good idea, with usability issues Rated 3 out of 5 stars
Few usability/user friendliness issues:
- It does not remember state sometimes
- you can't clearly tell if the dns caching is on or off
- mouse highlights icon, but left-click does nothing (it's not clear if it should - most icons do, so it'd be expected IMO)
And as suggested, it'd be good ot have a more distinctive icon for it. That gear is more of a 'settings' icon than a dns cache.