HTTP Error 303 See other Explained

303 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 the Web server:

• Obtain an IP address from the IP name of the site (the 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 the 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 '303'.

Fixing 303 errors - general

The 303 response from the Web server should always include an alternative URL to which redirection should occur. If it does, a Web browser will immediately retry the alternative URL. So you never actually see a 303 error in a Web browser, unless perhaps you have a corrupt redirection chain e.g. URL A redirects to URL B which in turn redirects back to URL A. If your client is not a Web browser, it should behave in the same way as a Web browser i.e. immediately retry the alternative URL.

If the Web server does not return an alternative URL with the 303 response, then either the Web server sofware itself is defective or the Webmaster has not set up the URL redirection correctly.

Fixing 303 errors - CheckUpDown

Redirection of URLs may occur for low-level URLs (specific URLs within the Web site) when you reorganise the web site, but is relatively uncommon for the top-level URLs which most CheckUpDown users ask us to check. So this type of error should be fairly infrequent. The 303 error should not actually occur at all, because it can really only be generated by the Web server in response to a POST command in a script, which we never use.

The most likely reason for 303 errors is defective programming by the vendor of the Web server software. Please contact us directly (email preferred) whenever you encounter 303 errors. Only we can resolve them for you. Unfortunately this may take some time, because we have to analyse the underlying HTTP data streams and may have to liaise with your ISP and the vendor of the Web server software to agree the exact source of the error.

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