Sorry, noob question:
one of the simplest ways to achieve this would be telling apache that the root url on that host should be redirected to the nmis url.
adding this to one of the apache config files
RedirectMatch "^/?$" /cgi-nmis8/nmiscgi.pl
should do the trick. if the cgi-... path is supposed to stay hidden, then you will likely have to resort to mod-rewrite or mod-proxy.
Fantastic, thanks Alexander working a treat. For Debian9.4: Added line to: /etc/apache2/apache2.conf Also had to: mv /var/www/html/index.html /var/www/html/index.html.bak