Information about the client's host is passed to CGI scripts in $ENV{REMOTE_HOST} (the client's host name) and/or $ENV{REMOTE_ADDR} (the client's ip address.) If the server doesn't set $ENV{REMOTE_HOST} it should set $ENV{REMOTE_ADDR}.

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In general it is not possible to get the "user name" (i.e. the identity used to log into the client's host) of the client. The concept of a user's identity is something that your application has to create.