Fix: Linux TCP Connection Refused

!Symptoms

• curl/wget returns 'Connection refused'
• Application unable to connect to a local or remote service
• Telnet to the port immediately returns 'Connection refused'
• Service appears to be running but not accepting connections

?Root Causes

• Service not listening on the expected port
• Service bound to 127.0.0.1 (localhost) instead of 0.0.0.0
• Firewall (iptables/nftables/ufw) blocking the port
• Service crashed and restarted but not yet accepting connections
• Wrong port number — service running on a different port

#Diagnosis Steps

1. 1Check if port is listening: `ss -tlnp | grep <port>`
2. 2Check what address it's bound to: `ss -tlnp | grep <port>` — look for 0.0.0.0 vs 127.0.0.1
3. 3Check firewall: `sudo iptables -L -n` or `sudo ufw status`
4. 4Check if service is running: `systemctl status <service>`
5. 5Try connecting locally: `curl http://127.0.0.1:<port>/`

>Fix

1. 1Start the service: `systemctl start <service>`
2. 2Change bind address from 127.0.0.1 to 0.0.0.0 in the service config
3. 3Open the port in the firewall: `sudo ufw allow <port>/tcp`
4. 4Fix the connection string to use the correct port
5. 5Wait for the service to fully start before connecting

*Prevention

• Use health check scripts that verify port availability after deployment
• Configure services to bind to 0.0.0.0 in production (or the specific interface)
• Document expected ports in service configuration
• Include firewall rules in infrastructure automation
• Set up monitoring for port availability on all critical services

Related Error Messages