forwarding domain name

[jmatz]

Scenario:

Have two old domain names which point to the same web site on a Linux/Apache server:
my-domain.com
mydomain.com

Any time a person types in one of the old domain names in the address bar of a browser window and goes to that URL, or clicks on an old link that uses one of the old domain names in the URL, I want to force the new domain name...

mynewdomain.com

...to appear in the browser address bar.

I have tried using redirect rules in an .htaccess file, but that does not work.

Is this a simple issue of needing to change something in the DNS records of the "old" domain names?

Thanks.