC7610], 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.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol",
RFC 2131, DOI 10.17487/RFC2131, March 1997,
<https://www.rfc-editor.org/info/rfc2131>.
[RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
<https://www.rfc-editor.org/info/rfc2132>.
[RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
IETF URN Sub-namespace for Registered Protocol
Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
2003, <https://www.rfc-editor.org/info/rfc3553>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC7227] 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, May 2014,
<https://www.rfc-editor.org/info/rfc7227>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, DOI 10.17487/RFC7234, June 2014,
<https://www.rfc-editor.org/info/rfc7234>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8415] 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, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
6.2. Informative References
[RFC3679] Droms, R., "Unused Dynamic Host Configuration Protocol
(DHCP) Option Codes", RFC 3679, DOI 10.17487/RFC3679,
January 2004, <https://www.rfc-editor.org/info/rfc3679>.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779,
DOI 10.17487/RFC3779, June 2004,
<https://www.rfc-editor.org/info/rfc3779>.
[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
DOI 10.17487/RFC6105, February 2011,
<https://www.rfc-editor.org/info/rfc6105>.
[RFC7113] Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)", RFC 7113,
DOI 10.17487/RFC7113, February 2014,
<https://www.rfc-editor.org/info/rfc7113>.
[RFC7610] Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
Protecting against Rogue DHCPv6 Servers", BCP 199,
RFC 7610, DOI 10.17487/RFC7610, August 2015,
<https://www.rfc-editor.org/info/rfc7610>.
[RFC7710] Kumari, W., Gudmundsson, O., Ebersman, P., and S. Sheng,
"Captive-Portal Identification Using DHCP or Router
Advertisements (RAs)", RFC 7710, DOI 10.17487/RFC7710,
December 2015, <https://www.rfc-editor.org/info/rfc7710>.
[RFC8908] Pauly, T., Ed. and D. Thakore, Ed., "Captive Portal API",
RFC 8908, DOI 10.17487/RFC8908, September 2020,
<https://www.rfc-editor.org/info/rfc8908>.
Appendix A. Changes from RFC 7710
This document incorporates the following changes from [RFC7710].
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.
Appendix B. Observations from IETF 106 Network Experiment
During IETF 106 in Singapore, an experiment
(https://tickets.meeting.ietf.org/wiki/IETF106network#Experiments)
enabling clients compatible with the Captive Portal API to discover a
venue-info-url (see experiment description
(https://tickets.meeting.ietf.org/wiki/CAPPORT) for more detail)
revealed that some Polycom devices on the same network made use of
DHCPv4 option code 160 for other purposes
(https://community.polycom.com/t5/VoIP-SIP-Phones/DHCP-
Standardization-160-vs-66/td-p/72577).
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 (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, and Steve
Sheng) and original contributors.
Also thanks to the CAPPORT WG for all of the discussion and
improvements, including contributions and review from Joe Clarke,
Lorenzo Colitti, Dave Dolson, Hans Kuhn, Kyle Larose, Clemens
Schimpe, Martin Thomson, Michael Richardson, Remi Nguyen Van, Subash
Tirupachur Comerica, Bernie Volz, and Tommy Pauly.
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
Email: warren@kumari.net
Erik Kline
Loon
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America