Rules Engine for Edge Firewall

The Rules Engine for Edge Firewall is a feature designed to implement security logic for your applications. You can find the Rules Engine settings tab within a created edge firewall.

The Rules Engine is programmable, allowing you to define the conditions (Criteria) and actions (Behaviors). If the Criteria are met, the Behaviors will be executed.

You can use the Rules Engine for Edge Firewall to configure all operational aspects of your application’s firewalls. Here are some implementations examples you can use it for:

Blocking a request.
Ignoring a request.
Limiting the access rate.
Applying a Web Application Firewall (WAF) policy.
Running an Edge Function for Edge Firewall with your own security code.
Monitoring traffic to identify threats.

An edge firewall can have as many rules as needed. These rules are interchangeable, allowing you to share the same rule across all other edge firewalls.

Go to Edge Firewall Reference Go to Work With Rules Engine Guide

Implementation

Scope	Resource
Secure an application	How to secure an application
Configuring main settings	How to configure Edge Firewall main settings
Creating a WAF rule set	How to create a WAF rule set
Blocking Tor exit node IP addresses	How to block Tor exit node IP addresses

How Rules Engine for Edge Firewall works

Each request from your users to your application is processed first by Edge Firewall. You can define a set of security rules within Edge Firewall.

The rules are composed by Criteria, which represent the conditions for executing the rules, and by Behaviors, which represent the actions to be executed.

The processing of the rules is sequential and you can use them as a powerful set of variables and comparison operators. If the conditions are met, the Behaviors of each rule are executed until all the rules are processed.

Name

Each rule must have a unique and easy-to-remember name, allowing you to easily reference and manage it within your system.

Description

Aside from the name of the rule, you can add a description to your rule using the Description field. Your description will be visible from the rule list and can be used to help you identify what the rule does.

Criteria

The Criteria section of Rules Engine for Edge Firewall is where you define the conditions for executing the rule. Criteria are composed of:

Variables
Comparison operators
Logic operators
Arguments, when applicable

The inclusion of arguments in a criteria depends on the comparison operators chosen. The format of the arguments is described in the variables and comparison operators sections below. You can also add logical operators to increase the amount of comparisons that the rule will execute.

For example, this criteria identifies requests where the URI starts with a forward slash (”/”) so that the defined behavior can then be applied to them.

	Variable	Comparison operator	Argument
If	`Request URI`	starts with	`/`

Variables

These are all the Criteria variables you can set:

Variable	Description	Example	Requirements
Header Accept	Header that tells you what types of media the customer accepts for the response.	`application/json`	Web Application Firewall
Header Accept Encoding	Header that tells you what types of content encoding, usually compression algorithms, that the client accepts for the response	`gzip`	Web Application Firewall
Header Accept Language	Header that informs the expected language	`en-US`	Web Application Firewall
Header Cookie	Header containing the cookies sent by the client in the request to the server	`session_id=abc123`	Web Application Firewall
Header Origin	Header that informs the origin of a cross-site access request or a preflight request. The source is a URI indicating the name of the server, with no path information	`https://example.com`	Web Application Firewall
Header Referer	Header indicating the address of the document, or element in a document, from which the request’s URI was obtained	`https://example.com/landing-page`	Web Application Firewall
Header User Agent	Header with a characteristic string that allows servers to identify the application, operating system, vendor, and/or version of the device	`Mozilla/5.0`	Web Application Firewall
Request Args	All arguments sent by the user in the request string (query string).	`page=1`	Web Application Firewall
Network	The IP address of the client making the HTTP request, which can be used for any network comparison (CIDR, ASN or Country)	`1xx.xxx.x.0`	Network Layer Protection
Host	In order of precedence: the hostname of the request line, or the value of the Host header field of the request, or the name of the server serving the request	`api.example.com`	-
Request Method	The request’s HTTP method. For example: `GET`, `POST`, `PUT`, etc.	`POST`	-
Request URI	This relates to the `${uri}` variable from Rules Engine for Edge Applications. The normalized (urldecoded) URI of the request. The value of `${uri}` can change during the processing of a request, for example, when an internal redirect occurs or when index files are used. It doesn’t carry the Query String parameters as `${request_uri}` do.	`/api/v1/`	-
Scheme	The scheme of the request: HHTP or HTTPS.	`https`	-
Client Certificate Validation	Server process to authenticate client digital certificate.	`true`	-
SSL Verification Status	Server result to client certificate validation. It can be `Success`, when the client certificate validation passed; `Certificate Verification Error`, when the client certificate wasn’t valid; and `Missing Client Certificate`, when the client certificate wasn’t sent in the request.	`Certificate Verification Error`	-

Comparison operators

The condition for the execution of a rule must be the comparison of a variable with an argument. The comparison operators are:

Operator	Description	Argument type
is equal	The value of the variable is equal to the argument, compared character by character	String
is not equal	The value of the variable isn’t exactly the same as the argument	String
starts with	The value of the variable starts with the argument	String
does not start with	The value of the variable doesn’t start with the argument	String
matches	The value of the variable matches the regular expression or list entered as an argument	Regular expression or list
does not match	The value of the variable doesn’t match the regular expression entered as an argument	Regular expression or list
exists	The variable has a defined value. For example, `Request Args` exists if an argument is sent in the request’s query string	-
does not exist	The variable doesn’t have a defined value. For example, `Request Args` doesn’t exist if an argument is sent in the request’s query string	-

Logic operators

Multiple criteria can be defined using the logical operators And and Or.

If explicit precedence is required, you can add multiple criteria groups under the And logic.

Behaviors

In the Behaviors section of the Rules Engine, you must add the behaviors you want your rule to perform if the conditions defined in the criteria are met.

For example, this behavior runs the specified function, allowing you to execute custom logic for handling requests.

Behavior	Argument
`Run Function`	`<name-of-the-instantiated-function>`

Deny (403 Forbidden)

It closes the request with an HTTP 403 Forbidden response. No arguments are required.

Drop (Close Without Response)

It closes the request without responding to the customer. No arguments are required.

Set Rate Limit

It defines an access rate limit that, if exceeded, will result in an HTTP 429 Too Many Requests response. To configure the Rate Limit, you must inform:

Type is the field to select the type of request for the Rate Limit, choosing Req/s (requests per second) or Req/min (requests per minute).
Average Rate Limit, which is the limit rate itself by second.
Client IP address, if you want the access rate to be counted by the client’s IP address or Global, if you want total access rate counted.
Maximum burst size, which indicates the maximum burst size of HTTP requests sent simultaneously, which will be queued and dispatched gradually, respecting the limit rate.
- The configured value will be the Rate Limit for each Azion Edge Node, implemented using the Leaky Bucket algorithm. It’s recommended to use Maximum burst size at a maximum of 10 times of the value configured in Average Rate Limit, which would result in penalizing the last request for a burst with up to a 10 second delay.

As the Rate Limit is applied by IP address of Global and by rule, if a rule has more than one URI specified and uses the conditional Or, the rate limit will be shared between the URIs. Create different rules if you wish to have a Rate Limite for each URI.

Set WAF Rule Set

It associates the WAF Rule Set to be used in the request. WAF policies must be previously configured in Edge Libraries > WAF Rules. You must also define the WAF mode: Learning or Blocking. Read more on WAF Rule Sets.

Run Function

It runs a function specified as a parameter. The function must have been previously instantiated and parameterized in the Functions tab in order to be used.

Set Custom Response

It allows a customized response when the request matches the criteria. You can customize the Status Code by changing it from 200 to 499, the Content Type header, and Content Body of your request with a maximum of 500 characters.

Behaviors execution and logic

While behaviors and rules are executed in the order they’re arranged, some behaviors can’t be stacked.

If a behavior of the type Set, such as Set Custom Response in Edge Firewall and Set Origin in Edge Application, is added multiple times to the rules in Rules Engine, only the last behavior from the last rule in which the criteria was met will be executed.

Behaviors of the type Add are cumulative and can be added multiple times to rules. This means that if Add Cookie and Add Header are executed multiple times for the same key-value pairs, multiple identical entries will be added.

However, for unique headers such as Host, the last behavior of type Add Header that is executed will overwrite the previous value, since there can’t be more than one Host header in a request.

Some behaviors, such as Deny, can finish the execution of rules. If a sequence of rules includes such behavior, any rules or behaviors that follow this type of finalizing behavior won’t be executed.

Debugging rules

You can debug rules created with Rules Engine for Edge Firewall through the GraphQL API, Data Stream, or Real-Time Events.

Watch a video tutorial about programming Edge Firewall rules for optimal WAF usage and cost optimization on Azion’s YouTube channel: