Add a DNS client example. (#10237)

Motivation:

It seems that there is no DNS client example in Netty project so far.

Modification:

Add a Netty DNS client example.

Result:

More examples

example/pom.xml

@@ -108,6 +108,11 @@
       <artifactId>netty-codec-haproxy</artifactId>
       <version>${project.version}</version>
     </dependency>
+    <dependency>
+      <groupId>${project.groupId}</groupId>
+      <artifactId>netty-codec-dns</artifactId>
+      <version>${project.version}</version>
+    </dependency>
 
     <dependency>
       <groupId>com.google.protobuf</groupId>

example/src/main/java/io/netty/example/dns/udp/DnsClient.java

@@ -0,0 +1,107 @@
+/*
+ * Copyright 2020 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:
+ *
+ *   http://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.example.dns.udp;
+
+import java.net.InetSocketAddress;
+import java.util.concurrent.TimeUnit;
+
+import io.netty.bootstrap.Bootstrap;
+import io.netty.buffer.ByteBufUtil;
+import io.netty.channel.Channel;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.channel.ChannelInitializer;
+import io.netty.channel.ChannelPipeline;
+import io.netty.channel.EventLoopGroup;
+import io.netty.channel.SimpleChannelInboundHandler;
+import io.netty.channel.nio.NioEventLoopGroup;
+import io.netty.channel.socket.DatagramChannel;
+import io.netty.channel.socket.nio.NioDatagramChannel;
+import io.netty.handler.codec.dns.DatagramDnsQuery;
+import io.netty.handler.codec.dns.DatagramDnsQueryEncoder;
+import io.netty.handler.codec.dns.DatagramDnsResponse;
+import io.netty.handler.codec.dns.DatagramDnsResponseDecoder;
+import io.netty.handler.codec.dns.DefaultDnsQuestion;
+import io.netty.handler.codec.dns.DnsQuery;
+import io.netty.handler.codec.dns.DnsQuestion;
+import io.netty.handler.codec.dns.DnsRawRecord;
+import io.netty.handler.codec.dns.DnsRecord;
+import io.netty.handler.codec.dns.DnsRecordType;
+import io.netty.handler.codec.dns.DnsSection;
+import io.netty.util.NetUtil;
+
+public final class DnsClient {
+
+    private static final String QUERY_DOMAIN = "www.example.com";
+    private static final int DNS_SERVER_PORT = 53;
+    private static final String DNS_SERVER_HOST = "8.8.8.8";
+
+    private DnsClient() { }
+
+    private static void handleQueryResp(DatagramDnsResponse msg) {
+        if (msg.count(DnsSection.QUESTION) > 0) {
+            DnsQuestion question = msg.recordAt(DnsSection.QUESTION, 0);
+            System.out.printf("name: %s%n", question.name());
+        }
+        for (int i = 0, count = msg.count(DnsSection.ANSWER); i < count; i++) {
+            DnsRecord record = msg.recordAt(DnsSection.ANSWER, i);
+            if (record.type() == DnsRecordType.A) {
+                //just print the IP after query
+                DnsRawRecord raw = (DnsRawRecord) record;
+                System.out.println(NetUtil.bytesToIpAddress(ByteBufUtil.getBytes(raw.content())));
+            }
+        }
+    }
+
+    public static void main(String[] args) throws Exception {
+        InetSocketAddress addr = new InetSocketAddress(DNS_SERVER_HOST, DNS_SERVER_PORT);
+        EventLoopGroup group = new NioEventLoopGroup();
+        try {
+            Bootstrap b = new Bootstrap();
+            b.group(group)
+             .channel(NioDatagramChannel.class)
+             .handler(new ChannelInitializer<DatagramChannel>() {
+                 @Override
+                 protected void initChannel(DatagramChannel ch) throws Exception {
+                     ChannelPipeline p = ch.pipeline();
+                     p.addLast(new DatagramDnsQueryEncoder())
+                     .addLast(new DatagramDnsResponseDecoder())
+                     .addLast(new SimpleChannelInboundHandler<DatagramDnsResponse>() {
+                        @Override
+                        protected void channelRead0(ChannelHandlerContext ctx, DatagramDnsResponse msg) {
+                            try {
+                                handleQueryResp(msg);
+                            } finally {
+                                ctx.close();
+                            }
+                        }
+                    });
+                 }
+             });
+            final Channel ch = b.bind(0).sync().channel();
+            DnsQuery query = new DatagramDnsQuery(null, addr, 1).setRecord(
+                    DnsSection.QUESTION,
+                    new DefaultDnsQuestion(QUERY_DOMAIN, DnsRecordType.A));
+            ch.writeAndFlush(query).sync();
+            boolean succ = ch.closeFuture().await(10, TimeUnit.SECONDS);
+            if (!succ) {
+                System.err.println("dns query timeout!");
+                ch.close().sync();
+            }
+        } finally {
+            group.shutdownGracefully();
+        }
+    }
+}