I have a dockerized Flask + MinIO application.

When creating the MinIO client with Flask, I use the Docker alias “minio“. If I use “localhost:9000“, it will fail to connect.

minio_client = Minio(
    "minio:9000",
    access_key="abc123",
    secret_key="123abc",
    secure=False
)

When I create a presigned URL to be able to access an object:

minio_client.presigned_get_object(bucket_name, file_name)

It will give me a URL like this, where I replace the “minio:9000” with “localhost:9000”, but that gives me the error:

http://minio:9000/images/images.jpeg?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=...&X-Amz-Date=...&X-Amz-Expires=...&X-Amz-SignedHeaders=host&X-Amz-Signature=..."


<Code>SignatureDoesNotMatch</Code>
<Message>The request signature we calculated does not match the signature you provided. Check your key and signing method.</Message>

From what I’ve been able to tell, the issue is that my client is signing using host “minio:9000” and I am trying to acces it using “localhost:9000”.
I have been able to make it work by adding this line to my /etc/hosts: 127.0.0.1 minio, but I wanted a more definitive solution.

Dockerfile

FROM python:3.9-slim

# Install dependencies for psycopg2-binary
RUN apt-get update && apt-get install -y 
    gcc 
    libpq-dev 
    && rm -rf /var/lib/apt/lists/*

WORKDIR /app

COPY . /app

RUN pip install --no-cache-dir -r requirements.txt

EXPOSE 5000

ENV FLASK_APP=server/server.py

ENV PYTHONPATH=/app

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

docker-compose.yml

version: '3.8'

services:
  flask_app:
    build:
      context: .
      dockerfile: Dockerfile
    environment:
      MINIO_ACCESS_KEY: abc123
      MINIO_SECRET_KEY: 123abc
      MINIO_BROWSER_REDIRECT_URL: localhost:9001
      MINIO_SERVER_URL: localhost:9001
    ports:
      - "5000:5000"
    depends_on:
      - minio
    volumes:
      - .:/app

  minio:
    image: minio/minio:latest
    environment:
      MINIO_ACCESS_KEY: abc123
      MINIO_SECRET_KEY: 123abc
    volumes:
      - minio-data:/data
    ports:
      - "9000:9000"
      - "9001:9001"
    command: server --address ":9000" --console-address ":9001" /data

volumes:
  minio-data: