# Amazon Config Custom Rules
<a name="evaluate-config_develop-rules"></a>

Amazon Config Custom Rules are rules that you create from scratch. There are two ways to create Amazon Config custom rules: with Lambda functions ([Amazon Lambda Developer Guide](https://docs.amazonaws.cn/lambda/latest/dg/gettingstarted-concepts.html#gettingstarted-concepts-function)) and with Guard ([Guard GitHub Repository](https://github.com/aws-cloudformation/cloudformation-guard)), a policy-as-code language.

Amazon Config custom rules created with Lambda are called *Amazon Config Custom Lambda Rules* and Amazon Config custom rules created with Guard are called *Amazon Config Custom Policy Rules*.

Before using custom rules, see [Considerations](evaluate-config.md#evaluate-config-considerations).

## Amazon Config Custom Policy Rules
<a name="evaluate-config_develop-rules-policy"></a>

Rules written using Guard can be created from the Amazon Config console or by using the Amazon Config rule APIs. Amazon Config Custom Policy rules allow you to create Amazon Config Custom rules without needing to use Java or Python to develop Lambda functions to manage your custom rules. Amazon Config Custom Policy rules are initiated by configuration changes. For more information about Guard, see the [Guard GitHub Repository](https://github.com/aws-cloudformation/cloudformation-guard).

## Amazon Config Custom Lambda Rules
<a name="evaluate-config_develop-rules-lambda"></a>

Custom Lambda rules provide you with the option to use Java or Python to create a Lambda function for a Amazon Config Custom rule. A * Lambda function* is custom code that you upload to Amazon Lambda, and it is invoked by events that are published to it by an event source. If the Lambda function is associated with an Amazon Config rule, Amazon Config invokes it when the rule is initiated. The Lambda function then evaluates the configuration information that is sent by Amazon Config, and it returns the evaluation results. For more information about Lambda functions, see [Function and Event Sources](https://docs.amazonaws.cn/lambda/latest/dg/intro-core-components.html) in the *Amazon Lambda Developer Guide*.

## Format differences for Amazon Config Custom Rules
<a name="evaluate-config_develop-schema"></a>

The following table displays the format differences in the fields for the [ConfigurationItem](https://docs.amazonaws.cn/config/latest/APIReference/API_ConfigurationItem.html) data type and for Amazon Config Custom Rules.


| ConfigurationItem | Amazon Config Custom Rule | 
| --- | --- | 
| version | configurationItemVersion | 
| accountId | awsAccountId | 
| arn | ARN | 
| configurationItemMD5Hash | configurationStateMd5Hash | 

**Topics**
+ [Custom Policy Rules](#evaluate-config_develop-rules-policy)
+ [Custom Lambda Rules](#evaluate-config_develop-rules-lambda)
+ [Format differences for Amazon Config Custom Rules](#evaluate-config_develop-schema)
+ [Creating Custom Policy Rules](evaluate-config_develop-rules_cfn-guard.md)
+ [Creating Custom Lambda Rules](evaluate-config_develop-rules_lambda-functions.md)
+ [Managing Deleted Resources for Custom Lambda Rules](evaluate-config_develop-rules-delete.md)