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   /*
    * Copyright 2015 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.util.internal.UnstableApi;
  
  /**
   * The default {@link DnsRecordDecoder} implementation.
   *
   * @see DefaultDnsRecordEncoder
   */
  @UnstableApi
  public class DefaultDnsRecordDecoder implements DnsRecordDecoder {
  
      static final String ROOT = ".";
  
      /**
       * Creates a new instance.
       */
      protected DefaultDnsRecordDecoder() { }
  
      @Override
      public final DnsQuestion decodeQuestion(ByteBuf in) throws Exception {
          String name = decodeName(in);
          DnsRecordType type = DnsRecordType.valueOf(in.readUnsignedShort());
          int qClass = in.readUnsignedShort();
          return new DefaultDnsQuestion(name, type, qClass);
      }
  
      @Override
      public final <T extends DnsRecord> T decodeRecord(ByteBuf in) throws Exception {
          final int startOffset = in.readerIndex();
          final String name = decodeName(in);
  
          final int endOffset = in.writerIndex();
          if (endOffset - in.readerIndex() < 10) {
              // Not enough data
              in.readerIndex(startOffset);
              return null;
          }
  
          final DnsRecordType type = DnsRecordType.valueOf(in.readUnsignedShort());
          final int aClass = in.readUnsignedShort();
          final long ttl = in.readUnsignedInt();
          final int length = in.readUnsignedShort();
          final int offset = in.readerIndex();
  
          if (endOffset - offset < length) {
              // Not enough data
              in.readerIndex(startOffset);
              return null;
          }
  
          @SuppressWarnings("unchecked")
          T record = (T) decodeRecord(name, type, aClass, ttl, in, offset, length);
          in.readerIndex(offset + length);
          return record;
      }
  
      /**
       * Decodes a record from the information decoded so far by {@link #decodeRecord(ByteBuf)}.
       *
       * @param name the domain name of the record
       * @param type the type of the record
       * @param dnsClass the class of the record
       * @param timeToLive the TTL of the record
       * @param in the {@link ByteBuf} that contains the RDATA
       * @param offset the start offset of the RDATA in {@code in}
       * @param length the length of the RDATA
       *
       * @return a {@link DnsRawRecord}. Override this method to decode RDATA and return other record implementation.
       */
      protected DnsRecord decodeRecord(
              String name, DnsRecordType type, int dnsClass, long timeToLive,
              ByteBuf in, int offset, int length) throws Exception {
  
          // DNS message compression means that domain names may contain "pointers" to other positions in the packet
          // to build a full message. This means the indexes are meaningful and we need the ability to reference the
          // indexes un-obstructed, and thus we cannot use a slice here.
          // See https://www.ietf.org/rfc/rfc1035 [4.1.4. Message compression]
          if (type == DnsRecordType.PTR) {
              return new DefaultDnsPtrRecord(
                      name, dnsClass, timeToLive, decodeName0(in.duplicate().setIndex(offset, offset + length)));
          }
          if (type == DnsRecordType.CNAME || type == DnsRecordType.NS) {
             return new DefaultDnsRawRecord(name, type, dnsClass, timeToLive,
                                            DnsCodecUtil.decompressDomainName(
                                                    in.duplicate().setIndex(offset, offset + length)));
         }
         return new DefaultDnsRawRecord(
                 name, type, dnsClass, timeToLive, in.retainedDuplicate().setIndex(offset, offset + length));
     }
 
     /**
      * Retrieves a domain name given a buffer containing a DNS packet. If the
      * name contains a pointer, the position of the buffer will be set to
      * directly after the pointer's index after the name has been read.
      *
      * @param in the byte buffer containing the DNS packet
      * @return the domain name for an entry
      */
     protected String decodeName0(ByteBuf in) {
         return decodeName(in);
     }
 
     /**
      * Retrieves a domain name given a buffer containing a DNS packet. If the
      * name contains a pointer, the position of the buffer will be set to
      * directly after the pointer's index after the name has been read.
      *
      * @param in the byte buffer containing the DNS packet
      * @return the domain name for an entry
      */
     public static String decodeName(ByteBuf in) {
         return DnsCodecUtil.decodeDomainName(in);
     }
 }