Introduction

Your Web server thinks that the data stream sent by the client (e.g. your Web browser or our CheckUpDown robot) was 'malformed' i.e. did not respect the HTTP protocol completely. So your Web server was unable to understand the request and process it.

It almost always means bad programming of the client system and/or your Web server.

400 errors in the HTTP cycle

Any client (e.g. your Web browser or our CheckUpDown robot) goes through the following cycle:

• 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 it recognises as '400'.

Resolving 400 errors - general

There is a low-level program bug in the client or the Web server or both. If you do not have access to the source code for these systems, the only thing you can do is refer the problem to technical support people at the companies that developed the systems.

Resolving 400 errors - CheckUpDown

You should never see this error at all when you use our CheckUpDown service. It indicates that the two systems (our robot and your Web server) fundamentally disagree on the syntax of HTTP data streams.

Please contact us directly (email preferred) whenever you encounter 400 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 your Web server software to agree the exact source of the error.