🐳 DAY 27: Docker Networking: Connectivity Testing Lab

Mastering container communication with Docker's default bridge network

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📖 Overview

This repository documents Task 2: Test Connectivity from Day 24: Docker Networking, part of a 100-day learning plan focused on Containers & Automation. The task explores Docker's default bridge network, testing inter-container communication and external access using Nginx containers. By following this lab, you'll verify connectivity using IP addresses, understand the limitations of DNS resolution, and expose a container to the host.

Objective: Demonstrate connectivity between containers (web1, web2, web3) on the default bridge network and external access via port mapping.

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🚀 Prerequisites

Before starting, ensure you have:

• Docker installed and running (docker --version and docker info).

• A web browser to test external access.

• Basic terminal knowledge (e.g., bash commands).

• Containers web1 and web2 running on the default bridge network:

    docker run -d --name web1 nginx
    docker run -d --name web2 nginx
  

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🛠️ Step-by-Step Process

Follow these steps to replicate the connectivity testing lab. All commands assume a single terminal unless specified. A second terminal may be needed for troubleshooting.

🛠 Step 1: Access web1’s Shell

Command:

docker exec -it web1 /bin/bash

• What it does: Opens an interactive Bash shell inside the web1 container.

• Terminal: Use your main terminal.

• Expected Outcome: You’re inside web1’s shell (prompt: root@<container_id>:/#).

🛠 Step 2: Install ping

Command (inside web1’s shell):

apt-get update && apt-get install -y iputils-ping

• What it does: Updates the package list and installs the ping utility in the minimal Nginx image.

• Terminal: Inside web1’s shell.

• Expected Outcome: ping is installed, with output showing package downloads.

🛠 Step 3: Ping web2 Using IP

Command (inside web1’s shell):

ping 172.17.0.3

• What it does: Pings web2 (IP: 172.17.0.3, from docker network inspect bridge) to test connectivity.

• Why it works: The default bridge network allows inter-container communication (enable_icc: "true").

• Terminal: Inside web1’s shell.

• Expected Outcome:

    PING 172.17.0.3 (172.17.0.3): 56 data bytes
    64 bytes from 172.17.0.3: icmp_seq=1 ttl=64 time=0.123 ms
    ...
  

  Press Ctrl+C to stop.

• Troubleshooting:

  • Verify web2 is running: Open a new terminal and run docker ps.

  • Confirm IP: Run docker network inspect bridge in a new terminal.

🛠 Step 4: Stay in the Shell

• Action: Remain in web1’s shell for the next step.

• Terminal: No change.

🛠 Step 5: Ping web2 by Name (Expect Failure)

Command (inside web1’s shell):

ping web2

• What it does: Attempts to ping web2 by its container name.

• Why it fails: The default bridge network doesn’t support DNS resolution by container name.

• Terminal: Inside web1’s shell.

• Expected Outcome:

    ping: web2: Name or service not known
  

• Troubleshooting: If a different error appears, verify web2 is running (docker ps in a new terminal).

🛠 Step 6: Exit the Shell

Command (inside web1’s shell):

exit

• What it does: Returns to the host terminal.

• Terminal: Back to the host terminal.

• Expected Outcome: Host prompt (e.g., user@machine:~$).

🛠 Step 7: Run a New Container with Port Mapping

Command (in host terminal):

docker run -d --name web3 -p 8080:80 nginx

• What it does: Starts a new Nginx container (web3), mapping host port 8080 to container port 80.

• Terminal: Host terminal.

• Expected Outcome: Container starts. Verify with:

    docker ps
  

  Look for web3 with 0.0.0.0:8080->80/tcp.

🛠 Step 8: Access web3 Externally

Action: Open http://localhost:8080 in a web browser.

• What it does: Accesses Nginx’s default welcome page via the port mapping.

• Terminal: None (use browser).

• Expected Outcome: Displays “Welcome to nginx!” page.

• Troubleshooting:

  • Check web3 is running (docker ps).

  • Ensure port 8080 is free (netstat -tuln on Linux or netstat -a -n -o on Windows).

  • Test with curl http://localhost:8080 in the host terminal.

🛠 Step 9: Inspect the Bridge Network

Command (in host terminal):

docker network inspect bridge

• What it does: Shows the bridge network’s details, including web3’s IP (e.g., 172.17.0.4).

• Terminal: Host terminal.

• Expected Outcome: web3 is listed in the Containers section:

    "Containers": {
        ...
        "<web3_id>": {
            "Name": "web3",
            "IPv4Address": "172.17.0.4/16",
            ...
        }
    }
  

• Troubleshooting: If web3 is missing, ensure it’s running and not on another network.

🛠 Step 10: Test Connectivity from web1 to web3

• Sub-step: Access web1’s Shell:

  • Command (in host terminal):

      docker exec -it web1 /bin/bash
    

  • Terminal: Host terminal to start, then inside web1’s shell.

  • Expected Outcome: Inside web1’s shell.

• Sub-step: Install curl:

  • Command (inside web1’s shell):

      apt-get update && apt-get install -y curl
    

  • What it does: Installs curl for HTTP testing.

  • Expected Outcome: curl is installed.

• Sub-step: Test Connectivity:

  • Command (inside web1’s shell):

      curl http://172.17.0.4
    

  • What it does: Sends an HTTP request to web3’s IP (from Step 9).

  • Terminal: Inside web1’s shell.

  • Expected Outcome: Returns Nginx’s HTML:

      <!DOCTYPE html>
      <html>
      <head>
      <title>Welcome to nginx!</title>
      ...
    

  • Troubleshooting:

    • Verify web3’s IP (docker network inspect bridge in a new terminal).

    • Ensure web3 is running (docker ps in a new terminal).

🛠 Step 11: Exit the Shell

Command (inside web1’s shell):

exit

• What it does: Returns to the host terminal.

• Terminal: Back to host terminal.

• Expected Outcome: Host prompt.

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🧹 Clean Up

To free resources, stop and remove containers:

docker stop web1 web2 web3
docker rm web1 web2 web3

• Terminal: Host terminal.

• Why: Prevents clutter and resource usage.

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🔑 Key Takeaways

• IP-Based Communication: Containers on the default bridge network communicate via IP addresses (e.g., 172.17.0.3 for web2).

• No DNS Resolution: The default bridge network doesn’t support name-based communication (e.g., ping web2 fails).

• Port Mapping: Exposes containers externally (e.g., -p 8080:80 for web3).

• Inter-Container Communication: Enabled by enable_icc: "true", allowing curl http://172.17.0.4.

• Relevance: Prepares you for user-defined networks (Day 24, Task 2) and Docker Compose (Day 28–30).

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🛠 Troubleshooting

• Ping Fails: Verify containers are running (docker ps) and IPs are correct (docker network inspect bridge).

• Browser Access Fails: Check port 8080 is free and web3 is running.

• Curl Fails: Ensure curl is installed and web3’s IP is correct.

• New Terminal: Use a second terminal for docker ps or docker network inspect while in web1’s shell.

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📚 Resources

• Docker Bridge Network Docs

• Docker Exec Command

• Nginx Documentation

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👨‍💻 Author

Ritesh Singh
🌐 LinkedIn | 📝 Hashnode | GITHUB