Lighttpd

I recognize that reverse DNS lookups are expensive, but in certain (low-traffic) cases, it would be very useful to be able to filter on an IP's reverse DNS name.

For instance, if I were to want to send high-speed internet users from a local ISP to one docroot and dialup users to another, where they are only detectably different by their reverse DNS (the ISP has not split highspeed users/modem users into fixed separate IP ranges), I could do something like this:

$HTTP["remotehost"] =~ "(^|\.)\.dialup\.mycrappyisp\.com$" {
  server.document-root = "/var/www/htdocs/dialupsite/pages/" 
}
else $HTTP["remotehost"] =~ "(^|\.)\.dsl\.mycrappyisp\.com$" {
  server.document-root = "/var/www/htdocs/dslsite/pages/" 
}

You'd never want this to be enabled by default, of course. You'd probably want to do some caching of reverse-dns lookups to help performance, but no-one who is serious about speed would be using this feature anyway.

-- gbauman