The ngx_http_addition_module module is a filter that adds a text before and after a response. This module is not built by default, it should be enabled with the --with-http_addition_module configuration parameter.

location / {
    add_before_body /before_action;
    add_after_body  /after_action;
}

Adds a text returned as a result of processing a given subrequest, before the response body. An empty string ("") as a parameter cancels addition inherited from the previous configuration level.

Adds a text returned as a result of processing a given subrequest, after the response body. An empty string ("") as a parameter cancels addition inherited from the previous configuration level.

addition_types text/html;

Allows to add text in responses with the specified MIME types, in addition to “text/html”. The special value “*” matches any MIME type (0.8.29).