Let's Play with Killercoda

I am BIG fan of Killercoda and especially their Ubuntu Playground that I am using of all sorts of exploration.

The Problem

There is only one ANNOYING anti-feature being that you can only access HTTP services running on all interfaces (like 0.0.0.0), which means that when you want to test an HTTP service running just on localhost, it does not work :-( This is very frustrating... until I find a workaround that I would like to share here ;-)

Let's first show the issue using the classic HTTP server you can get from a few lines of JavaScript using Node.js.

// server.mjs
import { createServer } from 'node:http';

const server = createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello World!\n');
});

// starts a simple http server locally on port 3000
server.listen(3000, 'localhost', () => {
  console.log('Listening on localhost:3000');
});

// run with `node server.mjs`

We first need to install Node.js on our Ubuntu Playground using the following commands:

curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.3/install.sh | bash

\. "$HOME/.nvm/nvm.sh"

nvm install 24

node --version # Should print "v24.12.0".

npm -v # Should print "11.6.2".

Then, we can create a file server.mjs and copy-paste the content above:

vi server.mjs

And, finally, we can run this simple HTTP server that runs on localhost only:

node server.mjs

We can verify that it runs correctly by using curl in another terminal:

curl localhost:3000

which should give the following response:

Hello World!

But, if you click on the little hamburger icon in the top-right corner, select "Traffic /Ports", type 3000 in the "Custom Ports" edit box and click on the "Access" button, you get an annoying "502 Bad Gateway" response, according to the documentation which says: "The services need to run on all interfaces (like 0.0.0.0) and not just localhost".

If you are the owner of the source code, like in this situation, the fix is easy, you just follow the documenation and change localhost to 0.0.0.0 and it will work. Have a try:

// server.mjs
import { createServer } from 'node:http';

const server = createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello World!\n');
});

// starts a simple http server locally on port 3000
server.listen(3000, '0.0.0.0', () => {
  console.log('Listening on 0.0.0.0:3000');
});

// run with `node server.mjs`

Let's now revert the situation to localhost and see how we can workaround the limitation in another way :-)

A realistic example would be if you want to test the Develop with containers hands-on guide from Docker. That's actually why I search for a workaround ;-)

The Workaround

The trick is to use a NGINX instance with the following reverse proxy configuration:

events {
  worker_connections 768;
}

http {
  server {
    listen 0.0.0.0:8080;

    location / {
      proxy_pass http://localhost:3000/;
    }

  }
}

You will then have an HTTP service running on 0.0.0.0 (port 8080, or the one you like) and the only thing it will do is to transfer the requests to localhost (port 3000).

Let's first install NGINX:

sudo apt update

sudo apt install -y nginx

systemctl status nginx

You can test the installation by clicking on the little hamburger icon in the top-right corner, select "Traffic /Ports" and click on 80 in the "Common Ports" section.

Let's backup the origniam nginx.conf file:

sudo mv /etc/nginx/nginx.conf /etc/nginx/nginx.conf.bak

and create our new nginx.conf file:

sudo vi /etc/nginx/nginx.conf

to copy-paste the content above.

Then, we still need to update NGINX with this new configuration:

sudo nginx -s reload

You can test this configuration by clicking on the little hamburger icon in the top-right corner, select "Traffic /Ports" and click on 8080 in the "Common Ports" section. You should now see the response coming from the HTTP service running just on localhost. Yes!

References

• https://nodejs.org/en/download