# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   rds_instance_public_access_check
# 
# Description:
#   This rule checks whether Amazon Relational Database Service (RDS) database (DB) instances are publicly accessible, as determined by checking the 'PubliclyAccessible' configuration property.
# 
# Reports on:
#   AWS::RDS::DBInstance
# 
# Evaluates:
#   AWS CloudFormation, AWS CloudFormation hook
# 
# Rule Parameters:
#   None
# 
# Scenarios:
#   Scenario: 1
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document does not contain any RDS DB instance resources
#       Then: SKIP
#   Scenario: 2
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an RDS DB instance resource
#        And: 'PubliclyAccessible' has not been specified
#       Then: FAIL
#   Scenario: 3
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an RDS DB instance resource
#        And: 'PubliclyAccessible' is present and is a value other than bool(false)
#       Then: FAIL
#   Scenario: 4
#      Given: The input document is an AWS CloudFormation or CloudFormation hook document
#        And: The input document contains an RDS DB instance resource
#        And: 'PubliclyAccessible' has been specified and set to bool(false)
#       Then: PASS

#
# Constants
#
let RDS_DB_INSTANCE_TYPE = "AWS::RDS::DBInstance"
let INPUT_DOCUMENT = this

#
# Assignments
#
let rds_db_instances = Resources.*[ Type == %RDS_DB_INSTANCE_TYPE ]

#
# Primary Rules
#
rule rds_instance_public_access_check when is_cfn_template(%INPUT_DOCUMENT)
                                           %rds_db_instances not empty {
    check(%rds_db_instances.Properties)
        <<
        [CT.RDS.PR.23]: Require an Amazon RDS database instance to not be publicly accessible
        [FIX]: Set the value of 'PubliclyAccessible' to 'false'.
        >>
}

rule rds_instance_public_access_check when is_cfn_hook(%INPUT_DOCUMENT, %RDS_DB_INSTANCE_TYPE) {
    check(%INPUT_DOCUMENT.%RDS_DB_INSTANCE_TYPE.resourceProperties)
        <<
        [CT.RDS.PR.23]: Require an Amazon RDS database instance to not be publicly accessible
        [FIX]: Set the value of 'PubliclyAccessible' to 'false'.
        >>
}

#
# Parameterized Rules
#
rule check(rds_db_instance) {
    %rds_db_instance{
       #Scenario: 2
       PubliclyAccessible exists
       #Scenario: 3 and 4
       PubliclyAccessible == false

    }
}

#
# Utility Rules
#
rule is_cfn_template(doc) {
    %doc {
        AWSTemplateFormatVersion exists  or
        Resources exists
    }
}

rule is_cfn_hook(doc, RESOURCE_TYPE) {
    %doc.%RESOURCE_TYPE.resourceProperties exists
}