/*
    * 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.resolver.dns;
  
  import io.netty.channel.ChannelFactory;
  import io.netty.channel.EventLoop;
  import io.netty.channel.ReflectiveChannelFactory;
  import io.netty.channel.socket.DatagramChannel;
  import io.netty.channel.socket.InternetProtocolFamily;
  import io.netty.channel.socket.SocketChannel;
  import io.netty.resolver.HostsFileEntriesResolver;
  import io.netty.resolver.ResolvedAddressTypes;
  import io.netty.util.concurrent.Future;
  import io.netty.util.internal.EmptyArrays;
  import io.netty.util.internal.ObjectUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.net.InetSocketAddress;
  import java.net.SocketAddress;
  import java.util.Arrays;
  import java.util.LinkedHashSet;
  import java.util.Set;
  
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  import static io.netty.util.internal.ObjectUtil.intValue;
  
  /**
   * A {@link DnsNameResolver} builder.
   */
  public final class DnsNameResolverBuilder {
  
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(DnsNameResolverBuilder.class);
      static final SocketAddress DEFAULT_LOCAL_ADDRESS = new InetSocketAddress(0);
  
      volatile EventLoop eventLoop;
      private ChannelFactory<? extends DatagramChannel> datagramChannelFactory;
      private ChannelFactory<? extends SocketChannel> socketChannelFactory;
      private boolean retryOnTimeout;
  
      private DnsCache resolveCache;
      private DnsCnameCache cnameCache;
      private AuthoritativeDnsServerCache authoritativeDnsServerCache;
      private SocketAddress localAddress = DEFAULT_LOCAL_ADDRESS;
      private Integer minTtl;
      private Integer maxTtl;
      private Integer negativeTtl;
      private long queryTimeoutMillis = -1;
      private ResolvedAddressTypes resolvedAddressTypes = DnsNameResolver.DEFAULT_RESOLVE_ADDRESS_TYPES;
      private boolean completeOncePreferredResolved;
      private boolean recursionDesired = true;
      private int maxQueriesPerResolve = -1;
      private boolean traceEnabled;
      private int maxPayloadSize = 4096;
      private boolean optResourceEnabled = true;
      private HostsFileEntriesResolver hostsFileEntriesResolver = HostsFileEntriesResolver.DEFAULT;
      private DnsServerAddressStreamProvider dnsServerAddressStreamProvider =
              DnsServerAddressStreamProviders.platformDefault();
      private DnsServerAddressStream queryDnsServerAddressStream;
      private DnsQueryLifecycleObserverFactory dnsQueryLifecycleObserverFactory =
              NoopDnsQueryLifecycleObserverFactory.INSTANCE;
      private String[] searchDomains;
      private int ndots = -1;
      private boolean decodeIdn = true;
  
      private int maxNumConsolidation;
      private DnsNameResolverChannelStrategy datagramChannelStrategy = DnsNameResolverChannelStrategy.ChannelPerResolver;
  
      /**
       * Creates a new builder.
       */
      public DnsNameResolverBuilder() {
      }
  
      /**
       * Creates a new builder.
       *
       * @param eventLoop the {@link EventLoop} which will perform the communication with the DNS
       * servers.
       */
      public DnsNameResolverBuilder(EventLoop eventLoop) {
          eventLoop(eventLoop);
      }
  
      /**
       * Sets the {@link EventLoop} which will perform the communication with the DNS servers.
      *
      * @param eventLoop the {@link EventLoop}
      * @return {@code this}
      */
     public DnsNameResolverBuilder eventLoop(EventLoop eventLoop) {
         this.eventLoop = eventLoop;
         return this;
     }
 
     @Deprecated
     protected ChannelFactory<? extends DatagramChannel> channelFactory() {
         return this.datagramChannelFactory;
     }
 
     ChannelFactory<? extends DatagramChannel> datagramChannelFactory() {
         return this.datagramChannelFactory;
     }
 
     /**
      * Sets the {@link ChannelFactory} that will create a {@link DatagramChannel}.
      * <p>
      * If <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> should be supported as well it is required
      * to call the {@link #socketChannelFactory(ChannelFactory)} or {@link #socketChannelType(Class)} method.
      *
      * @param datagramChannelFactory the {@link ChannelFactory}
      * @return {@code this}
      * @deprecated use {@link #datagramChannelFactory(ChannelFactory)}
      */
     @Deprecated
     public DnsNameResolverBuilder channelFactory(ChannelFactory<? extends DatagramChannel> datagramChannelFactory) {
         datagramChannelFactory(datagramChannelFactory);
         return this;
     }
 
     /**
      * Sets the {@link ChannelFactory} that will create a {@link DatagramChannel}.
      * <p>
      * If <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> should be supported as well it is required
      * to call the {@link #socketChannelFactory(ChannelFactory)} or {@link #socketChannelType(Class)} method.
      *
      * @param datagramChannelFactory the {@link ChannelFactory}
      * @return {@code this}
      */
     public DnsNameResolverBuilder datagramChannelFactory(
             ChannelFactory<? extends DatagramChannel> datagramChannelFactory) {
         this.datagramChannelFactory = datagramChannelFactory;
         return this;
     }
 
     /**
      * Sets the {@link ChannelFactory} as a {@link ReflectiveChannelFactory} of this type.
      * Use as an alternative to {@link #channelFactory(ChannelFactory)}.
      * <p>
      * If <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> should be supported as well it is required
      * to call the {@link #socketChannelFactory(ChannelFactory)} or {@link #socketChannelType(Class)} method.
      *
      * @param channelType the type
      * @return {@code this}
      * @deprecated use {@link #datagramChannelType(Class)}
      */
     @Deprecated
     public DnsNameResolverBuilder channelType(Class<? extends DatagramChannel> channelType) {
         return datagramChannelFactory(new ReflectiveChannelFactory<DatagramChannel>(channelType));
     }
 
     /**
      * Sets the {@link ChannelFactory} as a {@link ReflectiveChannelFactory} of this type.
      * Use as an alternative to {@link #datagramChannelFactory(ChannelFactory)}.
      * <p>
      * If <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> should be supported as well it is required
      * to call the {@link #socketChannelFactory(ChannelFactory)} or {@link #socketChannelType(Class)} method.
      *
      * @param channelType the type
      * @return {@code this}
      */
     public DnsNameResolverBuilder datagramChannelType(Class<? extends DatagramChannel> channelType) {
         return datagramChannelFactory(new ReflectiveChannelFactory<DatagramChannel>(channelType));
     }
 
     /**
      * Sets the {@link ChannelFactory} that will create a {@link SocketChannel} for
      * <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> if needed.
      * <p>
      * TCP fallback is <strong>not</strong> enabled by default and must be enabled by providing a non-null
      * {@link ChannelFactory} for this method.
      *
      * @param channelFactory the {@link ChannelFactory} or {@code null}
      *                       if <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> should not be supported.
      *                       By default, TCP fallback is not enabled.
      * @return {@code this}
      */
     public DnsNameResolverBuilder socketChannelFactory(ChannelFactory<? extends SocketChannel> channelFactory) {
         return socketChannelFactory(channelFactory, false);
     }
 
     /**
      * Sets the {@link ChannelFactory} as a {@link ReflectiveChannelFactory} of this type for
      * <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> if needed.
      * Use as an alternative to {@link #socketChannelFactory(ChannelFactory)}.
      * <p>
      * TCP fallback is <strong>not</strong> enabled by default and must be enabled by providing a non-null
      * {@code channelType} for this method.
      *
      * @param channelType the type or {@code null} if <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a>
      *                    should not be supported. By default, TCP fallback is not enabled.
      * @return {@code this}
      */
     public DnsNameResolverBuilder socketChannelType(Class<? extends SocketChannel> channelType) {
         return socketChannelType(channelType, false);
     }
 
     /**
      * Sets the {@link ChannelFactory} that will create a {@link SocketChannel} for
      * <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> if needed.
      * <p>
      * TCP fallback is <strong>not</strong> enabled by default and must be enabled by providing a non-null
      * {@link ChannelFactory} for this method.
      *
      * @param channelFactory the {@link ChannelFactory} or {@code null}
      *                       if <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> should not be supported.
      *                       By default, TCP fallback is not enabled.
      * @param retryOnTimeout if {@code true} the {@link DnsNameResolver} will also fallback to TCP if a timeout
      *                       was detected, if {@code false} it will only try to use TCP if the response was marked
      *                       as truncated.
      * @return {@code this}
      */
     public DnsNameResolverBuilder socketChannelFactory(
             ChannelFactory<? extends SocketChannel> channelFactory, boolean retryOnTimeout) {
         this.socketChannelFactory = channelFactory;
         this.retryOnTimeout = retryOnTimeout;
         return this;
     }
 
     /**
      * Sets the {@link ChannelFactory} as a {@link ReflectiveChannelFactory} of this type for
      * <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a> if needed.
      * Use as an alternative to {@link #socketChannelFactory(ChannelFactory)}.
      * <p>
      * TCP fallback is <strong>not</strong> enabled by default and must be enabled by providing a non-null
      * {@code channelType} for this method.
      *
      * @param channelType the type or {@code null} if <a href="https://tools.ietf.org/html/rfc7766">TCP fallback</a>
      *                    should not be supported. By default, TCP fallback is not enabled.
      * @param retryOnTimeout if {@code true} the {@link DnsNameResolver} will also fallback to TCP if a timeout
      *                       was detected, if {@code false} it will only try to use TCP if the response was marked
      *                       as truncated.
      * @return {@code this}
      */
     public DnsNameResolverBuilder socketChannelType(
             Class<? extends SocketChannel> channelType, boolean retryOnTimeout) {
         if (channelType == null) {
             return socketChannelFactory(null, retryOnTimeout);
         }
         return socketChannelFactory(new ReflectiveChannelFactory<SocketChannel>(channelType), retryOnTimeout);
     }
 
     /**
      * Sets the cache for resolution results.
      *
      * @param resolveCache the DNS resolution results cache
      * @return {@code this}
      */
     public DnsNameResolverBuilder resolveCache(DnsCache resolveCache) {
         this.resolveCache  = resolveCache;
         return this;
     }
 
     /**
      * Sets the cache for {@code CNAME} mappings.
      *
      * @param cnameCache the cache used to cache {@code CNAME} mappings for a domain.
      * @return {@code this}
      */
     public DnsNameResolverBuilder cnameCache(DnsCnameCache cnameCache) {
         this.cnameCache  = cnameCache;
         return this;
     }
 
     /**
      * Set the factory used to generate objects which can observe individual DNS queries.
      * @param lifecycleObserverFactory the factory used to generate objects which can observe individual DNS queries.
      * @return {@code this}
      */
     public DnsNameResolverBuilder dnsQueryLifecycleObserverFactory(DnsQueryLifecycleObserverFactory
                                                                            lifecycleObserverFactory) {
         this.dnsQueryLifecycleObserverFactory = checkNotNull(lifecycleObserverFactory, "lifecycleObserverFactory");
         return this;
     }
 
     /**
      * Sets the cache for authoritative NS servers
      *
      * @param authoritativeDnsServerCache the authoritative NS servers cache
      * @return {@code this}
      * @deprecated Use {@link #authoritativeDnsServerCache(AuthoritativeDnsServerCache)}
      */
     @Deprecated
     public DnsNameResolverBuilder authoritativeDnsServerCache(DnsCache authoritativeDnsServerCache) {
         this.authoritativeDnsServerCache = new AuthoritativeDnsServerCacheAdapter(authoritativeDnsServerCache);
         return this;
     }
 
     /**
      * Sets the cache for authoritative NS servers
      *
      * @param authoritativeDnsServerCache the authoritative NS servers cache
      * @return {@code this}
      */
     public DnsNameResolverBuilder authoritativeDnsServerCache(AuthoritativeDnsServerCache authoritativeDnsServerCache) {
         this.authoritativeDnsServerCache = authoritativeDnsServerCache;
         return this;
     }
 
     /**
      * Configure the address that will be used to bind too. If {@code null} the default will be used.
      * @param localAddress the bind address
      * @return {@code this}
      */
     public DnsNameResolverBuilder localAddress(SocketAddress localAddress) {
         this.localAddress = localAddress;
         return this;
     }
 
     /**
      * Sets the minimum and maximum TTL of the cached DNS resource records (in seconds). If the TTL of the DNS
      * resource record returned by the DNS server is less than the minimum TTL or greater than the maximum TTL,
      * this resolver will ignore the TTL from the DNS server and use the minimum TTL or the maximum TTL instead
      * respectively.
      * The default value is {@code 0} and {@link Integer#MAX_VALUE}, which practically tells this resolver to
      * respect the TTL from the DNS server.
      *
      * @param minTtl the minimum TTL
      * @param maxTtl the maximum TTL
      * @return {@code this}
      */
     public DnsNameResolverBuilder ttl(int minTtl, int maxTtl) {
         this.maxTtl = maxTtl;
         this.minTtl = minTtl;
         return this;
     }
 
     /**
      * Sets the TTL of the cache for the failed DNS queries (in seconds).
      *
      * @param negativeTtl the TTL for failed cached queries
      * @return {@code this}
      */
     public DnsNameResolverBuilder negativeTtl(int negativeTtl) {
         this.negativeTtl = negativeTtl;
         return this;
     }
 
     /**
      * Sets the timeout of each DNS query performed by this resolver (in milliseconds).
      * {@code 0} disables the timeout. If not set or a negative number is set, the default timeout is used.
      *
      * @param queryTimeoutMillis the query timeout
      * @return {@code this}
      */
     public DnsNameResolverBuilder queryTimeoutMillis(long queryTimeoutMillis) {
         this.queryTimeoutMillis = queryTimeoutMillis;
         return this;
     }
 
     /**
      * Compute a {@link ResolvedAddressTypes} from some {@link InternetProtocolFamily}s.
      * An empty input will return the default value, based on "java.net" System properties.
      * Valid inputs are (), (IPv4), (IPv6), (Ipv4, IPv6) and (IPv6, IPv4).
      * @param internetProtocolFamilies a valid sequence of {@link InternetProtocolFamily}s
      * @return a {@link ResolvedAddressTypes}
      */
     public static ResolvedAddressTypes computeResolvedAddressTypes(InternetProtocolFamily... internetProtocolFamilies) {
         if (internetProtocolFamilies == null || internetProtocolFamilies.length == 0) {
             return DnsNameResolver.DEFAULT_RESOLVE_ADDRESS_TYPES;
         }
         if (internetProtocolFamilies.length > 2) {
             throw new IllegalArgumentException("No more than 2 InternetProtocolFamilies");
         }
 
         switch(internetProtocolFamilies[0]) {
             case IPv4:
                 return (internetProtocolFamilies.length >= 2
                         && internetProtocolFamilies[1] == InternetProtocolFamily.IPv6) ?
                         ResolvedAddressTypes.IPV4_PREFERRED: ResolvedAddressTypes.IPV4_ONLY;
             case IPv6:
                 return (internetProtocolFamilies.length >= 2
                         && internetProtocolFamilies[1] == InternetProtocolFamily.IPv4) ?
                         ResolvedAddressTypes.IPV6_PREFERRED: ResolvedAddressTypes.IPV6_ONLY;
             default:
                 throw new IllegalArgumentException(
                         "Couldn't resolve ResolvedAddressTypes from InternetProtocolFamily array");
         }
     }
 
     /**
      * Sets the list of the protocol families of the address resolved.
      * You can use {@link DnsNameResolverBuilder#computeResolvedAddressTypes(InternetProtocolFamily...)}
      * to get a {@link ResolvedAddressTypes} out of some {@link InternetProtocolFamily}s.
      *
      * @param resolvedAddressTypes the address types
      * @return {@code this}
      */
     public DnsNameResolverBuilder resolvedAddressTypes(ResolvedAddressTypes resolvedAddressTypes) {
         this.resolvedAddressTypes = resolvedAddressTypes;
         return this;
     }
 
     /**
      * If {@code true} {@link DnsNameResolver#resolveAll(String)} will notify the returned {@link Future} as
      * soon as all queries for the preferred address-type are complete.
      *
      * @param completeOncePreferredResolved {@code true} to enable, {@code false} to disable.
      * @return {@code this}
      */
     public DnsNameResolverBuilder completeOncePreferredResolved(boolean completeOncePreferredResolved) {
         this.completeOncePreferredResolved = completeOncePreferredResolved;
         return this;
     }
 
     /**
      * Sets if this resolver has to send a DNS query with the RD (recursion desired) flag set.
      *
      * @param recursionDesired true if recursion is desired
      * @return {@code this}
      */
     public DnsNameResolverBuilder recursionDesired(boolean recursionDesired) {
         this.recursionDesired = recursionDesired;
         return this;
     }
 
     /**
      * Sets the maximum allowed number of DNS queries to send when resolving a host name.
      *
      * @param maxQueriesPerResolve the max number of queries
      * @return {@code this}
      */
     public DnsNameResolverBuilder maxQueriesPerResolve(int maxQueriesPerResolve) {
         this.maxQueriesPerResolve = maxQueriesPerResolve;
         return this;
     }
 
     /**
      * Sets if this resolver should generate the detailed trace information in an exception message so that
      * it is easier to understand the cause of resolution failure.
      *
      * @param traceEnabled true if trace is enabled
      * @return {@code this}
      * @deprecated Prefer to {@linkplain #dnsQueryLifecycleObserverFactory(DnsQueryLifecycleObserverFactory) configure}
      * a {@link LoggingDnsQueryLifeCycleObserverFactory} instead.
      */
     @Deprecated
     public DnsNameResolverBuilder traceEnabled(boolean traceEnabled) {
         this.traceEnabled = traceEnabled;
         return this;
     }
 
     /**
      * Sets the capacity of the datagram packet buffer (in bytes).  The default value is {@code 4096} bytes.
      *
      * @param maxPayloadSize the capacity of the datagram packet buffer
      * @return {@code this}
      */
     public DnsNameResolverBuilder maxPayloadSize(int maxPayloadSize) {
         this.maxPayloadSize = maxPayloadSize;
         return this;
     }
 
     /**
      * Enable the automatic inclusion of a optional records that tries to give the remote DNS server a hint about
      * how much data the resolver can read per response. Some DNSServer may not support this and so fail to answer
      * queries. If you find problems you may want to disable this.
      *
      * @param optResourceEnabled if optional records inclusion is enabled
      * @return {@code this}
      */
     public DnsNameResolverBuilder optResourceEnabled(boolean optResourceEnabled) {
         this.optResourceEnabled = optResourceEnabled;
         return this;
     }
 
     /**
      * @param hostsFileEntriesResolver the {@link HostsFileEntriesResolver} used to first check
      *                                 if the hostname is locally aliased.
      * @return {@code this}
      */
     public DnsNameResolverBuilder hostsFileEntriesResolver(HostsFileEntriesResolver hostsFileEntriesResolver) {
         this.hostsFileEntriesResolver = hostsFileEntriesResolver;
         return this;
     }
 
     protected DnsServerAddressStreamProvider nameServerProvider() {
         return this.dnsServerAddressStreamProvider;
     }
 
     /**
      * Set the {@link DnsServerAddressStreamProvider} which is used to determine which DNS server is used to resolve
      * each hostname.
      * @return {@code this}.
      */
     public DnsNameResolverBuilder nameServerProvider(DnsServerAddressStreamProvider dnsServerAddressStreamProvider) {
         this.dnsServerAddressStreamProvider =
                 checkNotNull(dnsServerAddressStreamProvider, "dnsServerAddressStreamProvider");
         return this;
     }
 
     protected DnsServerAddressStream queryServerAddressStream() {
         return this.queryDnsServerAddressStream;
     }
 
     /**
      * Set the {@link DnsServerAddressStream} which provides the server address for DNS queries.
      * @return {@code this}.
      */
     public DnsNameResolverBuilder queryServerAddressStream(DnsServerAddressStream queryServerAddressStream) {
         this.queryDnsServerAddressStream =
                 checkNotNull(queryServerAddressStream, "queryServerAddressStream");
         return this;
     }
 
     /**
      * Set the list of search domains of the resolver.
      *
      * @param searchDomains the search domains
      * @return {@code this}
      */
     public DnsNameResolverBuilder searchDomains(Iterable<String> searchDomains) {
         checkNotNull(searchDomains, "searchDomains");
 
         final Set<String> domains = new LinkedHashSet<String>(4);
 
         for (String f : searchDomains) {
             if (f == null) {
                 break;
             }
 
             // Avoid duplicate entries.
             domains.add(f);
         }
 
         this.searchDomains = domains.toArray(EmptyArrays.EMPTY_STRINGS);
         return this;
     }
 
   /**
    * Set the number of dots which must appear in a name before an initial absolute query is made.
    * The default value is {@code 1}.
    *
    * @param ndots the ndots value
    * @return {@code this}
    */
     public DnsNameResolverBuilder ndots(int ndots) {
         this.ndots = ndots;
         return this;
     }
 
    DnsCache getOrNewCache() {
         if (this.resolveCache != null) {
             return this.resolveCache;
         }
         return new DefaultDnsCache(intValue(minTtl, 0), intValue(maxTtl, Integer.MAX_VALUE), intValue(negativeTtl, 0));
     }
 
    AuthoritativeDnsServerCache getOrNewAuthoritativeDnsServerCache() {
         if (this.authoritativeDnsServerCache != null) {
             return this.authoritativeDnsServerCache;
         }
         return new DefaultAuthoritativeDnsServerCache(
                 intValue(minTtl, 0), intValue(maxTtl, Integer.MAX_VALUE),
                 // Let us use the sane ordering as DnsNameResolver will be used when returning
                 // nameservers from the cache.
                 new NameServerComparator(DnsNameResolver.preferredAddressType(resolvedAddressTypes).addressType()));
     }
 
     private DnsServerAddressStream newQueryServerAddressStream(
             DnsServerAddressStreamProvider dnsServerAddressStreamProvider) {
         return new ThreadLocalNameServerAddressStream(dnsServerAddressStreamProvider);
     }
 
    DnsCnameCache getOrNewCnameCache() {
         if (this.cnameCache != null) {
             return this.cnameCache;
         }
         return new DefaultDnsCnameCache(
                 intValue(minTtl, 0), intValue(maxTtl, Integer.MAX_VALUE));
     }
 
     /**
      * Set if domain / host names should be decoded to unicode when received.
      * See <a href="https://tools.ietf.org/html/rfc3492">rfc3492</a>.
      *
      * @param decodeIdn if should get decoded
      * @return {@code this}
      */
     public DnsNameResolverBuilder decodeIdn(boolean decodeIdn) {
         this.decodeIdn = decodeIdn;
         return this;
     }
 
     /**
      * Set the maximum size of the cache that is used to consolidate lookups for different hostnames when in-flight.
      * This means if multiple lookups are done for the same hostname and still in-flight only one actual query will
      * be made and the result will be cascaded to the others.
      *
      * @param maxNumConsolidation the maximum lookups to consolidate (different hostnames), or {@code 0} if
      *                            no consolidation should be performed.
      * @return {@code this}
      */
     public DnsNameResolverBuilder consolidateCacheSize(int maxNumConsolidation) {
         this.maxNumConsolidation = ObjectUtil.checkPositiveOrZero(maxNumConsolidation, "maxNumConsolidation");
         return this;
     }
 
     /**
      * Set the strategy that is used to determine how a {@link DatagramChannel} is used by the resolver for sending
      * queries over UDP protocol.
      *
      * @param datagramChannelStrategy  the {@link DnsNameResolverChannelStrategy} to use when doing queries over
      *                                 UDP protocol.
      * @return {@code this}
      */
     public DnsNameResolverBuilder datagramChannelStrategy(DnsNameResolverChannelStrategy datagramChannelStrategy) {
         this.datagramChannelStrategy = ObjectUtil.checkNotNull(datagramChannelStrategy, "datagramChannelStrategy");
         return this;
     }
 
     /**
      * Returns a new {@link DnsNameResolver} instance.
      *
      * @return a {@link DnsNameResolver}
      */
     public DnsNameResolver build() {
         if (eventLoop == null) {
             throw new IllegalStateException("eventLoop should be specified to build a DnsNameResolver.");
         }
 
         if (resolveCache != null && (minTtl != null || maxTtl != null || negativeTtl != null)) {
             logger.debug("resolveCache and TTLs are mutually exclusive. TTLs are ignored.");
         }
 
         if (cnameCache != null && (minTtl != null || maxTtl != null || negativeTtl != null)) {
             logger.debug("cnameCache and TTLs are mutually exclusive. TTLs are ignored.");
         }
 
         if (authoritativeDnsServerCache != null && (minTtl != null || maxTtl != null || negativeTtl != null)) {
             logger.debug("authoritativeDnsServerCache and TTLs are mutually exclusive. TTLs are ignored.");
         }
 
         DnsCache resolveCache = getOrNewCache();
         DnsCnameCache cnameCache = getOrNewCnameCache();
         AuthoritativeDnsServerCache authoritativeDnsServerCache = getOrNewAuthoritativeDnsServerCache();
 
         DnsServerAddressStream queryDnsServerAddressStream = this.queryDnsServerAddressStream != null ?
                 this.queryDnsServerAddressStream : newQueryServerAddressStream(dnsServerAddressStreamProvider);
 
         return new DnsNameResolver(
                 eventLoop,
                 datagramChannelFactory,
                 socketChannelFactory,
                 retryOnTimeout,
                 resolveCache,
                 cnameCache,
                 authoritativeDnsServerCache,
                 localAddress,
                 dnsQueryLifecycleObserverFactory,
                 queryTimeoutMillis,
                 resolvedAddressTypes,
                 recursionDesired,
                 maxQueriesPerResolve,
                 traceEnabled,
                 maxPayloadSize,
                 optResourceEnabled,
                 hostsFileEntriesResolver,
                 dnsServerAddressStreamProvider,
                 queryDnsServerAddressStream,
                 searchDomains,
                 ndots,
                 decodeIdn,
                 completeOncePreferredResolved,
                 maxNumConsolidation,
                 datagramChannelStrategy);
     }
 
     /**
      * Creates a copy of this {@link DnsNameResolverBuilder}
      *
      * @return {@link DnsNameResolverBuilder}
      */
     public DnsNameResolverBuilder copy() {
         DnsNameResolverBuilder copiedBuilder = new DnsNameResolverBuilder();
 
         if (eventLoop != null) {
             copiedBuilder.eventLoop(eventLoop);
         }
 
         if (datagramChannelFactory != null) {
             copiedBuilder.datagramChannelFactory(datagramChannelFactory);
         }
 
         copiedBuilder.socketChannelFactory(socketChannelFactory, retryOnTimeout);
 
         if (resolveCache != null) {
             copiedBuilder.resolveCache(resolveCache);
         }
 
         if (cnameCache != null) {
             copiedBuilder.cnameCache(cnameCache);
         }
         if (maxTtl != null && minTtl != null) {
             copiedBuilder.ttl(minTtl, maxTtl);
         }
 
         if (negativeTtl != null) {
             copiedBuilder.negativeTtl(negativeTtl);
         }
 
         if (authoritativeDnsServerCache != null) {
             copiedBuilder.authoritativeDnsServerCache(authoritativeDnsServerCache);
         }
 
         if (dnsQueryLifecycleObserverFactory != null) {
             copiedBuilder.dnsQueryLifecycleObserverFactory(dnsQueryLifecycleObserverFactory);
         }
 
         copiedBuilder.queryTimeoutMillis(queryTimeoutMillis);
         copiedBuilder.resolvedAddressTypes(resolvedAddressTypes);
         copiedBuilder.recursionDesired(recursionDesired);
         copiedBuilder.maxQueriesPerResolve(maxQueriesPerResolve);
         copiedBuilder.traceEnabled(traceEnabled);
         copiedBuilder.maxPayloadSize(maxPayloadSize);
         copiedBuilder.optResourceEnabled(optResourceEnabled);
         copiedBuilder.hostsFileEntriesResolver(hostsFileEntriesResolver);
 
         if (dnsServerAddressStreamProvider != null) {
             copiedBuilder.nameServerProvider(dnsServerAddressStreamProvider);
         }
 
         if (queryDnsServerAddressStream != null) {
             copiedBuilder.queryServerAddressStream(queryDnsServerAddressStream);
         }
 
         if (searchDomains != null) {
             copiedBuilder.searchDomains(Arrays.asList(searchDomains));
         }
 
         copiedBuilder.ndots(ndots);
         copiedBuilder.decodeIdn(decodeIdn);
         copiedBuilder.completeOncePreferredResolved(completeOncePreferredResolved);
         copiedBuilder.localAddress(localAddress);
         copiedBuilder.consolidateCacheSize(maxNumConsolidation);
         copiedBuilder.datagramChannelStrategy(datagramChannelStrategy);
         return copiedBuilder;
     }
 }