code: func getForwardingKernelParam(ipVer ipVersion, iface string) string {
  	switch ipVer {
  	case ipv4:
  		return "/proc/sys/net/ipv4/ip_forward"
  	case ipv6:
  		if iface == "" {
  			iface = "default"
  		}
  		return fmt.Sprintf("/proc/sys/net/ipv6/conf/%s/forwarding", iface)
  	default:
  		return ""
  	}
  }
docs: // Get kernel param path saying whether IPv${ipVer} traffic is being forwarded
  // on particular interface. Interface may be specified for IPv6 only. If
  // `iface` is empty, `default` will be assumed, which represents default value
  // for new interfaces.