// Copyright 2018 Google Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Package tun contains methods to open TAP and TUN devices.
package tun

import (
	"syscall"
	"unsafe"
)

// Open opens the specified TUN device, sets it to non-blocking mode, and
// returns its file descriptor.
func Open(name string) (int, error) {
	return open(name, syscall.IFF_TUN|syscall.IFF_NO_PI)
}

// OpenTAP opens the specified TAP device, sets it to non-blocking mode, and
// returns its file descriptor.
func OpenTAP(name string) (int, error) {
	return open(name, syscall.IFF_TAP|syscall.IFF_NO_PI)
}

func open(name string, flags uint16) (int, error) {
	fd, err := syscall.Open("/dev/net/tun", syscall.O_RDWR, 0)
	if err != nil {
		return -1, err
	}

	var ifr struct {
		name  [16]byte
		flags uint16
		_     [22]byte
	}

	copy(ifr.name[:], name)
	ifr.flags = flags
	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), syscall.TUNSETIFF, uintptr(unsafe.Pointer(&ifr)))
	if errno != 0 {
		syscall.Close(fd)
		return -1, errno
	}

	if err = syscall.SetNonblock(fd, true); err != nil {
		syscall.Close(fd)
		return -1, err
	}

	return fd, nil
}