Detailed explanation of the pitfalls of add_header in nginx configuration tutorial

Preface

add_header is a directive defined in the headers module. As the name suggests, it is used to add http response headers. But please note that it is just "added", not rewritten. So if a header already exists, using add_header will cause problems. Moreover, in earlier versions of nginx, add_header does not support use in error pages.

This is a command with many pitfalls. Its processing stage is later than location processing. Although it can be written in location, if another location is rewritten, the add_header that has not been processed in the previous location will be lost. for example:

location = /a {
 add_header a 1;
 rewrite //b;
}
location = /b {
 add_header b 2;
 return 204;
} 

There is no head a 1, right? This is a pit!

Another pitfall is the duplication problem mentioned at the beginning. For example, I want to set Content-Type for a content, but because a default_type is set globally, it is repeated.

default_type 'text/plain';

location = /a {
 add_header Content-Type application/json;
 return 200 '"OK"';
} 

There are of course many solutions, such as leaving the default_type empty for this location, or simply not using add_header and directly modifying the default_type for this location.

The last big pitfall is that it does not work on error pages, which is also clearly defined in the document. For example, the following example:

location = /a {
 add_header Content-Type application/json;
 return 404 '"Not Found"';
}

I want to respond with a JSON, but since the status code is 404, add_header here does not take effect.

Although this example can solve the problem with default_type, what if it is other headers? For example, what should I do with Access-Control-Allow-Origin? Then there is no solution except using Lua or other third-party modules. Of course, nginx is aware of this problem, so the document also states that a parameter called always is supported after version 1.7.5. Although nginx itself solves this problem, tengine based on 1.6.2 will have problems.

Summarize

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