tomfitzhenry/MakeTwoDnsRequests.java

## README.md

      
    Raw
  

              README.md
            
          
    Submitted to Oracle, but posted here too, because I'm not sure if this bug will make its way to the OpenJDK bug tracker.
Versions

$ java -version
openjdk version "1.8.0_121"
OpenJDK Runtime Environment (build 1.8.0_121-8u121-b13-0ubuntu1.16.04.2-b13)
OpenJDK 64-Bit Server VM (build 25.121-b13, mixed mode)
$ javac -version
javac 1.8.0_121

Steps to reproduce

(You might have to run tshark and kill as root.)
$ javac MakeTwoDnsRequests.java
$ tshark -i any -f "src port 53" -Y "dns.qry.type eq 1" -n -T fields -e dns.qry.name -e dns.a > without-jmx.txt 2>/dev/null &
[1] 17460
$ java MakeTwoDnsRequests
$ kill 17460
$ cat without-jmx.txt
www.example.com 93.184.216.34
www.example.com 93.184.216.34
$ 
$ tshark -i any -f "src port 53" -Y "dns.qry.type eq 1" -n -T fields -e dns.qry.name -e dns.a > with-jmx.txt 2>/dev/null &
[1] 17495
$ java -Dcom.sun.management.jmxremote MakeTwoDnsRequests
$ kill 17495
$ cat with-jmx.txt

Expected

I expect two DNS requests are sent:
$ cat with-jmx.txt
www.example.com 93.184.216.34
www.example.com 93.184.216.34

Actual

But actually one DNS request is sent!
$ cat with-jmx.txt
www.example.com 93.184.216.34

Workaround

For Hotspot, specify sun.net.inetaddr.ttl=0 when starting the JVM:
$ java -Dsun.net.inetaddr.ttl=0 -Dcom.sun.management.jmxremote MakeTwoDnsRequests

My guess at the cause

networkaddress.cache.ttl is cached in the static initializer of InetAddressCachePolicy[1]. JMX presumably causes InetAddressCachePolicy's static initialiser to be executed, before main gets a chance to set networkaddress.cache.ttl.

http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/8c93eb3fa1c0/src/share/classes/sun/net/InetAddressCachePolicy.java#l92


## MakeTwoDnsRequests.java
import java.security.Security;
import java.net.InetAddress;

public class MakeTwoDnsRequests {

	public static void main(String[] args) throws Exception {
	    // Disable JVM DNS caching
	    Security.setProperty("networkaddress.cache.ttl", "0");

	    System.out.println(InetAddress.getByName("www.example.com").getHostAddress());
	    System.out.println(InetAddress.getByName("www.example.com").getHostAddress());
	}
}
	import java.security.Security;
	import java.net.InetAddress;

	public class MakeTwoDnsRequests {

	public static void main(String[] args) throws Exception {
	// Disable JVM DNS caching
	Security.setProperty("networkaddress.cache.ttl", "0");

	System.out.println(InetAddress.getByName("www.example.com").getHostAddress());
	System.out.println(InetAddress.getByName("www.example.com").getHostAddress());
	}
	}