What HTTP status should I return during temporary site outage/downtime?

Question

I'm going to be taking down my website for an upgrade to the code. I'd like to have a temporary downtime page display during the upgrade. For the sake of preventing issues with bots attempting to spider my site during the downtime, what HTTP response should I make sure to return during this window?

Answer 1

According to the RFC, the correct response to return is 503 - Service Unavailable

10.5.4 503 Service Unavailable

The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.

  Note: The existence of the 503 status code does not imply that a
  server must use it when becoming overloaded. Some servers may wish
  to simply refuse the connection.