502 Bad Gateway

What HTTP 502 Bad Gateway Means

The server received an invalid response from the upstream server.

Gateway/proxy received invalid response from upstream server.

Common Causes

• Upstream server down
• PHP-FPM not running
• Application server crashed
• Network issue between servers
• Firewall blocking connection

How to Fix It (For Visitors)

• Refresh the page
• Try again in a few minutes
• Contact website administrator

How to Fix It (For Developers/Admins)

• Check upstream server status
• Restart PHP-FPM/application server
• Check Nginx proxy_pass configuration
• Verify firewall rules
• Check application server logs
• Increase timeout settings

Read the nginx Error Log Message

When nginx is the proxy, its error log names the exact problem:

Log message	Meaning
connect() failed (111: Connection refused) while connecting to upstream	Nothing is listening on the upstream address/port - the app or PHP-FPM is down or on another port.
connect() to unix:/run/php/php-fpm.sock failed (13: Permission denied)	nginx cannot access the PHP-FPM socket - fix listen.owner/listen.group.
upstream prematurely closed connection	The app crashed or was killed mid-request (often memory limits).
upstream sent too big header	Response headers exceed the buffer - raise proxy_buffer_size / fastcgi_buffer_size.
no live upstreams	All servers in the upstream group are marked as failed.

Step-by-step fixes are in the nginx 502 guide. Behind Cloudflare, origin problems usually show up as 520-526 errors instead.

Returning a 502 Bad Gateway (Code Examples)

If you build APIs or web apps, here is how to send an HTTP 502 response and how to test for it:

Node.js (Express)

app.get('/resource', (req, res) => {
  res.status(502).json({ error: 'Bad Gateway' });
});

Python (Flask)

@app.route('/resource')
def resource():
    return jsonify(error='Bad Gateway'), 502

PHP

<?php
http_response_code(502);
header('Content-Type: application/json');
echo json_encode(['error' => 'Bad Gateway']);

Check the status with curl

curl -I https://example.com/resource
# Look for: HTTP/1.1 502 Bad Gateway

Frequently Asked Questions

What does HTTP 502 Bad Gateway mean?

Gateway/proxy received invalid response from upstream server. In short: The server received an invalid response from the upstream server.

Is a 502 Bad Gateway error my fault or the website's?

502 is a 5xx server-error code, so the problem is on the server side, not your browser or device. As a visitor you can usually only retry; if you run the site, investigate the server.

How do I fix a 502 Bad Gateway error?

Check upstream server status. Restart PHP-FPM/application server. Check Nginx proxy_pass configuration.

Official Specification

The 502 Bad Gateway status code is defined in RFC 9110 Section 15.6.3.

View the IANA HTTP Status Code Registry →