Why am I getting the \"upstream sent too big header while reading response header from upstream\" error in NGINX?

NGINX Upstream Error: "upstream sent too big header while reading response header from upstream"

When encountering the error message "upstream sent too big header while reading response header from upstream," it indicates that the size of the headers received from the upstream server exceeds the configured buffer limit. This can cause the request to fail with a 502 Bad Gateway error.

Potential Causes and Solution:

The error typically occurs due to the following reasons:

• Large HTTP Headers: The headers returned by the upstream server may be too large for the configured buffer size.
• Faulty Upstream Configuration: The upstream server may be misconfigured and sending malformed or excessively large headers.

To resolve this issue, consider the following solutions:

1. Increase Header Buffer Sizes:

Modify the NGINX configuration file and increase the fastcgi_buffer_size and fastcgi_buffers directives to allow for larger header sizes. For example:

fastcgi_buffers 16 16k; 
fastcgi_buffer_size 32k;

2. Check Upstream Server Configuration:

Verify the configuration of the upstream server to ensure it is sending headers within the expected size limits. Check for any misconfigurations, such as unusually long user-agent strings or excessive headers.

3. Disable Cache:

The error can sometimes occur due to caching issues. Try disabling the fastcgi cache temporarily to see if it resolves the problem. Add the following line to your NGINX configuration:

fastcgi_cache_bypass $skip_cache;

4. Logging:

Enable logging in NGINX to capture more detailed information about the error. Add the following line to your NGINX configuration:

error_log /var/log/nginx/error.log error;

Inspect the error log for any additional clues about the cause of the problem.

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