What HTTP status code is most search-engine-friendly during a planned outage?

Question

If you have to take a site down for some type of unavoidable maintenance task (and it's not a big enough site that you have a backup server), what HTTP status code should you have your server return to minimize the possibility that search engines will think the site is gone?

I found this list of status codes from W3C, of which the following seem applicable:

• 503 Service Unavailable
• 500 Internal Server Error
• 408 Timeout
• 404 Not Found

I think 503 is the most appropriate, but I don't know what search engines might prefer.

Answer 1

According to Google the 503 code would be the way to go, since it means "the server is temporarily unavailable." Also check out the W3C page on the same.

Answer 2

From the horse's mouth:

If my site is down for maintenance, how can I tell Googlebot to come back later rather than to index the "down for maintenance" page?

You should configure your server to return a status of 503 (network unavailable) rather than 200 (successful). That lets Googlebot know to try the pages again later.

Answer 3

Don't send a 404 -- they may remove you from their index.

I'd probably send a 503 and an appropriate Retry-After, although I don't know if anything actually uses the header.