mod_xsendfile is a small Apache2 module that processes X-SENDFILE headers
registered by the original output handler.

If it encounters the presence of such header it will discard all output and
send the file specified by that header instead using Apache internals
including all optimizations like caching-headers and sendfile or mmap if
configured.

It is useful for processing script-output of e.g. php, perl or any cgi.