/*
 * Copyright 2018 The Outline Authors
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the author nor the
 *    names of its contributors may be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * Definitions for the DNS protocol.
 */

#ifndef BADVPN_MISC_DNS_PROTO_H
#define BADVPN_MISC_DNS_PROTO_H

#include <stdint.h>
#include <misc/byteorder.h>

B_START_PACKED
struct dns_header {
  uint16_t id;
  uint8_t qr_opcode_aa_tc_rd;
  uint8_t ra_z_rcode;
  uint16_t qdcount;
  uint16_t ancount;
  uint16_t nscount;
  uint16_t arcount;
} B_PACKED;
B_END_PACKED

// DNS header field masks
#define DNS_QR 0x80
#define DNS_TC 0x02
#define DNS_Z  0x70
#define DNS_RCODE 0x0F

#define DNS_ID_STRLEN 6

static void dns_get_header_id_str(char* id_str, uint8_t* data) {
  struct dns_header* dnsh = (struct dns_header*)data;
  sprintf(id_str, "%u", dnsh->id);
  id_str[DNS_ID_STRLEN - 1] = '\0';
}

static int dns_check(const uint8_t *data, int data_len,
                     struct dns_header *out_header) {
  ASSERT(data_len >= 0)
  ASSERT(out_header)

  // parse DNS header
  if (data_len < sizeof(struct dns_header)) {
    return 0;
  }
  memcpy(out_header, data, sizeof(*out_header));

  // verify DNS header is request
  return (out_header->qr_opcode_aa_tc_rd & DNS_QR) == 0 /* query */
            && (out_header->ra_z_rcode & DNS_Z) == 0 /* Z is Zero */
            && out_header->qdcount > 0 /* some questions */
            && !out_header->nscount && !out_header->ancount /* no answers */;
}

// Synthesizes a truncated DNS response by modifying |query|.
static void dns_synthesize_truncated_response(struct dns_header *query) {
  query->qr_opcode_aa_tc_rd |= DNS_QR;
  query->qr_opcode_aa_tc_rd |= DNS_TC;
  query->ra_z_rcode &= ~DNS_RCODE;
  query->ancount = query->qdcount;
  query->nscount = 0;
  query->arcount = 0;
}

 #endif  // BADVPN_MISC_DNS_PROTO_H