How to fix nginx throws 400 bad request headers on any header testing tools?

Question

I have my site which is using nginx, and testing site with header testing tools e.g. http://www.webconfs.com/http-header-check.php but every time it says 400 bad request bel

Answer 1

normally, Maxim Donnie's method can find the reason. But I encountered one 400 bad request will not log to err_log. I found the reason with the help with tcpdump