Maybe this will help. Its from the Red Hat Network Administration Tool.

http://people.redhat.com/harald/redhat-config-network/help/network-config-dns.html

This can also be accessed through the DNS dialogue as follows:

Red Hat Start Menu --> System Settings --> Network

This brings up the dialogue window titled "Network Configuration".

Click on the DNS tab and add appropriate nodes. Also, you can drop down the help menu for more info.

Here is a link for Virtual Domain regarding sendmail:

http://www.sendmail.org/virtual-hosting.html

This may be helpful. It present "Virual Domain" from the POV of the person wishing to establish the Virual Domain as opposed to the ISP that is offering Virtual Hosting on his computers.

http://www.northstarnet.org/virtualdomains.html

LelandJ