You need to write an application that would block access to certain sites. Where to start and where to dig?

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    In short, no, you cannot write a full-fledged Java firewall. Java does not provide such low-level APIs for working with the network. You could write modules that work with the C network and access them via JNI, but why do you need Java then?

    But you have the option to write a local proxy in Java that filters access to sites. In this case, the browser will have to use this proxy. And access via HTTP by proxy can be closed by means of OS / firewall / router.