---
title: "How to Put a Web App Online With a Public URL"
description: "Running a web app or API on a server and want it reachable from a browser? Which port to listen on, how HTTPS works, and how to use your own domain."
url: https://fps.ms/docs/containers/public-url
---
# Put a web app online with a Public URL

Every Container on FPS.ms gets a Public URL, like `faint-forge.hostedbyfps.com`, with HTTPS already set up. To make your web app open on it, your app must listen on the port in the `SERVER_PORT` environment variable, on all addresses (`0.0.0.0`). Requests to your Public URL then reach your app, and anyone who has the link can open it.

## Find your Public URL

Your Public URL is on the **Console** page of your server, with **Copy link** and **Open** buttons. To manage it:

1. Click **Manage** on your server.
2. Click **More**, then **Public URL**.

![The More menu on a server page with Public URL highlighted](https://assets.docs.fps.ms/containers/public-url/01-open-public-url-from-more-b5d9b669.webp)

![The Your Public URL card showing faint-forge.hostedbyfps.com with Copy link and Open buttons, and the Subdomain card below it](https://assets.docs.fps.ms/containers/public-url/02-your-public-url-and-subdomain-7e90483e.webp)

## Make your app listen on the right port

FPS.ms sends the traffic for your Public URL to one port of your server. Your app finds that port in the `SERVER_PORT` environment variable. There is no `PORT` variable, so frameworks that look for `PORT` need you to pass the port yourself.

Listen on `0.0.0.0`, not on `localhost` or `127.0.0.1`. An app that listens on `localhost` only answers inside the server, so your Public URL shows an error.

### Python (Flask)

```python title="app.py"
import os
from flask import Flask

app = Flask(__name__)

@app.get("/")
def home():
    return "Hello from Cobalt Works"

if __name__ == "__main__":
    app.run(host="0.0.0.0", port=int(os.environ["SERVER_PORT"]))
```

For FastAPI, start Uvicorn the same way: `uvicorn.run(app, host="0.0.0.0", port=int(os.environ["SERVER_PORT"]))`.

### Node.js (Express)

```js title="index.js"
const express = require('express');

const app = express();
app.get('/', (req, res) => res.send('Hello from Cobalt Works'));

app.listen(Number(process.env.SERVER_PORT), '0.0.0.0', () => {
    console.log(`Listening on port ${process.env.SERVER_PORT}`);
});
```

### Go

```go title="main.go"
package main

import (
	"fmt"
	"net/http"
	"os"
)

func main() {
	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprintln(w, "Hello from Cobalt Works")
	})
	http.ListenAndServe("0.0.0.0:"+os.Getenv("SERVER_PORT"), nil)
}
```

Other runtimes read `SERVER_PORT` the way they read any [environment variable](https://fps.ms/docs/containers/environment-variables.md).

!!! info "Serve plain HTTP" "Your app does not need a certificate. FPS.ms handles HTTPS for your Public URL and passes the requests on to your app as plain HTTP."

## Choose your own name

Free servers get a random name. On a paid server, the **Subdomain** card lets you pick any free name on `hostedbyfps.com`, using lowercase letters, numbers and dashes. Your old link stops working when you save a new name.

## Use your own domain

The **Your own domain** card connects a domain to your server. HTTPS is set up for you.

![The Your own domain card with the FPS.ms domain and External domain tabs](https://assets.docs.fps.ms/containers/public-url/03-connect-your-own-domain-15b49f54.webp)

- **FPS.ms domain**, on every plan: a domain you bought on FPS.ms that uses FPS DNS. Pick it, optionally type a subdomain like `api`, and connect. FPS.ms adds the DNS record for you.
- **External domain**, on paid plans: a domain from another registrar.
  1. In your domain's DNS settings, add an A record pointing to the IP address the panel shows.
  2. Type your domain, for example `api.cobaltworks.nl`.
  3. Click **Verify and connect**. New DNS records can take a few minutes to work. If it fails, wait and try again.

## Common problems

- **The Public URL shows an error page:** your app is not running, or it listens on another port or on `localhost`. Check the **Console** for the line your app prints when it starts, and use `SERVER_PORT` and `0.0.0.0`.
- **`EADDRINUSE` or "address already in use":** your app tries to open a port that is already taken. See [fix an app that will not start](https://fps.ms/docs/containers/troubleshooting.md).
- **A Discord or Telegram bot:** a bot that connects out to Discord, or a Telegram bot that uses polling, does not need a Public URL. A Telegram bot in webhook mode does: set your Public URL as its webhook address and listen on `SERVER_PORT`.

## Next steps

- [Environment variables and .env files](https://fps.ms/docs/containers/environment-variables.md)
- [Deploy from GitHub](https://fps.ms/docs/containers/deploy-from-github.md)
- [Supported runtimes](https://fps.ms/docs/containers/supported-runtimes.md)

---

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