docker host and port info from the container - docker

Question

I am deploying an application in a Docker container. The application sends requests to another server with a callback URL. The callback URL contains the host and port name where actually the app runs.
To configure this callback URL in a "stable, non-dynamic" test environment is easy because we know the IP and port where the app runs. But in Docker, the callback URL is the IP address of the host machine + the port that was configured in the docker-compose.yml file. So both parameter is dynamic, can not be hardcoded in the Docker image.
I need the docker host IP and the exposed port by the container info somehow in the container.
This is how my container gets the docker host machine IP:
version: '3'
services:
my-server:
image: ...
container_name: my-server
hostname: my-server
ports:
- "1234:9876"
environment:
- DOCKER_HOST_IP=${HOST_IP}
I set the host IP when I spin up the container:
HOST_IP=$(hostname -i) docker-compose up
Maybe this is not an elegant way but this is the best that I could do so far.
But I have no idea, how to get the exposed port info inside the container.
My idea was that once I know the host IP in the container, I can use nmap $HOST_IP to get the opened port list and grep for the proper line somehow. But this does not work, because I run many Docker containers on this host, and I am not able to select the proper line with grep.
here is the result of th nmap:
PORT STATE SERVICE
22/tcp open ssh
111/tcp open rpcbind
443/tcp open https
5001/tcp open commplex-link
5002/tcp open rfe
7201/tcp open dlip
1234/tcp open vcom-tunnel
1235/tcp open vcom-tunnel
1236/tcp open teradataordbms
60443/tcp open unknown
So when I execute nmap from the container then I can see all of the opened ports in my host machine. But I have no idea, how to select the line which belongs to the container where I am.
Can I can customize somehow the service name before docker spin-up the containers?
What is the best way to get the port number that was opened on the host machine by the container?

Answer 1

You should pass the complete externally-visible callback URL to the application.
ports:
- "1234:9876"
environment:
- CALLBACK_URL=http://physical-host.example.com:1234/path
You can imagine an interesting variety of scenarios where the host IP address isn't directly routable either. As a basic example, say you're running the container, on your laptop, at home. The laptop's IP address might be 192.168.1.2/24 but that's still a "private" address; you need your router's externally-visible IP address, and there's no easy way to discover that.
xx.xx.xx.xx /--------\ 192.168.1.1 192.168.1.2 /----------------\
------------| Router |---------------------------| Laptop |
\--------/ | Docker |
| 172.17.1.2 |
Callback address must be | Server |
http://xx.xx.xx.xx/path \----------------/
In a cloud environment, you can imagine a similar setup using load balancers. Your container might run on some cloud-hosted instance. The container might listen on port 11111, and you remap that to port 22222 on the instance. But then in front of this you have a load balancer that listens on the ordinary HTTPS port 443, does TLS termination, and then forwards to the instance, and you have a DNS name connected to that load balancer; the callback address would be https://service.example.com/path, but without explicitly telling the container this, there's no way it can figure this out.