WMS query parameters disappear behind the proxy

• ServiceException code MissingParameterValue with the text Could not find the LAYERS parameter.
• GetMap returns an XML error document instead of a PNG.
• GeoServer access logs show no BBOX for a request the client clearly sent.

1. Enable an access log with $request and $upstream_addr inside the location to compare inbound and forwarded requests.
2. Check whether proxy_pass ends with a URI such as http://backend/wms rather than only host and port.
3. Check for a rewrite ending in a question mark, which clears the query string.
4. Check whether the frontend encoded the whole query string so ampersands became %26.
5. curl the same URL through the proxy and inspect the upstream log.

Use proxy_pass http://127.0.0.1:8080 without a URI so the original path and query are preserved; when a path rewrite is genuinely needed, end with proxy_pass http://127.0.0.1:8080$uri$is_args$args. Build query strings with URLSearchParams so only values are encoded.

<ServiceException code="MissingParameterValue">
  Could not find the LAYERS parameter
</ServiceException>

location /geoserver/ {
    proxy_pass http://127.0.0.1:8080/geoserver/;
    access_log /var/log/nginx/gis_proxy.log custom;
}

# log_format custom '$remote_addr "$request" -> $upstream_addr $upstream_status';

• Appending /geoserver to proxy_pass to make the path look right, which is exactly what discards the parameters.
• Running encodeURIComponent over the whole string, turning ampersands into %26 so the server sees a single parameter.