Deploying a minimal flask app in docker – server connection issues

When using the flask command instead of app.run, you can pass the --host option to change the host. The line in Docker would be:

CMD ["flask", "run", "--host", "0.0.0.0"]

or

CMD flask run --host 0.0.0.0

Your Docker container has more than one network interface. For example, my container has the following:

$ ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
32: eth0@if33: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default 
    link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0
       valid_lft forever preferred_lft forever

if you run docker network inspect bridge, you can see that your container is connected to that bridge with the second interface in the above output. This default bridge is also connected to the Docker process on your host.

Therefore you would have to run the command:

CMD flask run --host 172.17.0.2

To access your Flask app running in a Docker container from your host machine. Replace 172.17.0.2 with whatever the particular IP address is of your container.

First of all in your python script you need to change code from

app.run()

to

app.run(host="0.0.0.0")

Second, In your docker file, last line should be like

CMD ["flask", "run", "-h", "0.0.0.0", "-p", "5000"]

And on host machine if 0.0.0.0:5000 doesn’t work then you should try with localhost:5000

Note – The CMD command has to be proper. Because CMD command provide defaults for executing container.

To build on other answers:

Imagine you have two computers. Each computer has a network interface (WiFi, say), which is its public IP. Each computer has a loopback/localhost interface, at 127.0.0.1. This means "just this computer."

If you listed on 127.0.0.1 on computer A, you would not expect to be able to connect to that via 127.0.0.1 when running on computer B. After all, you asked to listen on computer A’s local, private address.

Docker is similar setup; technically it’s the same computer, but the Linux kernel is allowing each container to run with its own isolated network stack. So 127.0.0.1 in a container is the same as 127.0.0.1 on a different computer than your host—you can’t connect to it.

Longer version, with diagrams: https://pythonspeed.com/articles/docker-connection-refused/

You need to modify the host to 0.0.0.0 in the docker file. This is a minimal example

# Example of Dockerfile

FROM python:3.8.5-alpine3.12

WORKDIR /app

EXPOSE 5000
ENV FLASK_APP=app.py

COPY . /app
RUN pip install -r requirements.txt

ENTRYPOINT [ "flask"]
CMD [ "run", "--host", "0.0.0.0" ]

and the file app.py is

# app.py
from flask import Flask

app = Flask(__name__)

@app.route("/")
def home():
    return "Hello world"


if __name__ == "__main__":
    app.run()

Then compile with

docker build . -t deploy_flask

and run with

docker run -p 5000:5000 -t -i deploy_flask:latest

You can check the response with curl http://127.0.0.1:5000/ -v

For fast readers, three quick things to check:

1. Make sure you have exposed the port in the Dockerfile.
2. Running the command in container using flask run --host=0.0.0.0
3. Specifying the port in your docker run command docker run -it -p5000:5000 yourImageName

In my case, binding the host to 0.0.0.0 only worked on my local environment, and it failed when deploying on a server.

Then it’s working when I replaced the port with --network=host:

Before:

docker run -d -p 5000:5000 <docker_image>

After:

docker run -d --network=host <docker_image>

ps. I still used the 0.0.0.0:5000 inside the container when running the flask app.