Reserved Domain or Dynamic Subdomain for Free-Tier Ngrok Tunnels

Answer to the question

With the current free tier of Ngrok you cannot obtain a reserved domain (e.g. myapp.ngrok.io). The only endpoint that persists across restarts is a paid‑plan reserved subdomain or a custom domain that points to Ngrok. For a free account the URL changes every time the tunnel is started or the client reconnects, so any client configuration that expects a fixed address will break unless you upgrade or automate the update.

Confirmed facts

• Free tier assigns a random subdomain under ngrok.io for each tunnel session.
• The --subdomain flag is only accepted when an authtoken is linked to a paid plan.
• Ngrok does not cache DNS responses; the URL is resolved on each request.
• When a tunnel is reused without a full process restart, the local TCP proxy can keep stale connections to the previous remote endpoint, which may cause unexpected timeouts or data corruption.

Practical steps for the free‑tier case

1. Start the tunnel normally:

   ngrok http 80

2. Copy the URL shown in the console (e.g. https://abc123.ngrok.io).
3. Use an automated script or CI job that queries the Ngrok API (https://api.ngrok.com/tunnels) to fetch the current public URL and updates your webhook or API client configuration at runtime.
4. If you need a truly static endpoint, consider upgrading to the Basic or Pro plan and request a reserved subdomain or set up a custom domain.

Stale TCP state and its mitigation

When you stop the Ngrok process with Ctrl+C or ngrok kill, the local TCP proxy closes all open connections. If you simply restart the same client process without a clean shutdown, the proxy may keep a half‑open session that points to the old tunnel, leading to timeouts or dropped packets. To avoid this:

• Always fully terminate the Ngrok process before starting a new tunnel.
• Use ngrok kill or send a SIGTERM to the process.
• If you run Ngrok behind a reverse proxy, configure the proxy to drop idle connections or use a health‑check that verifies the tunnel’s health before forwarding traffic.

When to ask for more information

If you need the tunnel to stay alive for longer than the free tier’s 8‑minute idle timeout, or if you are using a reverse proxy that might keep stale connections, let me know. That detail could change the recommendation for how you manage the tunnel lifecycle.