nginxintermediatenetworkingVerified

Nginx 502 Bad Gateway: Causes and Fixes

Last reviewed: 9/14/2026

3 solutions

Exact Error Message

502 Bad Gateway

Quick Fix

Check if upstream server is running and accessible: curl http://upstream-server:port

What This Error Means

A 502 Bad Gateway error occurs when Nginx acts as a reverse proxy and cannot receive a valid response from the upstream server. This typically means the upstream server is down, misconfigured, or refusing connections.

Common Symptoms

•Users see 502 Bad Gateway error
•Nginx error logs show upstream connection failures
•Partial or complete site outage
•Intermittent connection failures

Common Causes

•Upstream server is down or crashed
•Upstream server is not listening on expected port
•Firewall blocking connection to upstream
•Upstream server overloaded or slow
•DNS resolution issues
•Nginx misconfiguration

Diagnostic Steps

1Check Nginx error logs: tail -f /var/log/nginx/error.log
2Test upstream server connectivity directly
3Check upstream server status and logs
4Verify firewall rules
5Check DNS resolution
6Review Nginx configuration

Solutions

Solution 1: Start or restart upstream server

1Check if upstream service is running
2Start the service if stopped
3Restart if service is in bad state
4Verify service is listening on correct port

Commands to Run

Restarting production services may cause brief downtime

sudo systemctl status SERVICE_NAME

sudo systemctl restart SERVICE_NAME

sudo netstat -tlnp | grep PORT

Solution 2: Fix firewall rules

1Check firewall status
2Allow connections from Nginx to upstream
3Verify security group rules (AWS)
4Test connectivity after changes

Commands to Run

Firewall changes affect system security

sudo ufw status

sudo ufw allow from NGINX_IP to any port PORT

sudo iptables -L

Solution 3: Fix Nginx configuration

1Review upstream server configuration
2Check proxy_pass directive
3Verify server names and ports
4Test configuration: nginx -t
5Reload Nginx: nginx -s reload

Commands to Run

Invalid Nginx configuration will prevent reload

nginx -t

sudo nginx -s reload

Prevention Tips

✓Implement health checks for upstream servers
✓Use load balancing with multiple upstreams
✓Monitor upstream server health
✓Set appropriate proxy timeouts
✓Implement graceful degradation

Version Notes: Applies to Nginx 1.18+

Official Documentation

Nginx cannot connect to upstream server because it's not running or not accepting connections.

Was this helpful?