Jenkins and the mysterious Accept timed out error

Not so long ago I’ve been trying to set up a new Jenkins job on a freshly created Jenkins slave. Unfortunately for me the first attempt of running the Maven build resulted in a failure:

ERROR: Aborted Maven execution for InterruptedIOException
java.net.SocketTimeoutException: Accept timed out
 at java.net.PlainSocketImpl.socketAccept(Native Method)
 at java.net.AbstractPlainSocketImpl.accept(AbstractPlainSocketImpl.java:398)
 at java.net.ServerSocket.implAccept(ServerSocket.java:530)
 at java.net.ServerSocket.accept(ServerSocket.java:498)
 at hudson.maven.AbstractMavenProcessFactory$SocketHandler$AcceptorImpl.accept(AbstractMavenProcessFactory.java:211)
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
 at java.lang.reflect.Method.invoke(Method.java:606)
 at hudson.remoting.RemoteInvocationHandler$RPCRequest.perform(RemoteInvocationHandler.java:309)
 at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:290)
 at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:249)
 at hudson.remoting.UserRequest.perform(UserRequest.java:118)
 at hudson.remoting.UserRequest.perform(UserRequest.java:48)
 at hudson.remoting.Request$2.run(Request.java:328)
 at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
 at java.util.concurrent.FutureTask.run(FutureTask.java:262)
 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
 at java.lang.Thread.run(Thread.java:745)

Jenkins normally kicks off Maven builds by forking a Maven process on the system and then using Maven extensions to become part of the build process. So let’s have a look at an example command Jenkins runs for a Maven build:

[OpenAM] $ /jdk8/bin/java -cp /jenkins/maven31-agent.jar:/apache-maven-3.2.3/boot/plexus-classworlds-2.5.1.jar:/apache-maven-3.2.3/conf/logging jenkins.maven3.agent.Maven31Main /apache-maven-3.2.3 /jenkins/slave.jar /jenkins/maven31-interceptor.jar /jenkins/maven3-interceptor-commons.jar 57186

Here, that last parameter, 57186, looks quite suspiciously like a random port number. Could it be that Jenkins tries to connect to this local port and this connection fails resulting in the above stacktrace? Well, easy to test, let’s run the following iptables command on the box: