const HEADER_FORWARDED = 0b00001; // When using RFC 7239
const HEADER_X_FORWARDED_FOR = 0b00010;
const HEADER_X_FORWARDED_HOST = 0b00100;
const HEADER_X_FORWARDED_PROTO = 0b01000;
const HEADER_X_FORWARDED_PORT = 0b10000;
const HEADER_X_FORWARDED_ALL = 0b11110; // All "X-Forwarded-*" headers
const HEADER_X_FORWARDED_AWS_ELB = 0b11010; // AWS ELB doesn't send X-Forwarded-Host
我无法理解这一点，为什么他们要这样设置？