A captive portal is a web page accessed with a web browser that is displayed to newly connected users of a Wi-Fi 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, payment, acceptance of an end-user license agreement, acceptable use policy, 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 of attached clients can also be used to bypass the login process for specified devices.
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 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).
There is more than one way to implement a captive portal.
A common method is to direct all World Wide Web 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 of 204. If the device receives a HTTP 204 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.
Client traffic can also be redirected using ICMP redirect on the layer 3 level.
Redirect by DNS
When a client requests a World Wide Web resource, DNS is queried by the browser. In a captive portal, the firewall 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 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.
Circumvention of captive portals
Captive portals have been known to have incomplete firewall rule sets. 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.
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.
A captive portal that uses MAC addresses to track connected devices can sometimes be circumvented by connecting a router that allows setting of the router MAC address. Router firmware often calls this MAC cloning. Once a computer or tablet has been authenticated to the captive portal using a valid username and valid password, the MAC address of that computer or tablet can be entered into the router which will often continue to be connected through the captive portal as it shows to have the same MAC address as the computer or tablet that was previously connected.
Some of these implementations merely require users to pass a TLS or SSL encrypted login page, after which their IP and MAC address are allowed to pass through the gateway. This has been shown to be exploitable with a simple packet sniffer. 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.
Captive portals often require the use of a web browser; this is usually the first application that users start after connected to the Internet, but 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 rather than the hostname). A similar problem can occur if the client uses AJAX 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. Non-browser authentication is possible using WISPr, an XML-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. One such example is the 2005 deal between Nintendo and Wayport to provide free Wi-Fi access to Nintendo DS users at certain McDonald's restaurants. Also, VoIP SIP ports could be allowed to bypass the gateway to allow phones to work.
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