})
	ti.teardownCmds = append(ti.teardownCmds, &exec.Cmd{
		Path: xname,
		Args: []string{"ifconfig", ti.name, "destroy"},
	})
	return nil
}

func (ti *testInterface) setPointToPoint(suffix int) error {
	ti.name = fmt.Sprintf("gif%d", suffix)
	xname, err := exec.LookPath("ifconfig")
	if err != nil {
		return err
	}
	ti.setupCmds = append(ti.setupCmds, &exec.Cmd{
		Path: xname,

	})
	ti.teardownCmds = append(ti.teardownCmds, &exec.Cmd{
		Path: xname,
		Args: []string{"ip", "link", "delete", ti.name, "type", "dummy"},
	})
	return nil
}

func (ti *testInterface) setPointToPoint(suffix int) error {
	ti.name = fmt.Sprintf("gotest%d", suffix)
	xname, err := exec.LookPath("ip")
	if err != nil {
		return err
	}
	ti.setupCmds = append(ti.setupCmds, &exec.Cmd{
		Path: xname,

	// harm anyone.
	local, remote := "169.254.0.1", "169.254.0.254"
	ip := ParseIP(remote)
	for i := 0; i < 3; i++ {
		ti := &testInterface{local: local, remote: remote}
		if err := ti.setPointToPoint(5963 + i); err != nil {
			t.Skipf("test requires external command: %v", err)
		}
		if err := ti.setup(); err != nil {
			if e := err.Error(); strings.Contains(e, "No such device") && strings.Contains(e, "gre0") {