Reverse proxy with dynamic servers? [closed]

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http://serverfault.com – I'd like to have a public proxy (web-proxy?), and several servers behind that proxy. Now, assume the following problem (not a real case, but useful to clarify): The proxy should provide basic login mechanism. If "John" logs in, he should be redirected/forwarded to a specific search engine, let's say google.de And if "Bert" logs in, he should go to yahoo.com For sure I could do this by using Reverse-Proxy and RewriteRules. That would assume that the server has a static ip or known dns, like these search engines have. BUT what if the server is private application server that only has a dynami (HowTos)