HTTP Error 504 - Gateway timeout
A server (not necessarily a Web server) is acting as a gateway or proxy to fulfil the request by the client (e.g. your Web browser or our CheckUpDown robot) to access the requested URL. This server did not receive a timely response from an upstream server it accessed to deal with your HTTP request.
This usually means that the upstream server is down (no response to the gateway/proxy), rather than that the upstream server and the gateway/proxy do not agree on the protocol for exchanging data.
504 errors in the HTTP cycle
Any client (e.g. your Web browser or our CheckUpDown robot) goes through the following cycle when it communicates with your Web server:
Obtain an IP address from the IP name of your site (your site URL without the leading 'http://'). This lookup (conversion of IP name to IP address) is provided by domain name servers (DNSs).
Open an IP socket connection to that IP address.
Write an HTTP data stream through that socket.
Receive an HTTP data stream back from your Web server in response. This data stream contains status codes whose values are determined by the HTTP protocol. Parse this data stream for status codes and other useful information.
This error occurs in the final step above when the client receives an HTTP status code that it recognises as '504'.
Resolving 504 errors - general
This problem is entirely due to slow IP communication between back-end computers, possibly including your Web server. Only the people who set up the network at the site which hosts the Web server can fix this problem.
Resolving 504 errors - CheckUpDown
Use of proxies and caching is increasing on the Web. Our CheckUpDown robot will always try to 'drill through' to the real computer that actually hosts your Web site, but we do not have complete control over where our HTTP request actually ends up. If only one link in the chain of computers dealing with our HTTP request is broken, then an error such as 504 can easily occur.
Please contact us (email preferred) whenever you encounter 504 errors - there is nothing you can do to sort them out. We then have to liaise with your ISP and the vendor of your Web server software so that they can review the flow of IP data traffic between various computers under their control. However this is not an easy error to sort out, because the ebb and flow of Internet traffic makes this type of error very transient.