Communication between linked docker containers

Question

I have two docker containers in the following setup on a host machine:

• Container 1 - UDP Port 5043 is mapped to host port 5043 (0.0.0.0:5043:5043)
• Container 2 - Needs to send data to Container 1 on port 5043 as UDP.

Scenario 1

1. I start Container 1 and obtain it's IP address.
2. I use this IP address and configure Container 2 with it and start it.
3. Container 2 is able to send UDP data to Container 1 by calling udp://Container_1_IP:5043

EVERYTHING WORKS!!

Scenario 2

1. I start Container 1 by mapping 5043 UDP port to host's 5043 port (0.0.0.0:5043:5043)
2. I link Container 2 and Container 1 using '--links'.
3. Now, when Container 2 invokes the URL udp://Container_1_IP:5043, an error is thrown "Connection refused".
4. I did verify that I am able to ping the Container 1 from inside the Container 2 using the IP.

Any help to get the Scenario 2 working for me would be really appreciated!!

Answer 1

As mentioned in Docker links:

Docker also defines a set of environment variables for each port exposed by the source container.
Each variable has a unique prefix in the form:

<name>_PORT_<port>_<protocol>

The components in this prefix are:

• the alias specified in the --link parameter (for example, webdb)
• the <port> number exposed
• a <protocol> which is either TCP or UDP

That means you need to make sure that Container1 exposes the right port with the right protocol (in your case, UDP): see "How do I expose a UDP Port on Docker?"