O Varnish é um servidor  que funciona como proxy reverso e servidor de cache para aplicações/sites web.

Este post do Linux Journal mostra o que é , como usar e inclui também um exemplo de instalação via código fonte.

Varnish is a program that can greatly speed up a Web site while reducing
the load on the Web server. According to Varnish’s official site, Varnish
is a “Web application accelerator also known as a caching HTTP reverse

When you think about what a Web server does at a high level, it receives
HTTP requests and returns HTTP responses. In a perfect world, the server
would return a response immediately without having to do any real work. In
the real world, however, the server may have to do quite a bit of work
before returning a response to the client. Let’s first look at how a
typical Web server handles this, and then see what Varnish does
to improve the situation.

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