Connection Refused Error - Complete Troubleshooting Guide

Step 1: Check if Service is Running

Step 2: Test Port Connectivity

1. Service Not Running

Cause: The application/server is not started

Solutions:

• Web Development:
  npm start (Node.js/React)
  python manage.py runserver (Django)
  flask run (Flask)
  ng serve (Angular)
• Database Services:
  sudo systemctl start mysql (MySQL)
  sudo systemctl start postgresql (PostgreSQL)
  redis-server (Redis)
  mongod (MongoDB)

2. Wrong Port Number

Cause: Trying to connect to the wrong port

Solutions:

• Check your application's configuration files
• Look for port numbers in console output when starting the service
• Common default ports:
  • 3000 - React, Express, Rails
  • 8080 - Spring Boot, Tomcat
  • 5000 - Flask, .NET Core
  • 4200 - Angular
  • 3306 - MySQL
  • 5432 - PostgreSQL

3. Service Crashed or Failed to Start

Cause: The service started but then crashed

Solutions:

• Check application logs for error messages
• Look for syntax errors in your code
• Verify all dependencies are installed
• Check for configuration file errors
• Ensure required environment variables are set

4. Firewall Blocking Connection

Cause: Firewall is blocking the port

Solutions:

5. Service Binding to Wrong Interface

Cause: Service is only listening on specific network interface

Solutions:

• Configure service to bind to all interfaces (0.0.0.0)
• Examples:
  flask run --host=0.0.0.0
python manage.py runserver 0.0.0.0:8000
node server.js --host 0.0.0.0
• Try connecting to 127.0.0.1 instead of localhost

React Development Server

• Check if another React app is running on port 3000
• Look for the message "Something is already running on port 3000"
• Try: PORT=3001 npm start
• Kill existing processes: kill -9 $(lsof -t -i:3000)

Spring Boot Application

• Check application.properties for server.port setting
• Look for startup logs indicating the port
• Verify Java application started successfully
• Check for binding exceptions in logs

Database Connections

• MySQL: Check if mysqld service is running
• PostgreSQL: Verify postgresql service status
• MongoDB: Ensure mongod process is active
• Redis: Test with redis-cli ping