Unable to connect to host using DNS resolver on Java 9+

Expected behavior

Using Java 16/17 I can connect with a host by host name successfully, just as with Java 8

Actual behavior

When running the same code with Java 8, I can successfully connect to a remote host by name, but with Java 9, 16 and 17 I cannot.

Stacktrace: java.lang.ExceptionInInitializerError at xxx Caused by: java.net.UnknownHostException: failed to resolve ‘somehost’ after 10 queries at io.netty.resolver.dns.DnsResolveContext.finishResolve(DnsResolveContext.java:1046) at io.netty.resolver.dns.DnsResolveContext.tryToFinishResolve(DnsResolveContext.java:999) at io.netty.resolver.dns.DnsResolveContext.query(DnsResolveContext.java:417) at io.netty.resolver.dns.DnsResolveContext.onResponse(DnsResolveContext.java:628) at io.netty.resolver.dns.DnsResolveContext.access$400(DnsResolveContext.java:65) at io.netty.resolver.dns.DnsResolveContext$2.operationComplete(DnsResolveContext.java:461) at io.netty.util.concurrent.DefaultPromise.notifyListener0(DefaultPromise.java:578) at io.netty.util.concurrent.DefaultPromise.notifyListeners0(DefaultPromise.java:571) at io.netty.util.concurrent.DefaultPromise.notifyListenersNow(DefaultPromise.java:550) at io.netty.util.concurrent.DefaultPromise.notifyListeners(DefaultPromise.java:491) at io.netty.util.concurrent.DefaultPromise.setValue0(DefaultPromise.java:616) at io.netty.util.concurrent.DefaultPromise.setSuccess0(DefaultPromise.java:605) at io.netty.util.concurrent.DefaultPromise.trySuccess(DefaultPromise.java:104) at io.netty.resolver.dns.DnsQueryContext.trySuccess(DnsQueryContext.java:200) at io.netty.resolver.dns.DnsQueryContext.finish(DnsQueryContext.java:192) at io.netty.resolver.dns.DnsNameResolver$DnsResponseHandler.channelRead(DnsNameResolver.java:1301) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:103) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) at io.netty.channel.nio.AbstractNioMessageChannel$NioMessageUnsafe.read(AbstractNioMessageChannel.java:97) at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:719) at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:655) at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:581) at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:493) at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:986) at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) at java.base/java.lang.Thread.run(Thread.java:833)

Steps to reproduce

I am using Redisson, which is using Netty:

    Config config = new Config();
    config.useSingleServer()
            .setAddress("redis://somehost:5379");
    CLIENT = Redisson.create(config);

Minimal yet complete reproducer code (or URL to code)

Netty version

tried 4.1.68-FINAL and 4.1.70-FINAL (via Redisson)

JVM version (e.g. java -version)

9.0.4, 17.0.1

OS version (e.g. uname -a)

Window 10 21H2

[flupec]

Faced same issue with Spring WebClient that’s based on Netty. DNS suffixes cannot be resolved on Windows and Java 9+. I think this hack is responsible for such behaviour.

Why is reflection used here? You can acquire instance of ResolverConfiguration via static method, no?

[dlehammer]

I’ve experienced the symptom similarly to @flupec in a Spring Boot context, albeit in a Linux environment (Fedora 36 & Java 17). I.e. Netty unable to resolve the local machine alias ~ $HOSTNAME.

I found a workaround; explicitly adding the machine alias to /etc/hosts seemed to appease Netty (no other services has needed this, and ping etc. resolved the machine alias fine before I made any changes!)

127.0.0.1 my-machine-alias my-machine-alias.localdomain

I too am unable to understand why Netty tries to handle a responsibility owned by the OS, i.e. I somewhat understand the desire to cache resolved DNS-data, but caching it inside each service running on a OS is just asking for support tickets…

I’m not that familiar with the Windows DNS cache support, but on Linux most distributions provide a OS level cache for this specific purpose, ex. https://manpages.ubuntu.com/manpages/trusty/man8/dnscache.8.html 🤓