code: func getAllDefaultRoutes() ([]Route, error) {
  	routes, err := v4File.extract()
  	if err != nil {
  		return nil, err
  	}
  	v6Routes, _ := v6File.extract()
  	routes = append(routes, v6Routes...)
  	if len(routes) == 0 {
  		return nil, noRoutesError{
  			message: fmt.Sprintf("no default routes found in %q or %q", v4File.name, v6File.name),
  		}
  	}
  	return routes, nil
  }
docs: // getAllDefaultRoutes obtains IPv4 and IPv6 default routes on the node. If unable
  // to read the IPv4 routing info file, we return an error. If unable to read the IPv6
  // routing info file (which is optional), we'll just use the IPv4 route information.
  // Using all the routing info, if no default routes are found, an error is returned.