Giving more info to the redirector?

From the Squid docs,

    URL ip-address/fqdn ident method

is the info given to the redirector. I have got some questions here:

a) Is the ip-address/fqdn the one of the requestor or of the remote host?
b) if not, would it be possible to include this info ?
c) Especially interesting info to have would be the username when the request
   was authenticated using the proxy_auth method.
c) More interesting info would be the user agent, the Host header, basically
   everything possible.
   
(If you didn't guess already, I am trying to use the redirector for access
 control purposes)

Thanks in advance.

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