# AWS Config Custom Rules
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AWS Config Custom Rules are rules that you create from scratch. There are two ways to create AWS Config custom rules: with Lambda functions ([AWS Lambda Developer Guide](https://docs.aws.amazon.com/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.

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

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

## AWS Config Custom Policy Rules
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Rules written using Guard can be created from the AWS Config console or by using the AWS Config rule APIs. AWS Config Custom Policy rules allow you to create AWS Config Custom rules without needing to use Java or Python to develop Lambda functions to manage your custom rules. AWS 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).

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

## Format differences for AWS Config Custom Rules
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The following table displays the format differences in the fields for the [ConfigurationItem](https://docs.aws.amazon.com/config/latest/APIReference/API_ConfigurationItem.html) data type and for AWS Config Custom Rules.


| ConfigurationItem | AWS 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 AWS 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)