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If I'm sitting inside a corporate firewall and I'm seeing unexpected (ie, out of date) responses to HTTP requests. What are methods for determining whether there is a cache sitting somewhere between me and the origin server I am attempting to contact?

Should cache servers add extra HTTP headers to identify themselves?

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A well-behaved proxy will include the "Via:" http header with the response. Either use a browser add-on for looking at response headers, or just Wireshark the traffic.

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Your system administrator should be able to assist. Ask them to check the proxy logs after you've visited the website.

You might also want to delete your temporary files; there's a slim chance the old data is being returned from you PCs cache.

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