Why do you want to do this?

The best information available is the REMOTE_ADDR and REMOTE_HOST,
which tell you nothing about the user.   Techniques such as "finger@"
are not reliable, are widely disliked, and generally serve only to
introduce long delays in your CGI.   Better - as well as more polite -
just to ask your users to fill in a form.

BTW: the "From:" header line (HTTP_FROM variable) is usually only set
by robots, since human visitors to your webpage will not normally want
their addresses collected without permission, and browsers respect this.