Final answer:
The nginx: [warn] could not build optimal proxy_headers_hash error is a common error message encountered when setting up a reverse proxy server using Nginx.
Step-by-step explanation:
The nginx: [warn] could not build optimal proxy_headers_hash error is a common error message encountered when setting up a reverse proxy server using Nginx. This error occurs when the proxy_headers_hash_max_size and proxy_headers_hash_bucket_size values specified in the Nginx configuration file are not suitable for the server's workload.
To solve this error, you need to adjust the proxy_headers_hash_max_size and proxy_headers_hash_bucket_size values to fit the requirements of your server. These values control the size of the hash table used for caching headers, and can be adjusted based on the expected number of headers and their size.
For example, you can try increasing both values to a higher value, such as proxy_headers_hash_max_size 512; and proxy_headers_hash_bucket_size 64;. This should provide enough space in the hash table to accommodate the headers and prevent the error from occurring.