Think of it like this:
DNS is the phone directory/yellow pages. When someone wants to call your phone, they can look up your name and get your phone number and call that phone. DNS does the same but for computers – when someone wants to go to www.example.com they ask DNS for the IP address and then they can contact the computer that has that IP address. That is what resolve means. Resolving an IP address has nothing at all to do with Apache; it is strictly a DNS question.
The ServerName and ServerAlias is more like a company’s internal phone list. Your webserver is the switchboard; it will accept all incoming connections to the server. Then the client/caller will tell them what name they’re looking for, and it will look in the Apache configuration for how to handle that name.
If the name isn’t listed as a ServerName/ServerAlias in the apache configuration, apache will always give them the first VirtualHost listed. Or, if there’s no VirtualHost at all, it will give the same content no matter what hostname is given in the request.
ETA: So, step by step for a normal connection:
- You type
http://www.example.cominto your browser. - Your computer asks its DNS resolver which IP address it should use when it wants to talk to
www.example.com. - Your computer connects to that IP address, and says that it wants to talk to
www.example.com(that’s theHost:header in HTTP). - The webserver looks at its configuration to figure out what to do with a request for content from
www.example.com. Any one of the following may happen:www.example.comis listed as aServerNameorServerAliasfor aVirtualHost– if so, then it will use the configuration for that VirtualHost to deliver the content.- The server doesn’t have any VirtualHosts at all – if so, then it will use the configuration in its httpd.conf to deliver the content.
- The server has VirtualHosts but
www.example.comisn’t listed in any of them – if so, the first Virtualhost in the list will be used to deliver the content.