Security Group and Subnet Belongs to different networks

Question

I am creating a basic AWS CloudFormation Template with one VPC, 3 Security Group and 5 EC2 Instances my security group looks something like this -

{
  "WebApplicationServerSG": {
    "Type": "AWS::EC2::SecurityGroup",
    "Properties": {
      "VpcId": {
        "Ref": "DevVpc"
      },
      "GroupDescription": "Enable HTTP, HTTPS and SSH access",
      "Tags": [
        {
          "Key": "Name",
          "Value": "WebApplicationServer Service Group"
        }
      ],
      "SecurityGroupIngress": [
        {
          "IpProtocol": "tcp",
          "FromPort": "443",
          "ToPort": "443",
          "CidrIp": "0.0.0.0/0"
        },
        {
          "IpProtocol": "tcp",
          "FromPort": "80",
          "ToPort": "80",
          "CidrIp": "0.0.0.0/0"
        },
        {
          "IpProtocol": "tcp",
          "FromPort": "22",
          "ToPort": "22",
          "CidrIp": "0.0.0.0/0"
        }
      ],
      "SecurityGroupEgress": [
        {
          "IpProtocol": "tcp",
          "FromPort": "443",
          "ToPort": "443",
          "CidrIp": "0.0.0.0/0"
        },
        {
          "IpProtocol": "tcp",
          "FromPort": "80",
          "ToPort": "80",
          "CidrIp": "0.0.0.0/0"
        },
        {
          "IpProtocol": "tcp",
          "FromPort": "22",
          "ToPort": "22",
          "CidrIp": "0.0.0.0/0"
        }
      ]
    },
    "Metadata": {
      "AWS::CloudFormation::Designer": {
        "id": "a7977f00-48d6-488f-9e23-9bcd0785d399"
      }
    }
  }
}

And the VPC is something like below -

{
  "DevVpc": {
    "Type": "AWS::EC2::VPC",
    "Properties": {
      "CidrBlock": "172.31.0.0/16",
      "EnableDnsSupport": "false",
      "EnableDnsHostnames": "false",
      "InstanceTenancy": "dedicated",
      "Tags": [
        {
          "Key": "Name",
          "Value": "DevStackVpc"
        }
      ]
    }
  }
}

I am getting error while stack creation with the template -

Security group sg-31f91b5a and subnet subnet-ea0aa3a7 belong to different networks.

11:13:01 UTC+0550   CREATE_FAILED   AWS::EC2::Instance  WebApplicationServer    Security group sg-5147a53a and subnet subnet-ea0aa3a7 belong to different networks.

And here is a gist for complete template, any help would really be appreciated.

Answer 1

I got the above problem resolved by the pointers provided in comments.

The relationship between subnets, VPCs, security groups, and EC2 instances are below.

• The first thing which should get created is the VPC.
• The second is the subnet. Here, you mention the VpcId you created earlier.
• The third thing you create are security groups. Here, you mention the VpcId you created earlier as well.
• There is a property NetworkInterfaces, where you provide a SubnetId and GroupSet, which is an array of security group IDs. This is where you define the relationship between security groups, VPCs, and subnets.

Below is the sample template which actually worked:

{
"AWSTemplateFormatVersion": "2010-09-09",
"Parameters": {
    "DevServerKeyPair": {
        "Description": "Name of an existing EC2 KeyPair to enable SSH access to the instance",
        "Type": "AWS::EC2::KeyPair::KeyName",
        "ConstraintDescription": "Must be the name of an existing EC2 KeyPair."
    }
},
"Resources": {
    "DevVpc": {
        "Type": "AWS::EC2::VPC",
        "Properties": {
            "CidrBlock": "172.31.0.0/16",
            "EnableDnsSupport": "false",
            "EnableDnsHostnames": "false",
            "InstanceTenancy": "dedicated",
            "Tags": [
                {
                    "Key": "Name",
                    "Value": "DevStackVpc"
                }
            ]
        }
    },
    "DevSubnet": {
        "Type": "AWS::EC2::Subnet",
        "Properties": {
            "VpcId": {
                "Ref": "DevVpc"
            },
            "CidrBlock": "172.31.0.0/16",
            "AvailabilityZone": {
                "Fn::Select": [
                    0,
                    {
                        "Fn::GetAZs": ""
                    }
                ]
            }
        }
    },
    "WebApplicationServerSG": {
        "Type": "AWS::EC2::SecurityGroup",
        "Properties": {
            "VpcId": {
                "Ref": "DevVpc"
            },
            "GroupDescription": "Enable HTTP, HTTPS and SSH access",
            "Tags": [
                {
                    "Key": "Name",
                    "Value": "WebApplicationServer Service Group"
                }
            ],
            "SecurityGroupIngress": [
                {
                    "IpProtocol": "tcp",
                    "FromPort": "443",
                    "ToPort": "443",
                    "CidrIp": "0.0.0.0/0"
                },
                {
                    "IpProtocol": "tcp",
                    "FromPort": "80",
                    "ToPort": "80",
                    "CidrIp": "0.0.0.0/0"
                },
                {
                    "IpProtocol": "tcp",
                    "FromPort": "22",
                    "ToPort": "22",
                    "CidrIp": "0.0.0.0/0"
                }
            ],
            "SecurityGroupEgress": [
                {
                    "IpProtocol": "tcp",
                    "FromPort": "443",
                    "ToPort": "443",
                    "CidrIp": "0.0.0.0/0"
                },
                {
                    "IpProtocol": "tcp",
                    "FromPort": "80",
                    "ToPort": "80",
                    "CidrIp": "0.0.0.0/0"
                },
                {
                    "IpProtocol": "tcp",
                    "FromPort": "22",
                    "ToPort": "22",
                    "CidrIp": "0.0.0.0/0"
                }
            ]
        }
    },
    "WebApplicationServer": {
        "Type": "AWS::EC2::Instance",
        "Properties": {
            "ImageId": "ami-f3e5aa9c",
            "InstanceType": "t2.micro",
            "Tags": [
                {
                    "Key": "Name",
                    "Value": "WebApplicationServer"
                }
            ],
            "KeyName": {
                "Ref": "DevServerKeyPair"
            },
            "NetworkInterfaces": [
                {
                    "SubnetId": {"Ref": "DevSubnet"},
                    "AssociatePublicIpAddress": "true",
                    "DeviceIndex": "0",
                    "GroupSet": [{ "Ref" : "WebApplicationServerSG" }]
                }
            ]
        }
    }
  }
}

Hope this helps someone looking into a similar problem.