DevOps Day 42: Creating Custom Docker Networks

Today's task was an important step in building robust, multi-container applications. I moved beyond Docker's default networking and learned how to create a custom, user-defined bridge network with a specific IP address configuration.

This is a foundational skill for creating isolated environments where different services (like a web app and a database) can communicate with each other securely and predictably. This document is my first-person guide to that process.

The Task

My objective was to create a new, custom Docker network on App Server 2. The specific requirements were: - The network must be named ecommerce. - It must use the bridge driver. - It needed a specific IP configuration: - Subnet: 192.168.0.0/24 - IP Range: 192.168.0.0/24

My Step-by-Step Solution

1. Connect to the Server: I first logged into App Server 2 (ssh steve@stapp02).

2. Create the Custom Network: I used a single docker network create command, providing all the required configuration options as flags.

   sudo docker network create \
     --driver bridge \
     --subnet=192.168.0.0/24 \
     --ip-range=192.168.0.0/24 \
     ecommerce
   

3. Verification: The crucial final step was to inspect the network to ensure it was created with the correct settings.

   sudo docker network inspect ecommerce
   

   I examined the IPAM (IP Address Management) section of the JSON output, which confirmed that the Subnet and IPRange values matched my command perfectly. This was the definitive proof of success.

Key Concepts (The "What & Why")

• Docker Networks: For any real application, it's a best practice to create your own user-defined networks instead of using the default one. A custom network provides a secure, isolated environment for a group of containers to communicate.
• bridge Driver: This is the most common network driver for a single-host Docker setup. It creates a private, software-based network bridge on the host machine. Containers connected to this network can talk to each other but are isolated from containers on other networks.
• Automatic DNS Resolution: The biggest advantage of a custom bridge network is its built-in DNS server. This allows containers on the same network to find each other by using their container names as hostnames (e.g., a webapp container can connect to mysql://db_container:3306). This is a huge benefit for creating robust applications.
• Subnet and IP Range: These options provide precise control over the networking environment.
  • --subnet: I defined the overall IP address pool for my network. 192.168.0.0/24 provides a range of 256 addresses.
  • --ip-range: I specified which part of that subnet pool Docker should use to assign IPs to containers.

Commands I Used

• sudo docker network create [options] [network_name]: The primary command for this task.
  • --driver bridge: Specifies the network driver to use.
  • --subnet=[cidr]: Defines the IP address pool for the network.
  • --ip-range=[cidr]: Defines the range within the subnet from which container IPs will be allocated.
  • ecommerce: The name I assigned to my new network.
• sudo docker network inspect [network_name]: A powerful diagnostic command that outputs a detailed JSON object containing all the configuration and runtime information about a network. I used it to verify the IPAM section was correct.