AWS CloudFormation example that allows a security group rule to reference the same security group as the source.

aws-cfn-self-referencing-sg.json

{
  "Description": "Create a VPC with a SG which references itself",
  "AWSTemplateFormatVersion": "2010-09-09",
  "Resources": {
    "vpctester": {
      "Type": "AWS::EC2::VPC",
      "Properties": {
        "CidrBlock": "172.16.0.0/23",
        "EnableDnsSupport": false,
        "EnableDnsHostnames": false,
        "InstanceTenancy": "default",
        "Tags": [ { "Key": "Name", "Value": "vpctester" } ]
      }
    },
    "sgtester": {
      "Type": "AWS::EC2::SecurityGroup",
      "DependsOn": "vpctester",
      "Properties": {
        "GroupDescription": "vpc tester sg",
        "VpcId": { "Ref": "vpctester" }
      }
    },
    "sgtesteringress": {
      "Type": "AWS::EC2::SecurityGroupIngress",
      "DependsOn": "sgtester",
      "Properties": {
        "GroupId": { "Ref": "sgtester" },
        "IpProtocol": "tcp",
        "FromPort": "0",
        "ToPort": "65535",
        "SourceSecurityGroupId": { "Ref": "sgtester" }
      }
    }
  }
}

can you help me understand the difference between groupId and sourceSecurityGroupId?

Also, consider for eg, I have an ec2 bastion host, I have an RDS in the private subnet. I want to create a security group on ec2 that allows all inbound ssh traffic through the Internet gateway. I have another security group on RDS that allows inbound traffic from ec2 bastion. How can I do this? should I use sourceSecuritygroupId:<id of ec2's SG> in the ingress of RDS's security group?

@SwathiKanduri the groupId relates to the security group for which this AWS::EC2::SecurityGroupIngress resource is actually an ingress rule. The sourceSecurityGroupId relates to the security group which we want to allow inbound traffic from. In this case they both refer to sgtester because this is a self-referencing security group, but in the general case sourceSecurityGroupId would refer to some other security group that we want to allow inbound traffic from.

Thanks, it was helpful

Thanks!