Troubleshooting Guide: Nginx 'upstream sent too big header' Error

Locate and examine the Nginx error log to confirm the exact error message and identify the problematic upstream.

tail -f /var/log/nginx/error.log
grep -i "upstream sent too big header" /var/log/nginx/error.log

If using `proxy_pass`, increase the buffer size for reading the response header from the upstream server. Apply this within the relevant `location`, `server`, or `http` block.

location /api/ {
    proxy_pass http://backend_server;
    proxy_buffer_size 16k;
    proxy_buffers 4 16k;
}

If using PHP-FPM via `fastcgi_pass`, adjust the FastCGI buffer directives. This is a common fix for WordPress or Laravel applications.

location ~ \.php$ {
    fastcgi_pass unix:/var/run/php/php8.2-fpm.sock;
    fastcgi_buffer_size 16k;
    fastcgi_buffers 4 16k;
}

For Python applications using uWSGI, adjust the `uwsgi_buffer_size` and `uwsgi_buffers` directives.

location / {
    uwsgi_pass unix:/tmp/uwsgi.sock;
    uwsgi_buffer_size 16k;
    uwsgi_buffers 4 16k;
}

Always test your Nginx configuration syntax before applying changes to avoid taking down the server.

nginx -t
systemctl reload nginx

If increasing buffers doesn't resolve the issue, the upstream application itself may be generating excessively large headers. Use `curl -I` or browser dev tools to inspect the response headers.

curl -I https://your-domain.com/
# Look for large 'Set-Cookie', 'Location', or custom headers