Fly Dev Tunnel

Developers commonly use apps like ngrok, localtunnel, or cloudflared to expose a local web service at a publicly-accessible URL. This is useful for developing with HTTPS or sharing a site preview with a colleague or client.

By utilizing Fly, WireGuard, and a reverse proxy, you can achieve something similar with the added benefits of a custom domain and subdomains mapped to specific ports. Best of all, it’s free as long as you stay within Fly’s generous limits.

1. Install flyctl, the Fly command-line utility

Follow the instructions for your OS. After installation, run flyctl auth login to sign up or sign in.

2. Connect to Fly via WireGuard

Install WireGuard for your OS. Next, run flyctl wireguard create to create a WireGuard config. Use the following settings:

Region: dev
DNS name for peer: <any name for your machine>
Filename: fly.conf

Note: The default config sets Fly as your DNS resolver. If you don’t need Fly’s internal DNS features, edit fly.conf and comment out the DNS line with a pound sign (#).

Finally, setup the tunnel in WireGuard:

• On Mac or Windows, open the WireGuard app and click Import Tunnel(s) from File. Once imported, click Activate to connect.
• On Linux, use the command line.

3. Create the reverse proxy app

Run mkdir tunnel && cd tunnel to create an empty app folder. Run flyctl init to create a fly.toml config file. Use the following settings

App name: <any name for you app>
Select builder: Image
Select image: lukelambert/fly-dev-tunnel
Internal port: 8080

4. Configure the reverse proxy

The reverse proxy is configured using two environment variables:

• SUBDOMAINS: A comma-separated list in the format subdomain:local_port. An underscore (_) matches the default (catch all) domain.
• UPSTREAM: The IPv6 address of your local machine on the WireGuard network. To find this value, open the fly.conf file created in step 2 and copy the address starting with fd and ending just before the forward slash.

Edit fly.toml and add the following lines at the bottom, replacing the values with your own:

[env]
  SUBDOMAINS = "_:8000"
  UPSTREAM = "fdaa:0:ffff:ffff:ffff:0:0:1"

5. Deploy the reverse proxy

Run flyctl deploy. Once the app is deployed, you should have a tunnel from https://your-app-name.fly.dev to port 8000 on your local machine.

6. (Optional) Connect custom (sub)domains

Visit the Apps dashboard and select your app. Under the Certificates section, follow the instructions to add a custom domain. You can also add a wildcard subdomain, but this incurs a monthly fee. To map subdomains to ports, update your fly.toml and re-run flyctl deploy. Example:

[env]
  SUBDOMAINS = "_:8000,app1:9001,app2:9002"
  UPSTREAM = "fdaa:0:ffff:ffff:ffff:0:0:1"

Notes

All traffic is proxied over IPv6, so your web service should bind to an IPv6 address. To take down the tunnel and prevent traffic from reaching your machine, simply deactivate the WireGuard tunnel.