Hashing

Internally, when Varnish stores content in the cache indexed by a hash key used to find the object again. In the default setup this key is calculated based on URL, the Host: header, or if there is none, the IP address of the server::

sub vcl_hash {
    hash_data(req.url);
    if (req.http.host) {
        hash_data(req.http.host);
    } else {
        hash_data(server.ip);
    }
    return (lookup);
}

As you can see it first hashes req.url and then req.http.host if it exists. It is worth pointing out that Varnish doesn't lowercase the hostname or the URL before hashing it so in theory having "Varnish.org/" and "varnish.org/" would result in different cache entries. Browsers however, tend to lowercase hostnames.

You can change what goes into the hash. This way you can make Varnish serve up different content to different clients based on arbitrary criteria.

Let's say you want to serve pages in different languages to your users based on where their IP address is located. You would need some Vmod to get a country code and then put it into the hash. It might look like this.

In vcl_recv::

set req.http.X-Country-Code = geoip.lookup(client.ip);

And then add a vcl_hash::

sub vcl_hash {
    hash_data(req.http.X-Country-Code);
}

Because there is no return(lookup), the builtin VCL will take care of adding the URL, Host: or server IP# to the hash as usual.

If vcl_hash did return, ie::

sub vcl_hash {
    hash_data(req.http.X-Country-Code);
    return(lookup);
}

then only the country-code would matter, and Varnish would return seemingly random objects, ignoring the URL, (but they would always have the correct X-Country-Code).