Nginx - HTTP Configuration, Module Variables

The HTTP Core module introduces a large set of variables that you can use within the value of directives. Be careful though, as only a handful of directives accept variables in the definition of their value. If you insert a variable in the value of a directive that does not accept variables, no error is reported; instead the variable name appears as raw text.

There are three different kinds of variables that you will come across. The first set represents the values transmitted in the headers of the client request. The second set corresponds to the headers of the response sent to the client. Finally, the third set comprises variables that are completely generated by Nginx.

Request Headers

Nginx lets you access the client request headers under the form of variables that you will be able to employ later on in the configuration:

Variable	Description
$http_host	Value of the Host HTTP header, a string indicating the hostname that the client is trying to reach.
$http_user_agent	Value of the User-Agent HTTP header, a string indicating the web browser of the client.
$http_referer	Value of the Referer HTTP header, a string indicating the URL of the previous page from which the client comes.
$http_via	Value of the Via HTTP header, which informs us about possible proxies used by the client.
$http_x_forwarded_for	Value of the X-Forwarded-For HTTP header, which shows the actual IP address of the client if the client is behind a proxy.
$http_cookie	Value of the Cookie HTTP header, which contains the cookie data sent by the client.
$http_...	Additional headers sent by the client can be retrieved using $http_ followed by the header name in lowercase and with dashes (-) replaced by underscores (_).

Response headers

In a similar fashion, you are allowed to access the HTTP headers of the response that was sent to the client. These variables are not available at all times — they will only carry a value after the response is sent, for instance, at the time of writing messages in the logs.

Variable	Description
$sent_http_content_type	Value of the Content-Type HTTP header, indicating the MIME type of the resource being transmitted.
$sent_http_content_length	Value of the Content-Length HTTP header informing the client of the response body length.
$sent_http_location	Value of the Location HTTP header, which indicates that the location of the desired resource is different than the one specified in the original request.
$sent_http_last_modified	Value of the Last-Modified HTTP header corresponding to the modification date of the requested resource.
$sent_http_connection	Value of the Connection HTTP header, defining whether the connection will be kept alive or closed.
$sent_http_keep_alive	Value of the Keep-Alive HTTP header that defines the amount of time a connection will be kept alive.
$sent_http_transfer_encoding	Value of the Transfer-Encoding HTTP header, giving information about the response body encoding method (such as compress, gzip).
$sent_http_cache_control	Value of the Cache-Control HTTP header, telling us whether the client browser should cache the resource or not.
$sent_http_...	Additional headers sent to the client can be retrieved using $sent_http_ followed by the header name, in lowercase and with dashes (-) replaced by underscores (_).

Nginx Generated

Apart from the HTTP headers, Nginx provides a large amount of variables concerning the request, the way it was and will be handled, as well as settings in use with the current configuration.

Variable	Description
$arg_XXX	Allows you to access the query string (GET parameters), where XXX is the name of the parameter you want to utilize.
$args	All of the arguments of the query string combined together.
$binary_remote_addr	IP address of the client as binary data (4 bytes).
$body_bytes_sent	Amount of bytes sent in the body of the response.
$connection_requests	Amount of requests already served by the current connection.
$content_length	Equates to the Content-Length HTTP header.
$content_type	Equates to the Content-Type HTTP header.
$cookie_XXX	Allows you to access cookie data where XXX is the name of the parameter you want to utilize.
$document_root	Returns the value of the root directive for the current request.
$document_uri	Returns the current URI of the request. It may differ from the original request URI if internal redirects were performed. It is identical to the $uri variable.
$host	This variable equates to the Host HTTP header of the request. Nginx itself gives this variable a value for cases where the Host header is not provided in the original request.
$hostname	Returns the system hostname of the server computer
$https	Set to on for HTTPS connections, empty otherwise.
$is_args	If the $args variable is defined, $is_args equates to ?. If $args is empty, $is_args is empty as well. You may use this variable for constructing an URI that optionally comes with a query string, such as index.php$is_args$args. If there is any query string argument in the request, $is_args is set to ?, making this a valid URI.
$limit_rate	Returns the per-connection transfer rate limit, as defined by the limit_rate directive. You are allowed to edit this variable by using set (directive from the Rewrite Module): set $limit_rate 128k;
$nginx_version	Returns the version of Nginx you are running.
$pid	Returns the Nginx process identifier.
$query_string	Identical to $args.
$remote_addr	Returns the IP address of the client.
$remote_port	Returns the port of the client socket.
$remote_user	Returns the client username if they used authentication.
$realpath_root	Returns the document root in the client request, with symbolic links resolved into the actual path.
$request_body	Returns the body of the client request, or - if the body is empty.
$request_body_file	If the request body was saved (see the client_body_in_file_only directive) this variable indicates the path of the temporary file.
$request_completion	Returns OK if the request is completed, an empty string otherwise.
$request_filename	Returns the full filename served in the current request.
$request_method	Indicates the HTTP method used in the request, such as GET or POST.
$request_uri	Corresponds to the original URI of the request, remains unmodified all along the process (unlike $document_uri/$uri).
$scheme	Returns either http or https, depending on the request.
$server_addr	Returns the IP address of the server. Be aware as each use of the variable requires a system call, which could potentially affect overall performance in the case of high-traffic setups.
$server_name	Indicates the value of the server_name directive that was used while processing the request.
$server_port	Indicates the port of the server socket that received the request data.
$server_protocol	Returns the protocol and version, usually HTTP/1.0 or HTTP/1.1.
$tcpinfo_rtt,  $tcpinfo_rttvar,  $tcpinfo_snd_cwnd,  $tcpinfo_rcv_space	If your operating system supports the TCP_INFO socket option, these variables will be populated with information on the current client TCP connection.
$time_iso8601,  $time_local	Provides the current time respectively in ISO 8601 and local formats for use with the access_log directive.
$uri	Identical to $document_uri.