Some applications fail to establish websocket connections if that app has larger header values

[clord]

Checklist

• Have you pulled and found the error with jc21/nginx-proxy-manager:latest docker image?
  • Yes
• Are you sure you're not using someone else's docker image?
  • Yes
• Have you searched for similar issues (both open and closed)?
  • Yes

Describe the bug

Using an app built with zero-sync through nginx proxy leads to the following log and failure to set up websocket connection:

2025/07/26 15:37:55 [error] 240#240: *1323 upstream sent too big header while reading response header from upstream, client: 10.68.3.183, server: vite-proxy, request: "GET /sync/v18/connect?clientID=31hmlmpgukp4alk6&clientGroupID=gr3edb4abccfqda9&userID=anon&baseCookie=&ts=95628.09999046&lmid=0&wsid=7u0wuybbCMRuFCW HTTP/1.1", upstream: "http://10.68.3.1:4848/sync/v18/connect?clientID=ogukp&clientGroupID=gr3edc1&userID=anon&baseCookie=&ts=258.0999&lmid=0&wsid=WRuFCkEW", host: "vite-proxy"

Nginx Proxy Manager Version
v2.12.6

To Reproduce
Build an app with zero sync and try to proxy it through nginx proxy manager.

Expected behavior
proxy websocket connections.

Operating System
nix linux

Additional context
I think the core problem is that zero passes large header through a header, which is not out of spec for http but nginx filters. I suggest nginx proxy manager sets:

• large_client_header_buffers 4 32k - Allows larger headers
• client_header_buffer_size 32k - Increases the header buffer size