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A captive portal is a web page accessed with a web browser that is displayed to newly connected users of a Wi-Fi or wired network before they are granted broader access to network resources. Captive portals are commonly used to present a landing or log-in page which may require
authentication Authentication (from ''authentikos'', "real, genuine", from αὐθέντης ''authentes'', "author") is the act of proving an assertion, such as the identity of a computer system user. In contrast with identification, the act of indicati ...
, payment, acceptance of an end-user license agreement,
acceptable use policy An acceptable use policy (AUP), acceptable usage policy or fair use policy is a set of rules applied by the owner, creator or administrator of a computer network website, or service. That restricts the ways in which the network, website or system ...
, survey completion, or other valid credentials that both the host and user agree to adhere by. Captive portals are used for a broad range of mobile and pedestrian broadband services – including cable and commercially provided Wi-Fi and home hotspots. A captive portal can also be used to provide access to enterprise or residential wired networks, such as apartment houses, hotel rooms, and business centers. The captive portal is presented to the client and is stored either at the gateway or on a web server hosting the web page. Depending on the feature set of the gateway, websites or TCP ports can be white-listed so that the user would not have to interact with the captive portal in order to use them. The
MAC address A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking techno ...
of attached clients can also be used to bypass the login process for specified devices.
WISPr The Wide-Field Imager for Solar Probe (WISPR) is an imaging instrument of the '' Parker Solar Probe'' mission to the Sun, launched in August 2018. Imaging targets include visible light images of the corona, solar wind, shocks, solar ejecta, etc. De ...
refers to this web browser-based authentication method as the Universal Access Method (UAM).


Uses

Captive portals are primarily used in open wireless networks where the users are shown a welcome message informing them of the conditions of access (allowed ports, liability, etc.). Administrators tend to do this so that their own users take responsibility for their actions and to avoid any legal responsibility. Whether this delegation of responsibility is legally valid is a matter of debate. Often captive portals are used for marketing and commercial communication purposes. Access to the Internet over open Wi-Fi is prohibited until the user exchanges personal data by filling out a web-based registration form in a web browser. The web-based form either automatically opens in a web browser, or appears when the user opens a web browser and tries to visit any web page. In other words, the user is "captive" - unable to access the Internet freely until the user is granted access to the Internet and has "completed" the captive portal. This allows the provider of this service to display or send advertisements to users who connect to the Wi-Fi access point. This type of service is also sometimes known as "social Wi-Fi", as they may ask for a
social network A social network is a social structure made up of a set of social actors (such as individuals or organizations), sets of dyadic ties, and other social interactions between actors. The social network perspective provides a set of methods for an ...
account to login (such as Facebook). Over the past few years, such social Wi-Fi captive portals have become commonplace with various companies offering marketing centered around Wi-Fi data collection. The user can find many types of content in the captive portal, and it's frequent to allow access to the Internet in exchange for viewing content or performing a certain action (often, providing personal data to enable commercial contact); thus, the marketing use of the captive portal is a tool for lead generation (business contacts or potential clients).


Implementation

There are various ways to implement a captive portal.


HTTP redirect

A common method is to direct all
World Wide Web The World Wide Web (WWW), commonly known as the Web, is an information system enabling documents and other web resources to be accessed over the Internet. Documents and downloadable media are made available to the network through web se ...
traffic to a web server, which returns an HTTP redirect to a captive portal. When a modern, Internet-enabled device first connects to a network, it sends out an HTTP request to a detection URL predefined by its vendor and expects an
HTTP status code This is a list of Hypertext Transfer Protocol (HTTP) response status codes. Status codes are issued by a server in response to a client's request made to the server. It includes codes from IETF Request for Comments (RFCs), other specification ...
200 OK or 204 No Content. If the device receives a HTTP 200 status code, it assumes it has unlimited internet access. Captive portal prompts are displayed when you are able to manipulate this first HTTP message to return a HTTP status code of 302 (redirect) to the captive portal of your choice. specifies 511 Network Authentication Required code.


ICMP redirect

Client traffic can also be redirected using ICMP redirect on the layer 3 level.


Redirect by DNS

When a client requests a resource on a remote host by name,
DNS The Domain Name System (DNS) is a hierarchical and distributed naming system for computers, services, and other resources in the Internet or other Internet Protocol (IP) networks. It associates various information with domain names assigned to ...
is queried to resolve that hostname. In a captive portal, the
firewall Firewall may refer to: * Firewall (computing), a technological barrier designed to prevent unauthorized or unwanted communications between computer networks or hosts * Firewall (construction), a barrier inside a building, designed to limit the spre ...
will make sure that only the DNS server(s) provided by the network's DHCP can be used by unauthenticated clients (or, alternatively, it will forward all DNS requests by unauthenticated clients to that DNS server). This DNS server will return the IP address of the captive portal page as a result of all DNS lookups. In order to perform redirection by DNS the captive portal uses
DNS hijacking DNS hijacking, DNS poisoning, or DNS redirection is the practice of subverting the resolution of Domain Name System (DNS) queries. This can be achieved by malware that overrides a computer's TCP/IP configuration to point at a rogue DNS server unde ...
to perform an action similar to a man-in-the-middle attack. To limit the impact of DNS poisoning, a TTL of 0 is typically used.


Limitations


Security

Captive portals have been known to have incomplete firewall rule sets.


DNS tunneling

In some deployments, the rule set will route DNS requests from clients to the Internet, or the provided DNS server will fulfill arbitrary DNS requests from the client. This allows a client to bypass the captive portal and access the open Internet by tunneling arbitrary traffic within DNS packets.


Automatic submission

Some captive portals may be configured to allow appropriately equipped user agents to detect the captive portal and automatically authenticate. User agents and supplemental applications such as Apple's Captive Portal Assistant can sometimes transparently bypass the display of captive portal content against the wishes of the service operator as long as they have access to correct credentials, or they may attempt to authenticate with incorrect or obsolete credentials, resulting in unintentional consequences such as accidental account locking.


MAC spoofing

A captive portal that uses MAC addresses to track connected devices can sometimes be circumvented by re-using the MAC address of a previously authenticated device. Once a device has been authenticated to the captive portal using valid credentials, the gateway adds that device's MAC address to its allowlist; since MAC addresses can easily be spoofed, any other device can pretend to be the authenticated device and bypass the captive portal. Once the IP and MAC addresses of other connecting computers are found to be authenticated, any machine can spoof the MAC address and Internet Protocol (IP) address of the authenticated target, and be allowed a route through the gateway. For this reason some captive portal solutions created extended authentication mechanisms to limit the risk for usurpation.


Require Web Browser

Captive portals often require the use of a web browser; users who first use an email client or other application that relies on the Internet may find the connection not working without explanation, and will then need to open a web browser to validate. This may be problematic for users who do not have any web browser installed on their operating system. It is however sometimes possible to use email and other facilities that do not rely on DNS (e.g. if the application specifies the connection IP address rather than the hostname). A similar problem can occur if the client uses
AJAX Ajax may refer to: Greek mythology and tragedy * Ajax the Great, a Greek mythological hero, son of King Telamon and Periboea * Ajax the Lesser, a Greek mythological hero, son of Oileus, the king of Locris * ''Ajax'' (play), by the ancient Greek ...
or joins the network with pages already loaded into its web browser, causing undefined behavior (for example, corrupt messages appear) when such a page tries HTTP requests to its origin server. Similarly, as HTTPS connections cannot be redirected (at least not without triggering security warnings), a web browser that only attempts to access secure websites before being authorized by the captive portal will see those attempts fail without explanation (the usual symptom is that the intended website appears to be down or inaccessible). Platforms that have Wi-Fi and a TCP/IP stack but do not have a web browser that supports HTTPS cannot use many captive portals. Such platforms include the Nintendo DS running a game that uses
Nintendo Wi-Fi Connection Nintendo Wi-Fi Connection (WFC) was an online Multiplayer, multiplayer gaming service run by Nintendo to provide free online play in compatible Nintendo DS and Wii games. The service included the company's Wii Shop Channel and DSi Shop game d ...
. Non-browser authentication is possible using
WISPr The Wide-Field Imager for Solar Probe (WISPR) is an imaging instrument of the '' Parker Solar Probe'' mission to the Sun, launched in August 2018. Imaging targets include visible light images of the corona, solar wind, shocks, solar ejecta, etc. De ...
, an
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. T ...
-based authentication protocol for this purpose, or MAC-based authentication or authentications based on other protocols. It is also possible for a platform vendor to enter into a service contract with the operator of a large number of captive portal hotspots to allow free or discounted access to the platform vendor's servers via the hotspot's
walled garden A walled garden is a garden enclosed by high walls, especially when this is done for horticultural rather than security purposes, although originally all gardens may have been enclosed for protection from animal or human intruders. In temperate c ...
. For example, in 2005 Nintendo and Wayport partnered to provide free Wi-Fi access to Nintendo DS users at certain McDonald's restaurants. Also,
VoIP Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Internet ...
SIP ports could be allowed to bypass the gateway to allow phones to work.


See also

*
HTTP proxy The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, w ...
*
Proximity marketing Proximity marketing is the localized wireless distribution of advertising content associated with a particular place. Transmissions can be received by individuals in that location who wish to receive them and have the necessary equipment to do so. ...
* Mobile location analytics


References


External links


Android Captive Portal Setup
* {{IETF RFC, 7710 Captive-Portal Identification Using DHCP or Router Advertisements (RAs) Computer network security Web technology Wireless access points