docker linked container all ports closed - linker

Question

i try to connect from one docker container to another.
Container A has a Derby DB installed and started, listening on port 3301
Container B should connect to Container A
the docker files look like:
Container A
FROM java:8
# Install Derby
COPY db-derby-10.12.1.1-bin.tar.gz db-derby-10.12.1.1-bin.tar.gz
RUN mkdir /opt/Apache
RUN cp db-derby-10.12.1.1-bin.tar.gz /opt/Apache
RUN tar xzvf /opt/Apache/db-derby-10.12.1.1-bin.tar.gz
EXPOSE 3301
CMD ["/db-derby-10.12.1.1-bin/bin/startNetworkServer", "-p 3301"]
Container B
FROM java:8
# Install nmap
RUN apt-get update
RUN apt-get install -y nmap
COPY db-derby-10.12.1.1-bin.tar.gz db-derby-10.12.1.1-bin.tar.gz
RUN mkdir /opt/Apache
RUN cp db-derby-10.12.1.1-bin.tar.gz /opt/Apache
RUN tar xzvf /opt/Apache/db-derby-10.12.1.1-bin.tar.gz
EXPOSE 9080
I start both container and give them names
Container A
docker run -it --name derby <image>
Container B
docker run -it --link derby:derby <image> /bin/bash
Then i attach container B and
ping derby or ping 172.17.0.2
which is succesfull. But when i try to connect to the derby database via cli tool and giving a jdbc url like
connect 'jdbc:derby://172.17.0.2:3301/testdb;create=true';
I get an connection refused error.
Using nmap to scan the ports of container A results in "All ports are closed"
which is confusing because Docker References states:
So what does linking the containers actually do? You’ve learned that a
link allows a source container to provide information about itself to
a recipient container. In our example, the recipient, web, can access
information about the source db. To do this, Docker creates a secure
tunnel between the containers that doesn’t need to expose any ports
externally on the container; you’ll note when we started the db
container we did not use either the -P or -p flags. That’s a big
benefit of linking: we don’t need to expose the source container, here
the PostgreSQL database, to the network.
Has anybody maybe a hint or solution for me?
Regards

Answer 1

Allright i got it solved. For all others who might be interessted, you need to start Derby with command
startNetworkServer -h 0.0.0.0
that way you tell Derby to accept all connections from outside, restrict it if you need but the parameter has to be present otherwise connections are refused.
Regards

Answer 2

The port is not open as you have not told docker to open it.
EXPOSE in the Dockerfile instructs docker that the container listens on the specified port, but it does NOT open that port or expose it to the host.
--link doesn't open ports either, that is the benefit of link, you can securely connect 2 containers without having to expose any ports to the host.
So, to connect to the derby BD with your command line tool you have 2 options.
1) Open the port - (possible security implications?)
When you run the container, you specificy the port to open.
docker run -it --name derby -p 3301:3301 <image>
The above will map the 3301 port on the container to the 3301 port on the host.
Then you can use the host IP and port 3301 to connect to that container.
2) Connect to the container directly
You can effectively ssh in to the container and run the command on the container itself...
$ sudo docker exec -it derby bash
And then you have a bash session on the derby container directly, and can run commands on it.
UPDATE
To connect from one container to another over the link you can use the ENV vars that docker exposes on the container about the link. http://docs.docker.com/engine/userguide/networking/default_network/dockerlinks/#environment-variables
So, on container B you will have ENV vars containing the link name DERBY.
So, DERBY_PORT will be a IP and PORT to connect to the derby container.
However, if the "derby" container is restarted, the IP in the ENV var will be out of date. So it is better to connect to it by its link name.
Docker also sets up host names about the links, so you can connect to the derby container with
http://derby:3301
from within container B
So you could try..
connect 'jdbc:derby://derby:3301/testdb;create=true';