On a fresh new munin install, its web interface can be browsed using http://domain.fr/munin/ which is suboptimal. Moving it to a subdomain (such as http://munin.domain.fr) is quite easy actually.

  1. Disable default munin.conf from apache2 conf-enabled directory to get rid of the /munin behavior

    rm /etc/apache2/conf-enabled/munin.conf
    
  2. Create a new vhost, and copy/paste its content. Update it to match your needs, especially the ServerName part.
    Note: this vhost has been written for Apache v2.4. You may have to adapt it if you're using an earlier version.

    vim /etc/apache2/sites-available/munin.conf
    
  3. Enable this new vhost, and reload apache:

    a2ensite munin
    service apache2 reload
    
  4. Add a DNS record to your domain to match the vhost's ServerName