How to Fix 502 Bad Gateway Error

What is a 502 Bad Gateway Error?

A 502 Bad Gateway error occurs when one server (acting as a gateway or proxy) receives an invalid response from another server it's trying to reach. This is a server-side error, meaning the problem is with the website's infrastructure, not your computer or connection.

Common scenarios where you'll see 502 errors:

• A reverse proxy (like Nginx) can't reach the application server
• A CDN (like Cloudflare) can't connect to the origin server
• A load balancer receives a bad response from backend servers
• The upstream server is overloaded or crashed

Common Causes of 502 Bad Gateway

1. Origin Server Down - The backend server crashed or stopped
2. Server Overload - Too many requests overwhelming the server
3. PHP-FPM Issues - PHP process manager not responding
4. Timeout Issues - Upstream server taking too long to respond
5. Firewall Blocking - Firewall rules blocking internal communication
6. DNS Problems - Incorrect DNS resolution between servers
7. Bad Deployment - Recent code deploy broke the application
8. Memory Exhaustion - Server ran out of RAM

How to Fix 502 Bad Gateway: For Visitors

If you're just visiting a website and see a 502 error:

1. Refresh the Page

Sometimes 502 errors are temporary. Wait a few seconds and refresh (Ctrl+F5 or Cmd+Shift+R for a hard refresh).

2. Clear Browser Cache

Your browser might have cached a bad response. Clear your cache and cookies, then try again.

3. Try a Different Browser or Device

Rule out browser-specific issues by trying another browser or your phone.

4. Check if the Site is Down for Everyone

Use a service like URL Status Checker to see if the site returns 502 from our servers too.

How to Fix 502 Bad Gateway: For Website Owners

1. Check if Your Origin Server is Running

# Check if web server is running
sudo systemctl status nginx
sudo systemctl status apache2

# Check if application is running
sudo systemctl status php-fpm
pm2 status  # For Node.js apps

# Restart if needed
sudo systemctl restart nginx
sudo systemctl restart php-fpm

2. Check Server Logs

# Nginx error log
sudo tail -f /var/log/nginx/error.log

# Apache error log
sudo tail -f /var/log/apache2/error.log

# PHP-FPM log
sudo tail -f /var/log/php-fpm/error.log

# Application logs
sudo tail -f /var/www/myapp/logs/error.log

3. Check Server Resources

# Check CPU and memory usage
htop
free -m

# Check disk space
df -h

# Check for zombie processes
ps aux | grep -E 'Z|defunct'

4. Check PHP-FPM Configuration

Most 502 errors with PHP applications are caused by PHP-FPM issues:

# Check PHP-FPM status
sudo systemctl status php8.2-fpm

# Edit PHP-FPM pool config
sudo nano /etc/php/8.2/fpm/pool.d/www.conf

# Important settings to check:
pm = dynamic
pm.max_children = 50
pm.start_servers = 5
pm.min_spare_servers = 5
pm.max_spare_servers = 35
pm.max_requests = 500

# Restart after changes
sudo systemctl restart php8.2-fpm

5. Increase Timeout Values

If your application is slow, increase proxy timeouts in Nginx:

# In nginx.conf or site config
location / {
    proxy_pass http://127.0.0.1:3000;
    proxy_connect_timeout 60s;
    proxy_send_timeout 60s;
    proxy_read_timeout 60s;
}

# For PHP-FPM
location ~ \.php$ {
    fastcgi_read_timeout 300;
    fastcgi_send_timeout 300;
}

6. Check Firewall Rules

# Check if internal ports are open
sudo ufw status

# Allow internal communication
sudo ufw allow from 127.0.0.1

# Check iptables
sudo iptables -L -n

502 Error with Cloudflare

If you're using Cloudflare and see a 502 error:

• Error 502 - Cloudflare received a bad response from your origin
• Error 521 - Your origin server is down (Cloudflare-specific)
• Error 522 - Connection timed out to origin

Solutions:

1. Check your origin server is running
2. Whitelist Cloudflare IP ranges in your firewall
3. Temporarily enable "Development Mode" to bypass cache
4. Check Cloudflare's system status page

502 vs Other 5xx Errors

Error	Meaning	Likely Cause
500	Internal Server Error	Application code bug
502	Bad Gateway	Upstream server issue
503	Service Unavailable	Server overloaded/maintenance
504	Gateway Timeout	Upstream server too slow

Preventing 502 Errors

• Monitor your servers - Set up uptime monitoring to catch issues early
• Scale appropriately - Add more server resources before you need them
• Use health checks - Configure load balancers to detect unhealthy backends
• Implement graceful restarts - Don't kill processes during deployments
• Optimize your application - Slow code leads to timeouts and 502s