As an alternative, you could add a `TuntapRequirement` which detects if those are present, and installs the tuntap formula if not:

```ruby
class TuntapRequirement < Requirement
  default_formula 'tuntap'

  def satisfied?
    File.exist? '/dev/tun' # or some such
  end

  def message; <<-EOS.undent
    shrewsoft-vpn-client requires an installation of tuntap to function.
    EOS
  end
end

...

depends_on TuntapRequirement
```