Mapping Customer's Domain/Subdomain to my site
brk: customer1.mysite.com is easier to do.First, you setup a DNS record:customers.mysite.com IN   A   12.34.56.78Then, your customers setup CNAME records:sub.customer1.com  IN CNAME customers.mysite.comNext, depending on your webserver (going to assume Apache), you setup virthosts:<VirtualHost :80>
     ServerName customer1.mysite.com   (etc...)
</VirtualHost>
<VirtualHost :80>
     ServerName customer2.mysite.com   (etc...)
</VirtualHost>The reason you setup the customers.mysite.com record and have EVERYONE CNAME to that is because when you eventually change servers/hosts/IP addresses you simply update that single record (which is under your control) and then everything else follows.  Same logic for when you eventually need to insert a load balancer or an haproxy auto-failover maintenance page.