S        This expression returns the hostname for which the client is requesting data.

This is useful if the same server is used to host multiple sites with different hostnames.


The hostname is the domain and subdomain(s) in the request.


For example, in a request for 

GET https://subdomain.example.com/folder/myfile.html?getkey=getvalue

…this expression will return "subdomain.example.com".


The hostname is not read from the HTTP Host request header, so if the port is not 80, it will not be included in the hostname.