Handling Multiple Conditions with Local Nginx Variables

Nginx can struggle with rules which require multiple conditions to be checked. This guide will show how to set up multiple-condition EdgeRules.

The EdgeRules feature is available only on Enterprise plans.

Example Condition

An example requirement might be to redirect visitors from http://domain.com/ to http://domain.com/test/uri:

• if Nginx captures /test/uri AND
• the request header Accept is defined with value "test" AND
• there is a query string "debug=1"

The simplest solution with any programming language is to combine conditions within a single “if” statement (i.e. if something = 1 and something else = 2). However, this can’t be done with Nginx and therefore can’t be done with MaxCDN’s EdgeRules. Instead, we will use variables with multiple “if” statements to define different states for variables that can be used in the final action. In the example below, the final action is redirection.

EdgeRules Configuration

Nginx Configuration Block

set $tmp 0;
if ($http_accept = test) {                                  
	set $tmp 1;
}
if ($args = debug=1) {                                  
	set $tmp "${tmp}1";
}
if ($tmp = 11) {                                  
	rewrite ^ http://domain.com/$uri permanent;
}

What we are doing here is a bit of a hack, where we define variable “tmp” with value “0”. We then set the first condition of the same variable to “1” and the request header Accept to the value “test”. Since we have two conditions, we’re appending “1” to whatever the value was of “tmp” (with “${tmp}1”) variable and finally checking if the “tmp” variable contains the expected “11” value. If yes, we’re putting the rewrite directive into place.

cURL Examples

Default example request that meets none of the conditions:

$ curl -I cdn.domain.com/
HTTP/1.1 200 OK

New request with only query string defined:

$ curl -I cdn.domain.com/?debug=1
HTTP/1.1 200 OK

Another request with only request Accept header defined:

$ curl -I cdn.domain.com/ -H 'Accept: test'
HTTP/1.1 200 OK

Finally, request with both expected parameters satisfied:

$ curl -I cdn.domain.com/?debug=1 -H 'Accept: test'
HTTP/1.1 301 Moved Permanently
Location: http://domain.com/?debug=1

The “if” statement in Nginx can be dangerous if it’s improperly used within the location block (within location context). IF is a part of the rewrite module and, as such, it’s only ever 100% safe with rewrite and return directives. In other cases it can be unpredictable.

If you have any questions about handling multiple conditions with local Nginx variables, please feel free to reach out to support. Live chat and ticket support are available 24/7.