nginx add header conditional on an upstream_http_ variable

Solution 1:

This doesn't work because the if is evaluated before the request is passed to the backend, so the $upstream_http_ variables don't have any values yet. add_header with an empty value is ignored, so you can use a map to conditionally add the header like so:

map $upstream_http_strict_transport_security $sts {
  '' max-age=15552000;
}

server {
  location / {
    add_header Strict-Transport-Security $sts;
  }
}

Solution 2:

That's because if is executed before proxy take place and at this moment there is no variable $upstream_http_strict_transport_security.

You could use header_filter_by_lua directive from Lua module. E.g.

header_filter_by_lua 'if not ngx.header["X-Test"] then ngx.header["X-Test"] = "blah" end';