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We have a PHP application running on Ubuntu with a load balanacer and several servers. Some clients complain from time to time of Error 400 errors, but it is random. We are able to access the exact same pages without any problems.

We believe it is a DNS issue but we need to be absolutely sure of the error.

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What do the logs of your server(s) tell you? –  Flimzy Jun 27 '11 at 20:03
More details are needed... what load balancer? What web server? We have seen some mystery 400-Bad Request errors on nginx, and have tracked them down to buggy proxies at customer sites. People still running Microsoft ISA Server 2004 for example, or old WebSense proxies that haven't been updated in years. –  rmalayter Jun 27 '11 at 20:12
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