Week 6, Day 2 — Images, Port Mapping & Volumes

Lab 6B — Port mapping, bind mounts, logs, exec, and Docker networking

Part 1 — Port mapping with Nginx (20 min)

• Pull and run the official Nginx image with port mapping:
  docker run -d -p 8080:80 --name webserver nginx
  The -p flag maps: host port 8080 → container port 80. Any request to S1:8080 is forwarded to port 80 inside the container.
• Test: curl http://localhost:8080 — should return Nginx welcome HTML. From Windows browser: http://[S1-external-IP]:8080 — same result.
• View container logs: docker logs webserver — shows Nginx access log entries for each request. Follow live: docker logs -f webserver. Press a browser refresh and watch the entry appear.
• Inspect the running container: docker inspect webserver. Find in the output: the container's internal IP address (typically 172.17.x.x), the port mapping, the image used, the mount points.

Part 2 — Bind mounts (-v) for persistent content (25 min)

• Create a local HTML file to serve through the container:
  mkdir -p ~/webroot && echo "<h1>Served from a bind mount</h1>" > ~/webroot/index.html
• Stop the existing webserver and run a new one with the bind mount:
  docker stop webserver && docker rm webserver
docker run -d -p 8080:80 --name webserver -v ~/webroot:/usr/share/nginx/html:ro nginx
  The -v flag: host path : container path : options (ro = read-only).
• Test: curl http://localhost:8080 — returns the custom HTML. Modify ~/webroot/index.html on the host and refresh — the change appears immediately without restarting the container. The bind mount is a live link.
• Answer the lab question: if you stop and delete the webserver container, does ~/webroot/index.html still exist? (Yes — it's on the host filesystem, not inside the container.)

Part 3 — docker exec for live debugging (15 min)

• While the webserver is running, exec into it:
  docker exec -it webserver bash
  Inside the container: run ps aux — only Nginx processes. Run cat /etc/nginx/nginx.conf. Run ls /usr/share/nginx/html — see the bind-mounted index.html. Exit with exit — the container keeps running.
• docker exec is the primary debugging tool for running containers. Common uses: inspect config files, check log files not exposed via docker logs, run a quick diagnostic command without stopping the container.

Part 4 — Custom Docker network address range (15 min)

• Docker's default network uses 172.17.0.0/16 — this may conflict with other classroom networks. Change it by creating /etc/docker/daemon.json:
  sudo nano /etc/docker/daemon.json
  Add:
  {
"default-address-pools": [
{"base": "10.10.0.0/16", "size": 24}
]
}
  Restart Docker: sudo systemctl restart docker. New containers now use the 10.10.0.x range.
• Verify: run a new container and inspect its IP: docker inspect [container] | grep IPAddress — should show a 10.10.x.x address.

Part 5 — Running an Ubuntu container on a custom network (25 min)

• Create a custom Docker network:
  docker network create --driver bridge mynet
docker network ls — shows the new network alongside bridge, host, none.
• Run two containers on the same custom network:
  docker run -d -it --name ubuntu1 --network mynet ubuntu bash
docker run -d -it --name ubuntu2 --network mynet ubuntu bash
• Exec into ubuntu1 and ping ubuntu2 by name:
  docker exec -it ubuntu1 bash
ping ubuntu2
  Containers on the same custom network resolve each other by container name via Docker's built-in DNS. This is how docker-compose services find each other — by service name, without needing IPs.
• Clean up everything: stop and remove all containers, remove the custom network, remove images not needed for upcoming labs.