How to add a response header on nginx when using proxy_pass?

add_header works as well with proxy_pass as without. I just today set up a configuration where I've used exactly that directive. I have to admit though that I've struggled as well setting this up without exactly recalling the reason, though.

Right now I have a working configuration and it contains the following (among others):

server {
    server_name  .myserver.com
    location / {
        proxy_pass  http://mybackend;
        add_header  X-Upstream  $upstream_addr;
    }
}

Before nginx 1.7.5 add_header worked only on successful responses, in contrast to the HttpHeadersMoreModule mentioned by Sebastian Goodman in his answer.

Since nginx 1.7.5 you can use the keyword always to include custom headers even in error responses. For example:

add_header X-Upstream $upstream_addr always;

---

Limitation: You cannot override the server header value using add_header.

Hide response header and then add a new custom header value

Adding a header with add_header works fine with proxy pass, but if there is an existing header value in the response it will stack the values.

If you want to set or replace a header value (for example replace the Access-Control-Allow-Origin header to match your client for allowing cross origin resource sharing) then you can do as follows:

# 1. hide the Access-Control-Allow-Origin from the server response
proxy_hide_header Access-Control-Allow-Origin;
# 2. add a new custom header that allows all * origins instead
add_header Access-Control-Allow-Origin *;

So proxy_hide_header combined with add_header gives you the power to set/replace response header values.

Similar answer can be found here on ServerFault

UPDATE:

Note: proxy_set_header is for setting request headers before the request is sent further, not for setting response headers (these configuration attributes for headers can be a bit confusing).

There is a module called HttpHeadersMoreModule that gives you more control over headers. It does not come with Nginx and requires additional installation. With it, you can do something like this:

location ... {
  more_set_headers "Server: my_server";
}

That will "set the Server output header to the custom value for any status code and any content type". It will replace headers that are already set or add them if unset.