Render: Port scan timeout reached, no open ports detected (express)

Solved · 265 views · asked 3 weeks ago by rocco · edited

My express API deploys on Render, logs even say "Server running on http://localhost:3000", and then after a few minutes:

==> Port scan timeout reached, no open ports detected. Bind your service to at least one port. If you don't need to receive traffic on any port, create a background worker instead.

app.listen(3000, 'localhost', () => console.log('Server running on http://localhost:3000'));

What I’ve tried

Set the port to 3000 in Render environment settings, redeployed.

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chidi_eze3 weeks ago · edited

Two things in that line:

'localhost' means only processes inside the container can reach it. Render's router is outside, so it sees nothing. Bind to all interfaces.
Render tells you which port to use via PORT. Read it instead of hardcoding.

const port = process.env.PORT || 3000;
app.listen(port, '0.0.0.0', () => console.log(`Listening on ${port}`));

Then remove the PORT you set manually, let Render inject it.

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deployed, live. chatgpt wrote 'localhost' in every example it gave me rocco 3 weeks ago · edited

Same issue with uvicorn defaulting to 127.0.0.1, for the Python folks. grace_mw 3 weeks ago · edited

Same root cause on Railway and Fly, for anyone searching. anjali_p 3 weeks ago · edited

ollie_dev3 weeks ago · edited

Separate thing you'll notice next: Render's free tier spins the service down after inactivity, so the first request after a quiet period is slow. Not a bug in your code.

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grace_mw3 weeks ago · edited

Same fix for a Python backend on Render, since this comes up a lot:

uvicorn main:app --host 0.0.0.0 --port $PORT

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Render: Port scan timeout reached, no open ports detected (express)

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Render: Port scan timeout reached, no open ports detected (express)

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