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Editorial Team
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Asked: 2026-06-15T12:35:24+00:00

We are having problem connecting to our Java applications running in Amazon’s EC2 cluster.

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We are having problem connecting to our Java applications running in Amazon’s EC2 cluster. We definitely have allowed both the “JMX port” (which is usually the RMI registry port) and the server port (which does most of the work) to the security-group for the instances in question. Jconsole connects but seems to hang and never show any information.

We are running our java with something like the following:

java -server -jar foo.jar other parameters here > java.log 2>&1

We have tried:

  • Telnets to the ports connect but no information is displayed.
  • We can run jconsole on the instance itself using remote-X11 over ssh and it connects and shows information. So the JRE is exporting it locally.
  • Opening all ports in the security group. Weeee.
  • Using tcpdump to make sure the traffic is not going to other ports.
  • Simulating it locally. We can always connect to our local JREs or those running elsewhere on our network using the same application parameters.

java -version outputs:

OpenJDK Runtime Environment (IcedTea6 1.11.5) (amazon-53.1.11.5.47.amzn1-x86_64)
OpenJDK 64-Bit Server VM (build 20.0-b12, mixed mode)

As an aside, we are using my Simple JMX package which allows us to set both the RMI registry and server ports which are typically semi-randomly chosen by the RMI registry. You can also force this with something like the following JMX URI:

service:jmx:rmi://localhost:" + serverPort + "/jndi/rmi://:" + registryPort + "/jmxrmi"

These days we use the same port for both the server and the registry. In the past we have used X as the registry-port and X+1 for the server-port to make the security-group rules easy. You connect to the registry-port in jconsole or whatever JMX client you are using.

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1 Answer

  1. Editorial Team

    We are having problem connecting to our Java applications running in Amazon’s EC2 cluster.

    It turns out that the problem was a combination of two missing settings. The first forces the JRE to prefer ipv4 and not v6. This was necessary (I guess) since we are trying to connect to it via a v4 address:

    -Djava.net.preferIPv4Stack=true

    The real blocker was the fact that JMX works by first contacting the RMI port which responds with the hostname and port for the JMX client to connect. With no additional settings it will use the local IP of the box which is a 10.X.X.X virtual address which a remote client cannot route to. We needed to add the following setting which is the external hostname or IP of the server — in this case it is the elastic hostname of the server. 

    -Djava.rmi.server.hostname=ec2-107-X-X-X.compute-1.amazonaws.com

    The trick, if you are trying to automate your EC2 instances (and why the hell would you not), is how to find this address at runtime. To do that you need to put something like the following in our application boot script:

    # get our _external_ hostname
RMI_HOST=`wget -q -O - http://169.254.169.254/latest/meta-data/public-hostname`
...
java -server \
    -Djava.net.preferIPv4Stack=true -Djava.rmi.server.hostname=$RMI_HOST \
    -jar foo.jar other parameters here > java.log 2>&1

    The mysterious 169.254.169.254 IP in the wget command above provides information that the EC2 instance can request about itself. I’m disappointed that this does not include tags which are only available in an authenticated call.

    I initially was using the extern ipv4 address but it looks like the JDK tries to make a connection to the server-port when it starts up. If it uses the external IP then this was slowing our application boot time until that timed out. The public-hostname resolves locally to the 10-net address and to the public-ipv4 externally. So the application now is starting fast and JMX clients still work. Woo hoo!

    Hope this helps someone else. Cost me 3 hours today.

    To force your JMX server to start the server and the RMI registry on designated ports so you can block them in the EC2 Security Groups, see this answer:

    How to close rmiregistry running on particular port?

    Edit:

    We just had this problem re-occur. It seems that the Java JMX code is doing some hostname lookups on the hostname of the box and using them to try to connect and verify the JMX connection.

    The issue seems to be a requirement that the local hostname of the box should resolve to the local-ip of the box. For example, if your /etc/sysconfig/network has HOSTNAME=server1.foobar.com then if you do a DNS lookup on server1.foobar.com, you should get to the 10-NET virtual address. We were generating our own /etc/hosts file and the hostname of the local host was missing from the file. This caused our applications to either pause on startup or not startup at all.

    Lastly

    One way to simplify your JMX creation is to use my SimpleJMX package.

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