/*
    * Copyright 2019 The Netty Project
    *
    * The Netty Project licenses this file to you 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:
    *
    *   https://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 io.netty.handler.codec.dns;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufUtil;
  import io.netty.handler.codec.CorruptedFrameException;
  import io.netty.handler.codec.TooLongFrameException;
  import io.netty.util.CharsetUtil;
  
  import static io.netty.handler.codec.dns.DefaultDnsRecordDecoder.*;
  
  final class DnsCodecUtil {
  
      // See https://datatracker.ietf.org/doc/html/rfc1035#section-2.3.4
      static final int MAX_DOMAIN_NAME_LENGTH = 255;
  
      // The length of a message that is sent over TCP is encoded in two bytes.
      // See https://tools.ietf.org/html/rfc7766#section-8
      static final int MAX_TCP_MESSAGE_LENGTH = 0xFFFF;
  
      // A decoded name is at most 255 characters and every label contributes at least 2 ("x."), so a valid name has
      // at most 127 labels and therefore never needs to follow more compression pointers than that. Bounding the hops
      // per name keeps the decoding cost of each name constant instead of proportional to the message size.
      static final int MAX_COMPRESSION_POINTERS = 127;
  
      // The RFC does not define a maximum number of records per section (the count is a 16-bit value). The cost of
      // decoding a single record is bounded (see MAX_DOMAIN_NAME_LENGTH and MAX_COMPRESSION_POINTERS), so this only
      // limits the total work per message. It is high enough that a full 64 KiB TCP message of minimal records with
      // compressed names (e.g. A records, 16 bytes each) still decodes.
      static final int MAX_RECORDS_PER_SECTION = 4096;
  
      private DnsCodecUtil() {
          // Util class
      }
  
      /**
       * Writes the length of the message into the first two bytes of the given {@link ByteBuf} which were reserved for
       * it, as defined by <a href="https://tools.ietf.org/html/rfc7766#section-8">RFC 7766</a>.
       *
       * @throws IllegalArgumentException if the message is too large for the length to fit into two bytes.
       */
      static void setTcpMessageLength(ByteBuf buf) {
          int length = buf.readableBytes() - 2;
          if (length > MAX_TCP_MESSAGE_LENGTH) {
              throw new IllegalArgumentException("DNS message length " + length + " exceeds the maximum of " +
                      MAX_TCP_MESSAGE_LENGTH + " bytes that can be sent over TCP");
          }
          buf.setShort(0, length);
      }
  
      static void encodeDomainName(String name, ByteBuf buf) {
          if (ROOT.equals(name)) {
              // Root domain
              buf.writeByte(0);
              return;
          }
  
          int totalLength = 0;
          final String[] labels = name.split("\\.");
          for (int i = 0; i < labels.length; i++) {
              String label = labels[i];
              final int labelLen = label.length();
              if (labelLen == 0) {
                  if (i == labels.length - 1) {
                      // zero-length label at the end means the end of the name.
                      break;
                  } else {
                      throw new IllegalArgumentException("DNS name contains empty label: " + name);
                  }
              }
              if (labelLen > 63) {
                  throw new IllegalArgumentException(
                          "DNS label length " + labelLen + " exceeds maximum of 63: " + name);
              }
              int idx = label.indexOf('\0');
              if (idx != -1) {
                  throw new IllegalArgumentException(
                          "DNS label contains null byte at index " + idx);
              }
              totalLength += 1 + labelLen;
              if (totalLength > MAX_DOMAIN_NAME_LENGTH) {
                  throw new IllegalArgumentException(
                          "DNS name exceeds maximum length of " + MAX_DOMAIN_NAME_LENGTH + ": " + name);
              }
             buf.writeByte(labelLen);
             ByteBufUtil.writeAscii(buf, label);
         }
 
         buf.writeByte(0); // marks end of name field
     }
 
     /**
      * Returns the initial capacity to use for the {@link StringBuilder} that accumulates a decoded domain name.
      * A decoded name can never exceed {@link #MAX_DOMAIN_NAME_LENGTH} characters (see the check in
      * {@link #decodeDomainName(ByteBuf)}), so the capacity must not scale with {@code readable}, the number of
      * bytes left in the whole message: doing so would let a message with a huge amount of trailing data (e.g.
      * many more records after this name) force an oversized allocation for every single name decoded from it.
      */
     static int calculateMaxNameLength(int readable) {
         return Math.min(readable, MAX_DOMAIN_NAME_LENGTH);
     }
 
     /**
      * Ensures the record count of a section, as announced in the header of a DNS message, does not exceed
      * {@link #MAX_RECORDS_PER_SECTION}.
      *
      * @throws TooLongFrameException if the count is too large.
      */
     static int checkRecordCount(DnsSection section, int count) {
         if (count > MAX_RECORDS_PER_SECTION) {
             throw new TooLongFrameException(
                     section + " record count must be <= " + MAX_RECORDS_PER_SECTION + " but was " + count);
         }
         return count;
     }
 
     static String decodeDomainName(ByteBuf in) {
         int position = -1;
         int pointers = 0;
         final int end = in.writerIndex();
         final int readable = in.readableBytes();
 
         // Looking at the spec we should always have at least enough readable bytes to read a byte here but it seems
         // some servers do not respect this for empty names. So just workaround this and return an empty name in this
         // case.
         //
         // See:
         // - https://github.com/netty/netty/issues/5014
         // - https://www.ietf.org/rfc/rfc1035.txt , Section 3.1
         if (readable == 0) {
             return ROOT;
         }
 
         final StringBuilder name = new StringBuilder(calculateMaxNameLength(readable));
         while (in.isReadable()) {
             final int len = in.readUnsignedByte();
             final boolean pointer = (len & 0xc0) == 0xc0;
             if (pointer) {
                 if (position == -1) {
                     position = in.readerIndex() + 1;
                 }
 
                 if (!in.isReadable()) {
                     throw new CorruptedFrameException("truncated pointer in a name");
                 }
 
                 final int next = (len & 0x3f) << 8 | in.readUnsignedByte();
                 if (next >= end) {
                     throw new CorruptedFrameException("name has an out-of-range pointer");
                 }
                 in.readerIndex(next);
 
                 // Bounds the work per name, which also terminates loops.
                 if (++pointers > MAX_COMPRESSION_POINTERS) {
                     throw new CorruptedFrameException(
                             "name contains more than " + MAX_COMPRESSION_POINTERS + " compression pointers");
                 }
             } else if (len != 0) {
                 if (!in.isReadable(len)) {
                     throw new CorruptedFrameException("truncated label in a name");
                 }
                 // See https://datatracker.ietf.org/doc/html/rfc1035#section-2.3.4
                 if (len > 63) {
                     throw new TooLongFrameException("label must be <= 63 but was " + len);
                 }
                 name.append(in.toString(in.readerIndex(), len, CharsetUtil.UTF_8)).append('.');
                 in.skipBytes(len);
                 // See https://datatracker.ietf.org/doc/html/rfc1035#section-2.3.4
                 if (name.length() > MAX_DOMAIN_NAME_LENGTH) {
                     throw new TooLongFrameException(
                             "domain name must be <= " + MAX_DOMAIN_NAME_LENGTH + " but was " + name.length());
                 }
             } else { // len == 0
                 break;
             }
         }
 
         if (position != -1) {
             in.readerIndex(position);
         }
 
         if (name.length() == 0) {
             return ROOT;
         }
 
         if (name.charAt(name.length() - 1) != '.') {
             name.append('.');
         }
 
         return name.toString();
     }
 
     /**
      * Decompress pointer data.
      * @param compression compressed data
      * @return decompressed data
      */
     static ByteBuf decompressDomainName(ByteBuf compression) {
         String domainName = decodeDomainName(compression);
         ByteBuf result = compression.alloc().buffer(domainName.length() << 1);
         try {
             encodeDomainName(domainName, result);
         } catch (Throwable cause) {
             result.release();
             throw cause;
         }
         return result;
     }
 }