Those who have a wireless network setup in their organizations may also want to implement a captive portal. A captive portal is also referred to as a Service Selection Gateway in ISP-speak and is a software or hardware device used to regulate access via authentication, typically web-based, for all users who wish to use the network services. An example of an open-source software-based captive portal is wifidog (http://dev.wifidog .org). Another example that has been incorporated into various commercial products, as well as being able to run on most Linux-based routers and APs, is a tool by the name of NoCatAuth (http://nocat.net). There is also a C port of the tool by the name of NoCatSplash found on the same site. We will use wifidog for our illustration here.
The wifidog application is made up of two components: the client portion, which is a daemon process that gets installed on a router, and the auth server, which is a web application that gets installed in a central location. With the help of firewall rules, the client daemon controls traffic going through the router. When it detects a new user trying to access any protected resource, the client daemon sitting on the router will transparently redirect these users to the authentication server where they will be prompted to log in (for existing users) or sign up (for new users). The client and the authentication server then exchange information on whether the client is allowed or denied access to the client network's protected resource. The client also updates the authentication server every few minutes on uptime, load, traffic count per client, and so on, as to allow the server to know the client is still there.
So by plugging a Linux-based router running the wifidog client daemon between your AP and your network resources or by installing wifidog on a Linux-based AP directly, you would effectively implement a second layer of access controls and authentication, both of which serve to protect access to your network resources.
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