must auto subdomain set to www?
For the apache webserver, domain.tld and www.domain.tld
are different "domains", so if you add domain.tld as alias to a website, apache can not know that you want to add www.domain.tld
as well and assuming this automatically makes no sense for apache as www.domain.rld
can be routed to another site e.g. with a wildcard. So you have to tell apache that www.domain.tld
shall be routed to the same website. To do that, enable auto subdomain www as this adds a second alias internally for the www subdomain in the apache configuration.
and is a cname necessary?
A A-record is enough. You need either a a-record or a cname record.