URL unreachable errors

Google encountered an error when trying to access this URL. We may have encountered a DNS error or timeout, for instance. Your server may have been down or busy when we tried to access the page. Possible URL unreachable errors include:

5xx error	See RFC 2616 for a complete list of these status codes. Likely reasons for this error are an internal server error or a server busy error. If the server is busy, it may have returned an overloaded status to ask the Googlebot to crawl the site more slowly. In this case, we'll return again later to crawl additional pages.
DNS issue	We couldn't communicate with the DNS server when we tried to access the page. This could be because your server is down, or there is an issue with the DNS routing to your domain. Make sure that your domain is resolving correctly and try again.
robots.txt file unreachable	Before we crawled the pages of your site, we tried to check your robots.txt file to ensure we didn't crawl any pages that you had roboted out. However, your robots.txt file was unreachable. To make sure we didn't crawl any pages listed in that file, we postponed our crawl. When this happens, we return to your site later and crawl it once we can reach your robots.txt file. Note that this is different from a 404 response when looking for a robots.txt file. If we receive a 404, we assume that a robots.txt file does not exist and we continue the crawl.
Network unreachable	We encountered a network error when we tried to access the page. This can happen when Googlebot encounters a time-out or other network related issue when requesting a file from your site, and thus is forced to abandon the request. This can be caused by one or more of the following: Excessive page load times due to dynamic pages taking too long to respond Excessive page load times due to a site's hosting server being down, overloaded, or misconfigured The hosting server is blocking Google's web crawler A DNS configuration issue.
Failed to connect	A connection could not be established.
No response	The server closed the connection before we could receive a response.
Truncated response	The server closed the connection before we could receive a full response, and the body of the response appears to be truncated.
Connection refused	The server refused the connection.
Truncated headers	The server closed the connection before full headers were sent.