# ###################################
##       Rule Specification        ##
#####################################
# 
# Rule Identifier:
#   ec2_instance_multiple_eni_check
# 
# Description:
#   This control checks whether an AWS::EC2::Instance resource specifies multiple ENIs (Elastic Network Interfaces) in the NetworkInterfaces property.
# 
# Reports on:
#   AWS::EC2::Instance
# 
# 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 Amazon EC2 instance resources
#      Then: SKIP
#   Scenario: 2
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an Amazon EC2 instance resource
#       And: 'NetworkInterfaces' is not present or is present and contains 0 configurations
#      Then: SKIP
#   Scenario: 3
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an Amazon EC2 instance resource
#       And: 'NetworkInterfaces' is present and contains >1 configurations
#      Then: FAIL
#   Scenario: 4
#     Given: The input document is an AWS CloudFormation or CloudFormation hook document
#       And: The input document contains an Amazon EC2 instance resource
#       And: 'NetworkInterfaces' is present
#       And: 'NetworkInterfaces' is present and contains 1 configuration
#      Then: PASS

#
# Constants
#
let EC2_INSTANCE_TYPE = "AWS::EC2::Instance"
let INPUT_DOCUMENT = this

#
# Assignments
#
let ec2_instances = Resources.*[ Type == %EC2_INSTANCE_TYPE ]

#
# Primary Rules
#
rule ec2_instance_multiple_eni_check when is_cfn_template(%INPUT_DOCUMENT)
                                          %ec2_instances not empty {
    check(%ec2_instances.Properties)
        <<
        [CT.EC2.PR.12]: Require an Amazon EC2 instance to specify at most one network interface by means of the NetworkInterfaces property in the AWS::EC2::Instance resource
        [FIX]: Configure Amazon EC2 instances with only one ENI.
        >>

}

rule ec2_instance_multiple_eni_check when is_cfn_hook(%INPUT_DOCUMENT, %EC2_INSTANCE_TYPE) {

    check(%INPUT_DOCUMENT.%EC2_INSTANCE_TYPE.resourceProperties)
        <<
        [CT.EC2.PR.12]: Require an Amazon EC2 instance to specify at most one network interface by means of the NetworkInterfaces property in the AWS::EC2::Instance resource
        [FIX]: Configure Amazon EC2 instances with only one ENI.
        >>
}

#
# Parameterized Rules
#
rule check(ec2_instance) {
    %ec2_instance [
        # Scenario 2
        NetworkInterfaces exists
        NetworkInterfaces is_list
        NetworkInterfaces not empty
    ] {
        # Scenario 3 and 4
        NetworkInterfaces[1] not exists
    }
}

#
# 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
}