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A captive portal is a special web page that is shown before using the Internet normally. The portal is often used to present a login page. This is done by intercepting most packets, regardless of address or port, until the user opens a browser and tries to access the web. At that time the browser is redirected to a web page which may require authentication and/or payment, or simply display an acceptable use policy and require the user to agree. Captive portals are used at many Wi-Fi hotspots, and can be used to control wired access (e.g. apartment houses, hotel rooms, business centers, "open" Ethernet jacks) as well.
Since the login page itself must be presented to the client, either that login page is locally stored in the gateway, or the web server hosting that page must be "whitelisted" via a walled garden to bypass the authentication process. Depending on the feature set of the gateway, multiple web servers can be whitelisted (say for iframes or links within the login page). In addition to whitelisting the URLs of web hosts, some gateways can whitelist TCP ports. The MAC address of attached clients can also be set to bypass the login process.
This technique has occasionally been referred to as UAM (Universal Access Method) in implementations and standards forums
There is more than one way to implement a captive portal.
Redirection by HTTP
If an unauthenticated client requests a website, DNS is queried by the browser and the appropriate IP resolved as usual. The browser then sends an HTTP request to that IP address. This request, however, is intercepted by a firewall and forwarded to a redirect server. This redirect server responds with a regular HTTP response which contains HTTP status code 302 to redirect the client to the Captive Portal. To the client, this process is totally transparent. The client assumes that the website actually responded to the initial request and sent the redirect. A transparent proxy is often integrated to add flexibility such as exempting certain website domains from redirection.
This technique can also be implemented without cooperation from a firewall or proxy. A server given access to a network sniff of packet headers and allowed to spoof may send an HTTP redirect quickly enough to reach the client before the actual response.
Client traffic can also be redirected using ICMP redirect on the layer 3 level.
Redirection by DNS
When a client requests a website, DNS is queried by the browser. The firewall will make sure that only the DNS server(s) provided by 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.
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.
Captive portals are sometimes used to enforce payment or negotiate the level and duration of authorization with a prospective user. Emergency notification systems may use captive portals to interrupt users' browsing experience until they have acknowledged receipt of an emergency bulletin.
Institutions implementing NAC often use captive portals to collect machine information, to supply software assessment agents which the supplicant user must run before gaining admission to the network, to provide online assistance for self-remediation of security problems, and to inform quarantined users when their network access has been revoked.
Delivery of advertising content via captive portals is not uncommon.
Some of these implementations merely require users to pass an 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 IP 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 require the use of a browser; this is usually the first application that users start, but users who first use an email client or other will find the connection not working without explanation, and will need to open a browser to validate. A similar problem can occur if the client joins the network with pages already loaded into its browser, causing undefined behavior when such a page tries HTTP requests to its origin server.
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.
There also exists the option of the platform vendor entering 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, such as the deal between Nintendo and Wayport. For example, VoIP SIP ports could be allowed to bypass the gateway to allow phones to work.