-- little helper function
function file_exists(path)
  local attr = lighty.stat(path)
  if (attr) then
      return true
  else
      return false
  end
end
function removePrefix(str, prefix)
  return str:sub(1,#prefix+1) == prefix.."/" and str:sub(#prefix+2)
end

-- prefix without the trailing slash
local prefix = '/drupal'

-- the magic ;)
if (not file_exists(lighty.env["physical.path"])) then
    -- file still missing. pass it to the fastcgi backend
    request_uri = removePrefix(lighty.env["uri.path"], prefix)
    if request_uri then
      lighty.env["uri.path"]          = prefix .. "/index.php"
      local uriquery = lighty.env["uri.query"] or ""
      lighty.env["uri.query"] = uriquery .. (uriquery ~= "" and "&" or "") .. "q=" .. request_uri
      lighty.env["physical.rel-path"] = lighty.env["uri.path"]
      lighty.env["request.orig-uri"]  = lighty.env["request.uri"]
      lighty.env["physical.path"]     = lighty.env["physical.doc-root"] .. lighty.env["physical.rel-path"]
    end
end
-- fallthrough will put it back into the lighty request loop
-- that means we get the 304 handling for free. ;)

[...]
-- prefix without the trailing slash
local prefix = ''
[...]

[...]
## files to check for if .../ is requested
#index-file.names           = ( "index.php", "index.html",
#                               "index.htm", "default.htm" )
index-file.names           = ( "index.php" )

## Use the "Content-Type" extended attribute to obtain mime type if possible
# mimetype.use-xattr = "enable"

#### accesslog module
accesslog.filename         = "/var/log/lighttpd/access.log"

## deny access the file-extensions
#
# ~    is for backupfiles from vi, emacs, joe, ...
# .inc is often used for code includes which should in general not be part
#      of the document-root
#url.access-deny            = ( "~", ".inc" )
url.access-deny = ( "~", ".inc", ".engine", ".install", ".module", ".sh", "sql", ".theme", ".tpl.php", ".xtmpl", "Entries", "Repository", "Root" )

magnet.attract-physical-path-to = ( "/etc/lighttpd/drupal.lua" )
[...]