REST request from one docker container to another fails

Question

I have two applications, one of which has a RESTful interface that is used by the other. Both are running on the same machine.

Application A runs in a docker container. I am running it using the command line:

docker run -p 40000:8080 --name AppA image1

When I test Application B outside a docker container (in other words, before it is dockerized) Application B successfully executes all RESTful requests and receives responses without problems.

Unfortunately, when I dockerize and run Application B within a container:

docker run -p 8081:8081 --name AppB image2

whenever I attempt to send a RESTful request to Application A, I get the following:

Connect to localhost:40000 [localhost/127.0.0.1, localhost/0:0:0:0:0:0:0:1] failed: Connection refused

Of course, I also tried making Application B connect using my machine's IP address. When I do that, I get the following failure:

Connect to 192.168.1.101:40000 failed: No route to Host

Has anyone seen this kind of behavior before? What causes an application that communicates perfectly well with another dockerized application outside a docker container to fail to communicate with that same dockerized application once it is itself dockerized???

Someone please advise...

Answer 1

Simply linking B to A docker run -p 8081:8081 --link AppA --name AppB image2, then you can access the REST service using AppA:8080.

The reason is that Docker containers run on its own subnet (normally 172.17.0.0-255) and they cannot access the network that your host is on. Also localhost would be the container itself, not the host.