nginx关于http header有以下三个配置。

1 proxy_set_header

https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_set_header

配置命令proxy_set_header，是在ngx_http_proxy_module里边定义的。

用来对http request header进行修改。

2 add_header

https://nginx.org/en/docs/http/ngx_http_headers_module.html#add_header

配置命令 add_header，是在ngx_http_headers_module里边定义的。

用来对http server返回的http response header进行修改，

修改的方法是，追加的在header的最前面。

3 add_tailer

http://nginx.org/en/docs/http/ngx_http_headers_module.html#add_trailer

配置命令 add_tailer，是在ngx_http_headers_module里边定义的。

功能与add_header相同，区别是添加在header的结尾。

4 http header的顺序

既然有了添加到开头或者添加到结尾的区别。那么不禁要问，http header里边的KV结构是否存在顺序区别呢？

通过阅读rfc 2616，可以了解到如下内容：

rfc对http的header顺序没有强制要求，但是一个好的最佳实践一般是：

依次应该为：general-header,  request-header or response-header,  entity-header.

The order in which header fields with differing field names are received is not significant. However, it is "good practice" to send general-header fields first, followed by request-header or response- header fields, and ending with the entity-header fields.

另外， 如果有多个名字相同的header存在时，“实现”应该将他们的值压缩在一起，用逗号分隔。

逗号分隔后的内容是否包含了顺序信息应该有通讯两端定义，作为中间人的proxy不应该修改他们。

Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy MUST NOT change the order of these field values when a message is forwarded.