// https://tools.ietf.org/html/rfc1035

package main

import (
	"encoding/binary"
	"io/ioutil"
	"net"
)

// UDPDNSHeaderLen is the length of UDP dns msg header
const UDPDNSHeaderLen = 12

// TCPDNSHEADERLen is the length of TCP dns msg header
const TCPDNSHEADERLen = 2 + UDPDNSHeaderLen

// MaxUDPDNSLen is the max size of udp dns request.
// https://tools.ietf.org/html/rfc1035#section-4.2.1
// Messages carried by UDP are restricted to 512 bytes (not counting the IP
// or UDP headers).  Longer messages are truncated and the TC bit is set in
// the header.
// TODO: If the request length > 512 then the client will send TCP packets instead,
// so we should also serve tcp requests.
const MaxUDPDNSLen = 512

type dnstun struct {
	*proxy
	raddr string
}

// DNSTunProxy returns a dns forwarder. client -> dns.udp -> glider -> forwarder -> remote dns addr
func DNSTunProxy(addr, raddr string, upProxy Proxy) (Proxy, error) {
	s := &dnstun{
		proxy: newProxy(addr, upProxy),
		raddr: raddr,
	}

	return s, nil
}

// ListenAndServe .
func (s *dnstun) ListenAndServe() {
	l, err := net.ListenPacket("udp", s.addr)
	if err != nil {
		logf("failed to listen on %s: %v", s.addr, err)
		return
	}
	defer l.Close()

	logf("listening UDP on %s", s.addr)

	for {
		data := make([]byte, MaxUDPDNSLen)

		n, clientAddr, err := l.ReadFrom(data)
		if err != nil {
			logf("DNS local read error: %v", err)
			continue
		}

		data = data[:n]

		go func() {
			// TODO: check domain rules and get a proper upstream name server.
			domain := getDomain(data)

			rc, err := s.GetProxy(s.raddr).Dial("tcp", s.raddr)
			if err != nil {
				logf("failed to connect to server %v: %v", s.raddr, err)
				return
			}
			defer rc.Close()

			logf("proxy-dnstun %s, %s <-> %s", domain, clientAddr.String(), s.raddr)

			// 2 bytes length after tcp header, before dns message
			length := make([]byte, 2)
			binary.BigEndian.PutUint16(length, uint16(len(data)))
			rc.Write(length)
			rc.Write(data)

			resp, err := ioutil.ReadAll(rc)
			if err != nil {
				logf("error in ioutil.ReadAll: %s\n", err)
				return
			}

			// length is not needed in udp dns response. (2 bytes)
			// SEE RFC1035, section 4.2.2 TCP: The message is prefixed with a two byte length field which gives the message length, excluding the two byte length field.
			if len(resp) > 2 {
				msg := resp[2:]
				_, err = l.WriteTo(msg, clientAddr)
				if err != nil {
					logf("error in local write: %s\n", err)
				}
			}

		}()
	}
}

// getDomain from dns request playload, return []byte like:
// []byte{'w', 'w', 'w', '.', 'm', 's', 'n', '.', 'c', 'o', 'm', '.'}
// []byte("www.msn.com.")
func getDomain(p []byte) []byte {
	var ret []byte

	for i := UDPDNSHeaderLen; i < len(p); {
		l := int(p[i])

		if l == 0 {
			break
		}

		ret = append(ret, p[i+1:i+l+1]...)
		ret = append(ret, '.')

		i = i + l + 1
	}

	// TODO: check here
	// domain name could not be null, so the length of ret always >= 1?
	return ret[:len(ret)-1]
}