Apache2 mod_proxy: backend servers log frontend server as source of all requests

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http://serverfault.com – I have five servers running Ubuntu (LAMP). One of them (called Server1) receves all incomming requests from the internet, and uses VirtualHosts and mod_proxy to deliver the correct request to the correct server on the LAN. My problem is that in all the logs on my backend servers, all requests are logged as incomming from Server1's IP. Why is this, and how do I fix it? (HowTos)