"Ooooops: Network interface is invalid." from the experience here has always indicated there was something in one or more /etc/sysconfig/network-scripts/ifcfg-xxxx files the ClearOS validity checker didn't like. I suspect it only allows a certain number of limited options - have had that message before when I added lines to the files that are perfectly valid in the rhel world, but unfortunately the ClearOS checker know nothing about and considered it an issue. In the early days even adding a comment (a line with a '#' in the first column) would trigger this 'error' situation and the "Oooooops" message - thankfully that was fixed, but am still not convinced that it will correctly pass all possible combinations of rhel valid entries in the ifcfg-xxxxxx files.

See the relevant section in "/usr/share/doc/initscripts-9.37.2/sysconfig.txt" (or whatever doc version you have) for the very extensive list of options.

Have no idea what changes, if any, DNSThingy makes to the ifg-xxxx files - so cannot advise on this.. { must say the name 'DNSThingy' puts me off } However, as Nick alluded to, if you go outside whatever everybody else does, you will usually end up some time or another finding bug(s) or challenges that were previously unknown as no-one has trodden that exact route before... Not saying this is necessarily the case here, but this is what you are leaving yourself open to...

I have a feeling with all your testing that a full gateway may be more appropriate.