Why Java opens 3 ports when JMX is configured?

67
votes

I run my Java program with JDK7 on Centos6. I enable JMX using the following options:

JAVA_OPTS="${JAVA_OPTS} -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=9123 -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.local.only=true"

When I check what ports are opened I discover 2 additional random ports:

netstat -plunt | grep java
tcp        0      0 :::9123                     :::*                        LISTEN      13295/java
tcp        0      0 :::59927                    :::*                        LISTEN      13295/java
tcp        0      0 :::59928                    :::*                        LISTEN      13295/java

Please note that each restart only configured port 9123 remains same, and two additional ports change values.

netstat -plunt | grep java
tcp        0      0 :::9123                     :::*                        LISTEN      13331/java
tcp        0      0 :::59932                    :::*                        LISTEN      13331/java
tcp        0      0 :::59933                    :::*                        LISTEN      13331/java

What are 2 additional ports and why they are opened?

How can I configure 2 additional random ports?

How can I configure ::ffff:127.0.0.1 will appear before all ports opened by JMX?

Why one port is not used when connecting with JConsole?

Added to clarify the answer

Unfortunately, the additional random port is still opened To remind you, I use Centos 6. My Tomcat settings are look like this (Tomcat does not deploy any applications):

CATALINA_OPTS="${CATALINA_OPTS}  -XX:+DisableAttachMechanism -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.local.only=true -Djava.rmi.server.useLocalHostname=true -Djava.rmi.server.useCodebaseOnly=true -Dcom.sun.management.jmxremote.port=9123 -Dcom.sun.management.jmxremote.rmi.port=9123"

Tomcat process looks like this:

/usr/java/jdk1.7.0_51/bin/java -Djava.util.logging.config.file=/usr/tomcat-7.0.47/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager -XX:+DisableAttachMechanism -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.local.only=true -Djava.rmi.server.useLocalHostname=true -Djava.rmi.server.useCodebaseOnly=true -Dcom.sun.management.jmxremote.port=9123 -Dcom.sun.management.jmxremote.rmi.port=9123 -Djava.endorsed.dirs=/usr/tomcat-7.0.47/endorsed -classpath /usr/tomcat-7.0.47/bin/bootstrap.jar:/usr/tomcat-7.0.47/bin/tomcat-juli.jar -Dcatalina.base=/usr/tomcat-7.0.47 -Dcatalina.home=/usr/tomcat-7.0.47 -Djava.io.tmpdir=/usr/tomcat-7.0.47/temp org.apache.catalina.startup.Bootstrap start

Unfortunately, each time I see additional listening port:

tcp        0      0 :::38830                    :::*                        LISTEN      790/java
tcp        0      0 ::ffff:127.0.0.1:8080       :::*                        LISTEN      790/java
tcp        0      0 :::9123                     :::*                        LISTEN      790/java

Additional run:

tcp        0      0 ::ffff:127.0.0.1:8080       :::*                        LISTEN      2348/java
tcp        0      0 :::36252                    :::*                        LISTEN      2348/java
tcp        0      0 :::9123                     :::*                        LISTEN      2348/java

BTW, why I can not see ::ffff:127.0.0.1 before RMI ports?

Added second time to clarify the comment

It is not related to Tomcat. I have tried to run ant with similar settings: Ant process looks like this:

/usr/bin/java -XX:+DisableAttachMechanism -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.local.only=true -Djava.rmi.server.useLocalHostname=true -Djava.rmi.server.useCodebaseOnly=true -Dcom.sun.management.jmxremote.port=9123 -Dcom.sun.management.jmxremote.rmi.port=9123 -classpath /usr/apache-ant-1.9.2/lib/ant-launcher.jar -Dant.home=/usr/apache-ant-1.9.2 -Dant.library.dir=/usr/apache-ant-1.9.2/lib org.apache.tools.ant.launch.Launcher -cp  sleep

Unfortunately, each time I see additional listening port:

tcp        0      0 :::41200                    :::*                        LISTEN      13597/java
tcp        0      0 :::9123                     :::*                        LISTEN      13597/java

Additional run:

tcp        0      0 :::58356                    :::*                        LISTEN      13629/java
tcp        0      0 :::9123                     :::*                        LISTEN      13629/java

Answer: It is Java's bug

I success to open bug on Java: http://bugs.java.com/bugdatabase/view_bug.do?bug_id=8035404

4
javarmijmx
possible duplicate of JMX enabled Java application appears to open a random high order port when JMX client connects – Boris the Spider
Not. Nobody explains why 3-d port is opened (than not used by JConsole) – Michael
Try following the links in the answer - this one is particularly useful. The short answer is "that's how JMX works". – Boris the Spider
And what will change when you know? Its still going to open three ports because "that's how JMX works". IMO this is like asking why Excel keeps a lock on the file that you have open in it; it just works that way. Knowing the exact reason (which you'd have to ask the original designer) is not going to change anything in the way it works. – Gimby
If they open the port by mistake I will open a bug on Java :) With Excel it is not the bug. – Michael

4 Answers

125
votes

Contrary to common belief JMX/RMI doesn't need to open all these ports. You can actually force them to be same which will mean that at the end of the day you'll only need to punch one hole in the firewall (if firewall is your concern).

Try setting System Properties:

com.sun.management.jmxremote.port
com.sun.management.jmxremote.rmi.port

to the same value!!

Explicitly setting these will stop RMI from picking random ports. Setting them to the same value will make sure it opens less ports to listen on.

This will work in Java 7 update 25 or later.

What is the third port?

The third port that you see opened by your application (or the second if you followed my advice above) is used by the Java Attach API. It is what JConsole uses for connecting to "Local Process". The Java Attach API feature is enabled by default since Java 6 regardless of the com.sun.management.jmxremote property. This feature will use a random port (aka an OS ephemeral port) but it really doesn't matter because the feature only allows connections from the host itself. If you really dislike this feature then you can add -XX:+DisableAttachMechanism to the command line to disable the Java Attach API feature. Then you'll no longer see the java process (in this case Tomcat) listening on a random port.

How do I make JMX listen on the loopback interface only

With a custom made application you would use a RMIServerSocketFactory but this is Tomcat so you would have to do it using Tomcat's JMX Remote Lifecycle Listener.

On the other hand it doesn't matter now that you have the com.sun.management.jmxremote.local.only property since Java 7. It makes sure that only connections from the host itself are allowed. Mind you that JMX library doesn't achieve this by binding to loopback interface which would certainly be one way of doing it but also slight inaccurate as a host can potentially have several loopback interfaces.

In fact by and large (with the most recent additions to JDK wrt JMX) I would say that Tomcat's JMX Remote Lifecycle Listener is now redundant except if you want to bind to some really odd network interface.

8
votes

Using Oracle Java SE 1.8.0_121.

It's possible to set jmxremote.port and jmxremote.rmi.port to the same value, it's one less port opened. It's also possible to set jmxremote.host=127.0.0.1, to have that port (or those two ports, if you set them differently) bind to the loopback interface only.

Another port is still dynamically assigned though, and will bind to 0.0.0.0. I was not able to prevent this port with -XX+DisableAttachMechanism, and was also unable to make it bind to anything else than 0.0.0.0.

3
votes

Because jmx is encapsulated in rmi wich is very firewall and nat unfriendly. Avoid it if you can, there is an alternative encapsulation called jmxmp.

Have a look at, that might help you : http://blog.markfeeney.com/2010/10/jmx-through-ssh-tunnel.html http://jrds.fr/sourcetype/jmx/start#jmx_protocols

0
votes

The third port has nothing to do with how RMI (JRMP) works, as incorrectly noted in the bug.

Also has nothing to do with the Java Attach API.

It is the port used by the local JMX listener. Whenever remote JMX is started, for some reason the local one is started as well. This can be seen in this code from OpenJDK:

        /*
         * If the jmxremote.port property is set then we start the
         * RMIConnectorServer for remote M&M.
         *
         * If the jmxremote or jmxremote.port properties are set then
         * we start a RMIConnectorServer for local M&M. The address
         * of this "local" server is exported as a counter to the jstat
         * instrumentation buffer.
         */
        if (jmxremote != null || jmxremotePort != null) {
            if (jmxremotePort != null) {
                jmxServer = ConnectorBootstrap.
                        startRemoteConnectorServer(jmxremotePort, props);
                startDiscoveryService(props);
            }
            startLocalManagementAgent();
        }

It is also mentioned in the documentation:

To enable monitoring and management from remote systems, you must set the following system property when you start the Java VM: com.sun.management.jmxremote.port=portNum

In the property above, portNum is the port number through which you want to enable JMX RMI connections. Be sure to specify an unused port number. In addition to publishing an RMI connector for local access, setting this property publishes an additional RMI connector in a private read-only registry at the specified port using a well known name, "jmxrmi".

Also, since 2020 the local port can be configured via com.sun.management.jmxremote.local.port.