write it in virtual host conf

<VirtualHost *:80>
    ServerAdmin support@mydomain.local
    DocumentRoot /usr/share/nagios/html
    ServerName nagios1.mydomain.local
    ServerAlias nagios1
    ServerAlias monitoring

    RewriteEngine on
    RedirectMatch 301 ^/$ http://monitoring.mydomain.local/nagios
</VirtualHost>