nginx Magic Variables

Source

Let's say an upstream, proxied by nginx, sets a cookie foo=bar in its HTTP response. To use this cookie name as a variable in nginx configuration, use the magic prefix $cookie_ and the variable becomes $cookie_foo. But how did a cookie become a variable? I call it a magic variable. It could also be called an arbitrary variable.

nginx publishes its complete list of variables. Notice how some variables end in an underscore, which as of writing are,

These are variable name prefixes (for want of a better terminology). Their respective modules create magic or arbitrary variables from the suffix, which has a special meaning to the module.

In our example, the cookie name foo becomes the suffix which ngx_http_core_module uses to create the variable $cookie_foo. We don't have to explicitly create or set this variable; it is available for us to use.

Both $upstream_header_time and $upstream_http_server are examples of embedded variables (provided by ngx_http_upstream_module). Since $upstream_http_ is in our list above, it is an embedded and a magic (arbitrary) variable. The "magic" part comes from the suffix server which is the value from the HTTP response header, Server. Meanwhile, $upstream_header_time is only an embedded variable.

Read the official documentation for more information.