how to log ip behind proxy

[lemat]

does anybody know the header sent by the proxy servers to indicate the IP of real user?

I'd like to log it using custom log directive.

\"%{the_header}i\""

[Romeo]

... try using the following:
\"%{X-FORWARDED-FOR}i\" \"%{CLIENT-IP}i\"
The usage of these fields depends on the proxy.

Regards,
R.