I'm trying to add an additional DNS entry in my OSX network preferences but I want to keep the DNS provided by DHCP. These show up in gray, but when I click the plus button below them, they disappear to make way for the user-provided DNS. So it seems I can only have either static entries or DHCP entries but not both. Is there some way I can accomplish this by changing settings in a .conf file somewhere perhaps?
Since Mac OS X 10.6.3 you computer keeps a list of all DNS servers it has ever seen and automatically picks the fastest one for you. So you can simply add the new one and be happy with it, as your Mac will remember the ones provided by DHCP. If you want to define a strict order after which Mac OS does it´s lookups, best follow this how-to.
Additionally, to see the current DNS search order (and search domains), run: