X-Forwarded-For header value.

        This is the amended header so that it contains the previous IP address
        in the forwarding change.

def x_forwarded_for(self):
        """X-Forwarded-For header value.

        This is the amended header so that it contains the previous IP address
        in the forwarding change.

        """
        ip = self._request.META.get('REMOTE_ADDR')
        current_xff = self.headers.get('X-Forwarded-For')

        return '%s, %s' % (current_xff, ip) if current_xff else ip