What could cause 502 errors with reverse proxy SSL termination in my home lab?

[Brian27]

I’ve been trying to set up a reverse proxy for my home lab server to handle SSL termination, but I keep running into weird 502 errors from my backend service. The proxy itself seems fine, but the requests aren’t making it through cleanly. I’m wondering if my configuration for passing the correct headers is missing something, or if it’s a timeout issue I haven’t spotted.

[AaronZM]

Yeah, I chased a similar thing last month. the 502s started when the backend would occasionally close the connection under load, and the proxy just sat there with a bad gateway. I found that my upstream timeout values were too aggressive and the keepalive between proxy and upstream wasn't right. After bumping proxy_read_timeout and proxy_connect_timeout in Nginx, the errors dropped, but only when the backend got to steady state. It felt like a temporary mismatch between what the proxy expected and what the app would present during startup.

[MarkHC]

Another time I blamed headers and spent a day tweaking X-Forwarded-For and Host. Eventually I realized the backend is picky about the Host header; if you pass the host name that doesn't match the backend vhost, it just returns something odd and you end up with 502. I added a simple proxy_set_header Host $host and that cleared a chunk of the noise. Still not a full fix, but it helped sometimes.

[Kenneth81]

Sometimes the issue wasn't the proxy at all but the backend being unhealthy. I had a container that would crash after a long request, and the proxy kept trying to reuse a dead connection. We added a max_fails and a short fail_timeout, but that felt like treating symptoms. In the end we found a flaky startup script caused the backend to take longer than expected during deploys, which is when the 502s spiked.

[Ronald.M]

One thing to check: do you see the 502 tied to a timeout or to a particular path? If you can share what your proxy logs show, that might help.