HTTP 502 Bad Gateway — Causes and Fixes

What HTTP 502 means

HTTP 502 Bad Gateway comes from an intermediary — nginx, a load balancer, Cloudflare — that forwarded your request to an upstream server and received an invalid response: a connection refused, a crash mid-response, or bytes that do not parse as HTTP.

The named culprit is almost never the proxy itself; it is the application server behind it being down, overloaded or unreachable.

Common causes of 502 errors

The application server (PHP-FPM, Node, gunicorn, etc.) crashed or is not running.
The proxy points at the wrong port/socket, or a firewall blocks the upstream connection.
Upstream response exceeded buffers or timed out mid-transfer.
A deploy restarted backends while traffic was flowing.
With CDNs like Cloudflare: the origin server is down while the edge is fine.

How to fix it as a visitor

Reload after a minute — 502s during deploys resolve themselves.
Check the service's status page if it keeps happening.

How to fix it as a developer

Check whether the upstream process is alive and listening on the address the proxy expects.
Read the proxy's error log — nginx says exactly why (connect() failed, upstream prematurely closed, etc.).
Verify ports, unix socket paths and firewall rules between proxy and app.
Add health checks so the load balancer stops routing to dead instances.

Example response

HTTP/1.1 502 Bad Gateway
Server: nginx
Content-Type: text/html

<html><body><h1>502 Bad Gateway</h1></body></html>

FAQ

What is the difference between 502 and 504?

502 means the upstream answered with something invalid (or refused the connection); 504 means it did not answer within the timeout at all.

Why does Cloudflare show 502?

Cloudflare's edge could not get a valid response from your origin server — the origin is down, unreachable or returning malformed responses.

Is a 502 temporary?

Usually yes — deploys and restarts cause brief 502s. Persistent 502s mean the backend is genuinely down or misconfigured.