502 Bad Gateway in nginx - Rocketeers app

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502 Bad Gateway in nginx
========================

### [\#Errors](https://rocketeersapp.com/knowledge/errors)

A 502 Bad Gateway means nginx reached your application but got an invalid response back. In almost every case the backend (PHP-FPM) crashed, never started, or is unreachable.

 Published by [Mark van Eijk](https://rocketeersapp.com/author/mark-van-eijk) on June 23, 2026 · 1 minute read

1. [About error 502](#content-about-error-502)
2. [Why do I see this error](#content-why-do-i-see-this-error)
3. [Solution](#content-solution)

[\#](#content-about-error-502 "Permalink")About error 502
---------------------------------------------------------

A `502 Bad Gateway` is returned by nginx when it is acting as a reverse proxy and receives an invalid response from the upstream server it forwards requests to. For a PHP application that upstream is almost always **PHP-FPM**.

The important thing to understand: nginx itself is fine. The problem sits *behind* nginx, in the service it is trying to talk to.

[\#](#content-why-do-i-see-this-error "Permalink")Why do I see this error
-------------------------------------------------------------------------

The most common causes, roughly in order:

- PHP-FPM is not running, crashed, or was never started.
- nginx points to the wrong PHP-FPM socket or port (a typo in `fastcgi_pass`).
- The socket file exists but the nginx user can't read it (wrong permissions).
- The backend died halfway through the request (often an out of memory kill).

Always start by reading the nginx error log, it names the exact reason:

 ```
tail -f /var/log/nginx/error.log

```

You'll see a line ending in something like `connect() to unix:/run/php/php8.3-fpm.sock failed (2: No such file or directory)` or `(111: Connection refused)`. That bracketed code tells you everything.

[\#](#content-solution "Permalink")Solution
-------------------------------------------

First, check whether PHP-FPM is actually running and start it if not:

 ```
systemctl status php8.3-fpm
systemctl restart php8.3-fpm

```

Then confirm nginx is pointing at the socket that PHP-FPM actually listens on. The path in your nginx config must match the `listen` directive in the pool config (`/etc/php/8.3/fpm/pool.d/www.conf`):

 ```
location ~ \.php$ {
    fastcgi_pass unix:/run/php/php8.3-fpm.sock;
    include fastcgi_params;
}

```

If the socket exists but you still get `(13: Permission denied)`, make sure the pool runs as the same user as nginx (usually `www-data`):

 ```
user = www-data
group = www-data
listen.owner = www-data
listen.group = www-data

```

Reload both services after any change:

 ```
systemctl reload php8.3-fpm
systemctl reload nginx

```

If the backend keeps dying mid-request rather than refusing the connection, the cause is usually resources, not configuration. Check the [504 Gateway Timeout](/504-gateway-timeout-nginx) article for slow responses, and [PHP allowed memory size exhausted](/php-allowed-memory-size-exhausted) for out of memory kills. When the backend is up but every worker is busy, you'll see a [503 Service Unavailable](/503-service-unavailable) instead.

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