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[FIXED] Error 'curl: (7) Failed to connect to localhost port'

Read time: 4 min Updated: 2026-08-14
✨ Quick Answer

The error curl: (7) Failed to connect to localhost port 3000: Connection refused occurs when the curl CLI tool attempts to reach a local service endpoint, but no process is actively listening on that port or the server is bound strictly to IPv6 (::1) rather than IPv4 (127.0.0.1).

Quick Diagnostics

Cause
Service on the specified port is not listening
Solution
Verify listening ports: `netstat -tulnp
Cause
Docker container binding exclusively to 127.0.0.1 inside container
Solution
Bind service to 0.0.0.0 to accept external traffic

The error curl: (7) Failed to connect to localhost port 3000: Connection refused occurs when the curl CLI tool attempts to reach a local service endpoint, but no process is actively listening on that port or the server is bound strictly to IPv6 (::1) rather than IPv4 (127.0.0.1).

Quick Solution (1 Minute):

  1. Ensure your backend app is active in a separate terminal.
  2. Pass the explicit IPv4 address instead of the hostname localhost: curl http://127.0.0.1:3000

🚀 Step-by-Step Fixes

Step 1: Verify Port Listening Status

Check if your target port is open using ss:

BASH 
sudo ss -tulpn | grep :3000

If nothing returns, your backend process (Node.js, Python, Go) is not running. Start your dev server before issuing cURL requests.

Step 2: Resolve IPv6 vs IPv4 Hostname Conflict

On modern Linux distributions, localhost resolves to ::1 (IPv6) first. If your server app listens exclusively on 127.0.0.1 (IPv4), cURL fails upon trying IPv6.

Option A: Force cURL IPv4 mode:

BASH 
curl -4 http://localhost:3000

Option B: Use loopback IP directly:

BASH 
curl http://127.0.0.1:3000

Step 3: Configure Host Binding in Your Backend Code

Ensure your application binds to 0.0.0.0 or 127.0.0.1:

  • Express.js (Node.js):
    JAVASCRIPT 
    app.listen(3000, '0.0.0.0', () => {
  console.log('Server running on port 3000');
});
  • FastAPI / Uvicorn (Python):
    BASH 
    uvicorn main:app --host 0.0.0.0 --port 3000

Prevention Advice

Recommended security practices: