// +build linux

package obfssh

import (
	"encoding/binary"
	"fmt"
	"net"
	"syscall"
	"unsafe"
)

const (
	// SO_ORIGINAL_DST in linux/netfilter_ipv4.h
	soOriginalDst = 80
)

func getOriginDst(c net.Conn) (net.Addr, error) {
	var sockaddr syscall.RawSockaddrAny
	var len = unsafe.Sizeof(sockaddr)

	cc, ok := c.(*net.TCPConn)
	if !ok {
		return nil, fmt.Errorf("only tcp socket supported")
	}

	f, err := cc.File()
	if err != nil {
		return nil, err
	}

	defer f.Close()

	// get original ip destination, in C like this
	//
	// struct sockaddr addr;
	// memset(&addr, 0, sizeof(addr);
	// int len = sizeof(addr);
	// getsocketopt(fd, SOL_IP, SO_ORIGINAL_DST, &addr, &len);
	//
	_, _, errno := syscall.Syscall6(sysGetSockOpt, f.Fd(),
		uintptr(syscall.SOL_IP), uintptr(soOriginalDst),
		uintptr(unsafe.Pointer(&sockaddr)),
		uintptr(unsafe.Pointer(&len)), 0)

	if errno != 0 {
		return nil, fmt.Errorf("syscall error %d", errno)
	}

	var port uint16
	var ip net.IP

	switch sockaddr.Addr.Family {
	case syscall.AF_INET:
		a := (*syscall.RawSockaddrInet4)(unsafe.Pointer(&sockaddr))
		ip = net.IP(a.Addr[0:])
		port = ntohl(a.Port)
	case syscall.AF_INET6:
		a := (*syscall.RawSockaddrInet6)(unsafe.Pointer(&sockaddr))
		ip = net.IP(a.Addr[0:])
		port = ntohl(a.Port)
	default:
		return nil, fmt.Errorf("unknown socket family: %d",
			sockaddr.Addr.Family)
	}

	addr := &net.TCPAddr{IP: ip, Port: int(port)}
	return addr, nil
}

func ntohl(a uint16) uint16 {
	b := make([]byte, 2)
	binary.BigEndian.PutUint16(b, a)
	c := binary.LittleEndian.Uint16(b)
	return c
}