Docker Compose - container A "could not resolve host" of container B

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30-09-2024

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Docker Compose: "Could not resolve host" Errors Between Containers

When working with Docker Compose and multiple containers, you might encounter the error "could not resolve host" when trying to access services from one container to another. This error usually arises when the containers cannot communicate with each other due to incorrect network configuration.

Let's delve into this issue, analyze its causes, and explore solutions to ensure seamless communication between your Docker Compose services.

Scenario:

Consider a simple Docker Compose file with two services:

version: '3.8'
services:
  web:
    image: nginx:latest
    ports:
      - "80:80"
    depends_on:
      - db
  db:
    image: mysql:latest
    environment:
      MYSQL_ROOT_PASSWORD: "password"

Our web container tries to access the database container, but you encounter an error:

web_1 | connect to database: error: could not resolve host: db

Causes of the "Could not resolve host" Error:

1. Missing DNS Configuration: Docker Compose, by default, uses a simple network that doesn't automatically resolve hostnames. Your web container might not be able to find the "db" hostname.

2. Incorrect depends_on Usage: While depends_on ensures that the database container starts before the web container, it doesn't automatically guarantee network connectivity.

3. Network Isolation: Docker containers are isolated by default, limiting their ability to see other containers unless explicitly configured.

Solutions:

1. Use Docker Compose's built-in network:

   • Docker Compose automatically creates a network for its services. You can specify this network in the networks section of your docker-compose.yml file:

   version: '3.8'
   services:
     web:
       image: nginx:latest
       ports:
         - "80:80"
       networks:
         - app-net
       depends_on:
         - db
     db:
       image: mysql:latest
       environment:
         MYSQL_ROOT_PASSWORD: "password"
       networks:
         - app-net
   networks:
     app-net:
       driver: bridge
   

   • Now both containers share the same network, allowing them to communicate using their service names as hostnames.

2. Configure DNS within the container:

   • You can configure your container's DNS settings within the docker-compose.yml file:

   version: '3.8'
   services:
     web:
       image: nginx:latest
       ports:
         - "80:80"
       depends_on:
         - db
       dns:
         - 127.0.0.11 # Docker's internal DNS server
     db:
       image: mysql:latest
       environment:
         MYSQL_ROOT_PASSWORD: "password"
   

   • This solution tells your web container to resolve hostnames through Docker's internal DNS server.

3. Use environment variables for container-to-container communication:

   • You can pass the database container's IP address as an environment variable to the web container:

   version: '3.8'
   services:
     web:
       image: nginx:latest
       ports:
         - "80:80"
       depends_on:
         - db
       environment:
         DB_HOST: ${db_ip}
     db:
       image: mysql:latest
       environment:
         MYSQL_ROOT_PASSWORD: "password"
       expose:
         - 3306
   

   • expose makes the database's port available on the internal Docker network.
   • We need a way to find the database container's IP address and pass it as an environment variable to the web container. We could use a shell script or a custom Docker image for this.

4. Use a dedicated network:

   • Instead of relying on the default Docker Compose network, you can create a dedicated network and connect your containers to it. This offers more flexibility and allows you to fine-tune the network's configuration.

Conclusion:

Resolving the "could not resolve host" issue in Docker Compose requires understanding the fundamental concepts of container networking and the available configuration options. By properly defining your network, configuring DNS, or using environment variables for communication, you can effectively facilitate communication between your Docker Compose services and ensure your applications function seamlessly.