gistfile1.txt

  devise_for(
    :users,
    only: :sessions,
    controllers: { sessions: 'devise/sessions' }
  )

  constraints MainAppConstraint.new do
    devise_for(
      :users,
      skip: :sessions,
      controllers: {
        registrations: 'registrations',
        passwords: 'devise/passwords',
        omniauth_callbacks: 'tenon/omniauth_callbacks'
      }
    )
  end

gistfile2.txt

class MainAppConstraint #:nodoc:#
  def matches?(request)
    subdomain = request.subdomain
    !subdomain.present? || protected_subdomains.include?(subdomain)
  end

  private

  def protected_subdomains
    %w(
      www
      admin
      ftp
      gopher
    )
  end
end