//go:build linux
// +build linux

package tcp

import (
	"syscall"

	"github.com/v2fly/v2ray-core/v4/common/net"
	"github.com/v2fly/v2ray-core/v4/transport/internet"
)

const SO_ORIGINAL_DST = 80 // nolint: golint,stylecheck

func GetOriginalDestination(conn internet.Connection) (net.Destination, error) {
	sysrawconn, f := conn.(syscall.Conn)
	if !f {
		return net.Destination{}, newError("unable to get syscall.Conn")
	}
	rawConn, err := sysrawconn.SyscallConn()
	if err != nil {
		return net.Destination{}, newError("failed to get sys fd").Base(err)
	}
	var dest net.Destination
	err = rawConn.Control(func(fd uintptr) {
		var remoteIP net.IP
		switch addr := conn.RemoteAddr().(type) {
		case *net.TCPAddr:
			remoteIP = addr.IP
		case *net.UDPAddr:
			remoteIP = addr.IP
		default:
			newError("failed to call getsockopt").WriteToLog()
			return
		}
		if remoteIP.To4() != nil {
			// ipv4
			addr, err := syscall.GetsockoptIPv6Mreq(int(fd), syscall.IPPROTO_IP, SO_ORIGINAL_DST)
			if err != nil {
				newError("failed to call getsockopt").Base(err).WriteToLog()
				return
			}
			ip := net.IPAddress(addr.Multiaddr[4:8])
			port := uint16(addr.Multiaddr[2])<<8 + uint16(addr.Multiaddr[3])
			dest = net.TCPDestination(ip, net.Port(port))
		} else {
			// ipv6
			addr, err := syscall.GetsockoptIPv6MTUInfo(int(fd), syscall.IPPROTO_IPV6, SO_ORIGINAL_DST)
			if err != nil {
				newError("failed to call getsockopt").Base(err).WriteToLog()
				return
			}
			ip := net.IPAddress(addr.Addr.Addr[:])
			port := net.PortFromBytes([]byte{byte(addr.Addr.Port), byte(addr.Addr.Port >> 8)})
			dest = net.TCPDestination(ip, port)
		}
	})
	if err != nil {
		return net.Destination{}, newError("failed to control connection").Base(err)
	}
	if !dest.IsValid() {
		return net.Destination{}, newError("failed to call getsockopt")
	}
	return dest, nil
}