title: Log an HTTP Header in Apache2 Access Logs
body: <p>I have a website running behind Cloudflare, which is a reverse-proxy. This means that I only ever get one or two IP addresses in my access logs.</p>
  
  <p>However, Cloudflare does provide the visitor IP address in the request headers, which I assume I can log instead of the standard one in an access log.</p>
  
  <p>I know how to use <code>CustomLog</code>, but is there a way to save an HTTP header in an Apache access log?</p>
  
  <p>Thanks.</p>