    # File lib/fog/rackspace.rb, line 53
        def self.authenticate(options, connection_options = {})
          rackspace_auth_url = options[:rackspace_auth_url] || "auth.api.rackspacecloud.com"
          url = rackspace_auth_url.match(/^https?:/) ? \
                    rackspace_auth_url : 'https://' + rackspace_auth_url
          uri = URI.parse(url)
          connection = Fog::Connection.new(url, false, connection_options)
          @rackspace_api_key  = options[:rackspace_api_key]
          @rackspace_username = options[:rackspace_username]
          response = connection.request({
            :expects  => [200, 204],
            :headers  => {
              'X-Auth-Key'  => @rackspace_api_key,
              'X-Auth-User' => @rackspace_username
            },
            :host     => uri.host,
            :method   => 'GET',
            :path     =>  (uri.path and not uri.path.empty?) ? uri.path : 'v1.0'
          })
          response.headers.reject do |key, value|
            !['X-Server-Management-Url', 'X-Storage-Url', 'X-CDN-Management-Url', 'X-Auth-Token'].include?(key)
          end
        end