What causes 502 Bad Gateway with nginx reverse proxy to my backend service?

[JonathanOW]

I’ve been trying to set up a reverse proxy for my home server using Nginx, but I keep running into 502 Bad Gateway errors. My backend service is definitely running on the port I specified, but the proxy just won’t connect properly. I’m wondering if my configuration file syntax is wrong or if there’s a firewall rule on the server itself blocking the internal connection.

[Aaron_J]

I chased 502s for a while after I changed the port in the proxy config and forgot to reload. It finally started working once I restarted the proxy service and reloaded the config.

[Kenneth51]

The error log kept saying upstream connection refused, which pointed to the backend not listening on the interface the proxy was reaching. I verified the service was bound to localhost and the port, then tried a curl from the host to confirm it answered.

[Madison_B]

Did you check the error log for upstream errors? Sometimes the backend process crashes and comes back slowly, leaving the proxy with a 502 until it stabilizes.

[Justin75]

Another angle I stumbled on is the firewall or container networking. If the backend runs in a container, the proxy might be trying to reach the host port while the container guard blocks it. In my case opening the local port or using the container network made the difference.