Docker on Mac is running but refusing to expose port - macos

Question

Mac here, running Docker Community Edition Version 17.12.0-ce-mac49 (21995).
I have Dockerized a web app with a Dockerfile like so:
FROM openjdk:8
RUN mkdir /opt/myapp
ADD build/libs/myapp.jar /opt/myapp
ADD application.yml /opt/myapp
ADD logback.groovy /opt/myapp
WORKDIR /opt/myapp
EXPOSE 9200
ENTRYPOINT ["java", "-Dspring.config=.", "-jar", "myapp.jar"]
I then build that image like so:
docker build -t myapp .
I then run a container of that image like so:
docker run -it -p 9200:9200 --net="host" --env-file ~/myapp-local.env --name myapp myapp
In the console I see the app start up without any errors, and all seems to be well. Even my metrics publishes (which publish heartbeat and other health metrics every 20 seconds) are printing to the console as I would expect them to. Everything seems to be fine.
Except when I go to run a curl against my app from another terminal/session:
curl -i -H "Content-Type: application/json" -X POST -d '{"username":"heyitsme","password":"12345"}' http://localhost:9200/v1/auth/signIn
curl: (7) Failed to connect to localhost port 9200: Connection refused
Now, if this were a situation where the /v1/auth/signIn path wasn't valid, or if there was something wrong with my request entity/payload, the server would pick up on it and send an error (I assure you; as I can confirm this exact same curl works when I run the server outside of Docker as just a standalone service).
So this is definitely a situation where the curl command can't connect to localhost:9200. Again, when I run my app outside of Docker, that same curl command works perfectly, so I know my app is trying to standup on port 9200.
Any ideas as to what could be going wrong here, or how I could begin troubleshooting?

Answer 1

The way you run your container has 2 conflicting parts:
-p 9200:9200 says: "publish (bind) port 9200 of the container to port 9200 of the host"
--net="host" says: "use the host's networking stack"
According to Docker for Mac - Networking docs / Known limitations, use cases, and workarounds, you should only publish a port:
I want to connect to a container from the Mac
Port forwarding works for localhost; --publish, -p, or -P all work. Ports exposed from Linux are forwarded to the Mac.
Our current recommendation is to publish a port, or to connect from another container. This is what you need to do even on Linux if the container is on an overlay network, not a bridge network, as these are not routed.
The command to run the nginx webserver shown in Getting Started is an example of this.
$ docker run -d -p 80:80 --name webserver nginx

Answer 2

Check that your app bind to 0.0.0.0:9200 and not localhost:9200 or something similar

Answer 3

Problem seems to be in the network mode you are running the container.
Quick test: Login to your container and run the curl cmd there, hopefully it works. That would isolate the problem to request not being forwarded from host to container.
Try running your container on the default bridge network and test.
Refer to this blog for details on the network modes in docker
TLDR; You will need to add an IPtables entry to allow the traffic to enter your container.