http request custom header, underscores cannot be used

Custom headers cannot use underscores

! ! Say important things eight times! Just take it as what I said eight times.

The question is, the header using underscore cannot be obtained from the server. Example:

• If the header is header_parameter_test1, the server cannot obtain it;
• But if you change it headerParameterTest1, you can get it.

So, is this a limitation of nginx? Linux limitations? Or is it a restriction of http (probably not, because the browser Request Headershows that it has been added), so what is the restriction...