This is the patch I wrote for [Armeria](https://github.com/line/armeria):

https://github.com/line/armeria/pull/155/files

As shown in the PR above, we could write a decorating `NameResolver` that works for all `NameResolver` implementations. However, I'm not sure what the name of that class would be. Any suggestions?
 /cc @slandelle 
 InflightDnsNameResolver ? Not sure its the best name though

> Am 27.04.2016 um 12:18 schrieb Trustin Lee notifications@github.com:
> 
> This is the patch I wrote for Armeria:
> 
> https://github.com/line/armeria/pull/155/files
> 
> As shown in the PR above, we could write a decorating NameResolver that works for all NameResolver implementations. However, I'm not sure what the name of that class would be. Any suggestions?
> 
> —
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> Reply to this email directly or view it on GitHub
 @trustin WDYT ?
 FlyingQueriesAwareDnsNameResolver ? Pro is that such name will make Spring people jealous ;-) 
 InProgressAware?
 @trustin also we can just make it package private for now and find a good name later if we feel we not have a good one atm.
 Alright. Let me cook some tomorrow.
 any news ?
 Fixed by https://github.com/netty/netty/pull/5267

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DNS name resolution queries should not be sent while the same query is in progress already.
When you attempt to make a lot of connection attempts to the same target host at the same time and our DNS resolver does not have a record for it in the cache, the DNS resolver will send as many DNS queries as the number of connection attempts.

As a result, DNS server will reject or drop the requests, making the name resolution attempt fail.

To fix this, we could keep the list of name resolution queries and subscribe to the future of the matching query instead of sending a duplicate query.

Will work on a fix.

resolver-dns/src/main/java/io/netty/resolver/dns/DnsAddressResolverGroup.java
resolver-dns/src/main/java/io/netty/resolver/dns/DnsAddressResolverGroup.java resolver-dns/src/main/java/io/netty/resolver/dns/InflightNameResolver.java
diff --git a/resolver-dns/src/main/java/io/netty/resolver/dns/DnsAddressResolverGroup.java b/resolver-dns/src/main/java/io/netty/resolver/dns/DnsAddressResolverGroup.java
index 4fb88c44d8d..1f666076c74 100644
--- a/resolver-dns/src/main/java/io/netty/resolver/dns/DnsAddressResolverGroup.java
+++ b/resolver-dns/src/main/java/io/netty/resolver/dns/DnsAddressResolverGroup.java
@@ -22,13 +22,20 @@
 import io.netty.channel.socket.DatagramChannel;
 import io.netty.resolver.AddressResolver;
 import io.netty.resolver.AddressResolverGroup;
+import io.netty.resolver.InetSocketAddressResolver;
+import io.netty.resolver.NameResolver;
 import io.netty.util.concurrent.EventExecutor;
+import io.netty.util.concurrent.Promise;
 import io.netty.util.internal.StringUtil;
 import io.netty.util.internal.UnstableApi;
 
+import java.net.InetAddress;
 import java.net.InetSocketAddress;
+import java.util.List;
+import java.util.concurrent.ConcurrentMap;
 
 import static io.netty.resolver.dns.DnsNameResolver.ANY_LOCAL_ADDR;
+import static io.netty.util.internal.PlatformDependent.newConcurrentHashMap;
 
 /**
  * A {@link AddressResolverGroup} of {@link DnsNameResolver}s.
@@ -40,6 +47,9 @@ public class DnsAddressResolverGroup extends AddressResolverGroup<InetSocketAddr
     private final InetSocketAddress localAddress;
     private final DnsServerAddresses nameServerAddresses;
 
+    private final ConcurrentMap<String, Promise<InetAddress>> resolvesInProgress = newConcurrentHashMap();
+    private final ConcurrentMap<String, Promise<List<InetAddress>>> resolveAllsInProgress = newConcurrentHashMap();
+
     public DnsAddressResolverGroup(
             Class<? extends DatagramChannel> channelType, DnsServerAddresses nameServerAddresses) {
         this(channelType, ANY_LOCAL_ADDR, nameServerAddresses);
@@ -83,11 +93,16 @@ protected AddressResolver<InetSocketAddress> newResolver(
             EventLoop eventLoop, ChannelFactory<? extends DatagramChannel> channelFactory,
             InetSocketAddress localAddress, DnsServerAddresses nameServerAddresses) throws Exception {
 
-        return new DnsNameResolverBuilder(eventLoop)
-                .channelFactory(channelFactory)
-                .localAddress(localAddress)
-                .nameServerAddresses(nameServerAddresses)
-                .build()
-                .asAddressResolver();
+        final NameResolver<InetAddress> resolver = new InflightNameResolver<InetAddress>(
+                eventLoop,
+                new DnsNameResolverBuilder(eventLoop)
+                        .channelFactory(channelFactory)
+                        .localAddress(localAddress)
+                        .nameServerAddresses(nameServerAddresses)
+                        .build(),
+                resolvesInProgress,
+                resolveAllsInProgress);
+
+        return new InetSocketAddressResolver(eventLoop, resolver);
     }
 }
diff --git a/resolver-dns/src/main/java/io/netty/resolver/dns/InflightNameResolver.java b/resolver-dns/src/main/java/io/netty/resolver/dns/InflightNameResolver.java
new file mode 100644
index 00000000000..cb93f9a2127
--- /dev/null
+++ b/resolver-dns/src/main/java/io/netty/resolver/dns/InflightNameResolver.java
@@ -0,0 +1,131 @@
+/*
+ * Copyright 2016 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.resolver.dns;
+
+import io.netty.resolver.NameResolver;
+import io.netty.util.concurrent.EventExecutor;
+import io.netty.util.concurrent.Future;
+import io.netty.util.concurrent.FutureListener;
+import io.netty.util.concurrent.Promise;
+import io.netty.util.internal.StringUtil;
+
+import java.util.List;
+import java.util.concurrent.ConcurrentMap;
+
+import static io.netty.util.internal.ObjectUtil.checkNotNull;
+
+// FIXME(trustin): Find a better name and move it to the 'resolver' module.
+final class InflightNameResolver<T> implements NameResolver<T> {
+
+    private final EventExecutor executor;
+    private final NameResolver<T> delegate;
+    private final ConcurrentMap<String, Promise<T>> resolvesInProgress;
+    private final ConcurrentMap<String, Promise<List<T>>> resolveAllsInProgress;
+
+    InflightNameResolver(EventExecutor executor, NameResolver<T> delegate,
+                         ConcurrentMap<String, Promise<T>> resolvesInProgress,
+                         ConcurrentMap<String, Promise<List<T>>> resolveAllsInProgress) {
+
+        this.executor = checkNotNull(executor, "executor");
+        this.delegate = checkNotNull(delegate, "delegate");
+        this.resolvesInProgress = checkNotNull(resolvesInProgress, "resolvesInProgress");
+        this.resolveAllsInProgress = checkNotNull(resolveAllsInProgress, "resolveAllsInProgress");
+    }
+
+    @Override
+    public Future<T> resolve(String inetHost) {
+        return resolve(inetHost, executor.<T>newPromise());
+    }
+
+    @Override
+    public Future<List<T>> resolveAll(String inetHost) {
+        return resolveAll(inetHost, executor.<List<T>>newPromise());
+    }
+
+    @Override
+    public void close() {
+        delegate.close();
+    }
+
+    @Override
+    public Promise<T> resolve(String inetHost, Promise<T> promise) {
+        return resolve(resolvesInProgress, inetHost, promise, false);
+    }
+
+    @Override
+    public Promise<List<T>> resolveAll(String inetHost, Promise<List<T>> promise) {
+        return resolve(resolveAllsInProgress, inetHost, promise, true);
+    }
+
+    private <U> Promise<U> resolve(
+            final ConcurrentMap<String, Promise<U>> resolveMap,
+            final String inetHost, final Promise<U> promise, boolean resolveAll) {
+
+        final Promise<U> earlyPromise = resolveMap.putIfAbsent(inetHost, promise);
+        if (earlyPromise != null) {
+            // Name resolution for the specified inetHost is in progress already.
+            if (earlyPromise.isDone()) {
+                transferResult(earlyPromise, promise);
+            } else {
+                earlyPromise.addListener(new FutureListener<U>() {
+                    @Override
+                    public void operationComplete(Future<U> f) throws Exception {
+                        transferResult(f, promise);
+                    }
+                });
+            }
+        } else {
+            try {
+                if (resolveAll) {
+                    @SuppressWarnings("unchecked")
+                    final Promise<List<T>> castPromise = (Promise<List<T>>) promise; // U is List<T>
+                    delegate.resolveAll(inetHost, castPromise);
+                } else {
+                    @SuppressWarnings("unchecked")
+                    final Promise<T> castPromise = (Promise<T>) promise; // U is T
+                    delegate.resolve(inetHost, castPromise);
+                }
+            } finally {
+                if (promise.isDone()) {
+                    resolveMap.remove(inetHost);
+                } else {
+                    promise.addListener(new FutureListener<U>() {
+                        @Override
+                        public void operationComplete(Future<U> f) throws Exception {
+                            resolveMap.remove(inetHost);
+                        }
+                    });
+                }
+            }
+        }
+
+        return promise;
+    }
+
+    private static <T> void transferResult(Future<T> src, Promise<T> dst) {
+        if (src.isSuccess()) {
+            dst.trySuccess(src.getNow());
+        } else {
+            dst.tryFailure(src.cause());
+        }
+    }
+
+    @Override
+    public String toString() {
+        return StringUtil.simpleClassName(this) + '(' + delegate + ')';
+    }
+}
