Lab | Nginx in Ubuntu container — first without, then with port mapping

Containers have their own network stack. A service listening on localhost:80 inside a container isn’t reachable from the host unless you map a host port to a container port. With Docker this is done using -p HOST_PORT:CONTAINER_PORT (e.g., -p 8080:80). Without it, curl localhost on the host will fail, even though the service is healthy inside the container.

Prerequisites

• Docker installed and running.
• Internet access to install packages inside the container.

Part 1 — Run Nginx without port mapping (host curl should fail)

1. Start a plain Ubuntu container (no port mapping)

docker run -dit --name web-nomap --rm ubuntu

2. Install Nginx inside the container

docker exec web-nomap apt-get update
docker exec web-nomap apt-get install -y nginx curl

3. Change the default HTML

docker exec web-nomap bash -lc 'echo "<h1>Hello from INSIDE the container (no mapping)</h1>" > /var/www/html/index.html'

4. Start Nginx (foreground mode)

docker exec -d web-nomap nginx -g "daemon off;"

-d runs the process detached so your terminal doesn’t block.

5. Verify from inside the container (should work)

docker exec web-nomap curl -s http://localhost | head -n1

Expected: "<h1>Hello from INSIDE the container (no mapping)</h1>"

6. Try from the host (should FAIL)

curl -v http://localhost

Expected: Connection refused or similar. Why? Nginx is listening on the container’s localhost:80, which is not bound to your host network. No port mapping = no reachability from the host.

7. Clean up Part 1

docker rm -f web-nomap

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Part 2 — Run Nginx with port mapping (host curl should work)

1. Start a new Ubuntu container with port mapping

docker run -dit --name web-mapped --rm -p 8080:80 ubuntu

-p 8080:80 = host port 8080 → container port 80.

2. Install Nginx

docker exec web-mapped apt-get update
docker exec web-mapped apt-get install -y nginx curl

3. Change the HTML

docker exec web-mapped bash -lc 'echo "<h1>Hello from a mapped port (8080→80)</h1>" > /var/www/html/index.html'

4. Start Nginx (foreground)

docker exec -d web-mapped nginx -g "daemon off;"

5. Verify from inside the container

docker exec web-mapped curl -s http://localhost | head -n1

Expected: "<h1>Hello from a mapped port (8080→80)</h1>"

6. Verify from the host (should WORK now)

curl -s http://localhost:8080 | head -n1

Expected: "<h1>Hello from a mapped port (8080→80)</h1>"

7. (Optional) See the port binding

docker ps --filter name=web-mapped
# or
docker inspect -f '{{json .NetworkSettings.Ports}}' web-mapped | jq

8. Clean up

docker rm -f web-mapped

Submission Guidelines

To complete this lab:

1. Take screenshots of the Nginx container running and the port mapping.
   • part1-fail.png — screenshot showing curl localhost failing from the host (no port mapping).
   • part2-success.png — screenshot showing curl localhost:8080 succeeding from the host (with port mapping).
2. Paste the screenshots into a Google Doc.
3. Upload the document to Google Drive.
4. To submit the lab, paste the Google Drive link in the submission field in the Student Portal.

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Extra

Quick discussion prompts

• Why did curl localhost fail in Part 1 but succeed in Part 2?
• What happens if you use -p 80:80 instead and then visit http://localhost?
• How would multiple containers avoid clashing on host ports?

Common pitfalls & fixes

• Nginx “not found” → forgot apt-get update before apt-get install.
• curl localhost on host still fails → confirm you started the mapped container (docker ps) and you are curling the right host port (:8080).
• Nginx exits immediately → always start it with nginx -g "daemon off;" so it stays in the foreground (containers stop when the main process exits).