Why Am I Getting the \'upstream sent too big header while reading response header from upstream\' Error?

"upstream sent too big header while reading response header from upstream" Error: Identifying and Resolving the Issue

Encountering the error message "upstream sent too big header while reading response header from upstream" indicates that the header sent by the upstream server exceeds the maximum size specified by the client. To resolve this issue, it's essential to understand the factors contributing to the error.

Header Size Limit:
The Nginx server has a default header size limit of 4 KiB. If the header sent by the upstream server is larger than this limit, Nginx will issue the error message.

Possible Causes:

1. Excessive HTTP Headers: The upstream server may include unnecessary or oversized HTTP headers, causing the header size to exceed the limit.
2. Caching with FastCGI: When caching is enabled using FastCGI, additional headers are added to the response, potentially enlarging the header size.
3. Unfiltered Requests: If untrusted input is not properly validated, malicious requests could intentionally send large headers to exploit the server.
4. Configuration Errors: Incorrect configuration of the proxy_buffer_size, proxy_buffers, or proxy_busy_buffers_size directives can also lead to header size issues.

Resolution:

1. Increase Header Size Limit: To increase the maximum header size limit, adjust the fastcgi_buffers and fastcgi_buffer_size directives in the config file:

fastcgi_buffers 16 16k; 
fastcgi_buffer_size 32k;

2. Optimize HTTP Headers: Review the HTTP headers sent by the upstream server and remove unnecessary or redundant headers. Use compression for headers where applicable.
3. Disable Caching: If caching is not essential, consider disabling it for the affected requests by setting $skip_cache to 1.
4. Configure Proxying: Ensure proper configuration of the proxy_buffer_size, proxy_buffers, and proxy_busy_buffers_size directives to match the expected request and response sizes.
5. Filter Untrusted Input: Implement input validation techniques to prevent malicious requests from exploiting the server by sending excessive headers.

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