Security Group Examples in AWS CDK - Complete Guide

Table of Contents #

1. Creating a Security Group in AWS CDK
2. Importing an Existing Security Group in AWS CDK

Creating a Security Group in AWS CDK #

Security groups are virtual firewalls - they control the traffic that goes in and out of our EC2 instances.

They allow us to define inbound and outbound rules. Inbound traffic is traffic that comes into the EC2 instance, whereas Outbound traffic is traffic that goes out of the EC2 instance.

In this article, we are going to look at multiple examples of creating security groups and editing their inbound and outbound rules.

Let's start by creating a VPC and a security group for a web server:

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import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cdk from 'aws-cdk-lib';

export class CdkStarterStack extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    const vpc = new ec2.Vpc(this, 'my-cdk-vpc', {
      cidr: '10.0.0.0/16',
      natGateways: 0,
      maxAzs: 3,
      subnetConfiguration: [
        {
          name: 'public-subnet-1',
          subnetType: ec2.SubnetType.PUBLIC,
          cidrMask: 24,
        },
      ],
    });

    // 👇 Create a SG for a web server
    const webserverSG = new ec2.SecurityGroup(this, 'web-server-sg', {
      vpc,
      allowAllOutbound: true,
      description: 'security group for a web server',
    });

    webserverSG.addIngressRule(
      ec2.Peer.anyIpv4(),
      ec2.Port.tcp(22),
      'allow SSH access from anywhere',
    );

    webserverSG.addIngressRule(
      ec2.Peer.anyIpv4(),
      ec2.Port.tcp(80),
      'allow HTTP traffic from anywhere',
    );

    webserverSG.addIngressRule(
      ec2.Peer.anyIpv4(),
      ec2.Port.tcp(443),
      'allow HTTPS traffic from anywhere',
    );

    webserverSG.addIngressRule(
      ec2.Peer.ipv4('123.123.123.123/16'),
      ec2.Port.allIcmp(),
      'allow ICMP traffic from a specific IP range',
    );
  }
}

Let's go over what we did in the code snippet.

1. We created a VPC, by instantiating and configuring the Vpc class. We set the natGateways prop to 0 to avoid getting charged unnecessarily.

   If you want to read more about creating VPCs, I've written another article on creating VPCs in CDK.

2. We created a security group for a web server. The props we passed when instantiating the SecurityGroup class are:

• vpc - the VPC, the security group will be created in
• allowAllOutbound - whether the security group should allow all outbound traffic. By default allowAllOutbound is set to true
• description - a short description of the security group

3. To allow inbound traffic we used the addIngressRule method on an instance of the SecurityGroup class. The parameters we passed to the method are:

• peer - the Source in a security group inbound rule
• connection - the Port, Protocol and Type in a security group inbound rule
• description - a short description of the security group rule

4. These are the inbound rules we added to our security group:

Let's provision the VPC and the security group:

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npx aws-cdk deploy

After the resources have been deployed, we can see that the inbound security group rules have been applied:

The outbound rules allow all traffic because we've set the allowAllOutbound to true, which is also the default value:

Let's add 2 more security groups - 1 for a backend server, and 1 for a database server.

The security group of the backend server will only allow requests on port 8000, made from instances in the webserverSG security group.

Whereas the security group for the database server will only allow requests on port 3306, made from instances in the backendServerSG security group.

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import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cdk from 'aws-cdk-lib';

export class CdkStarterStack extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    // ... rest

    // 👇 Create a SG for a backend server
    const backendServerSG = new ec2.SecurityGroup(this, 'backend-server-sg', {
      vpc,
      allowAllOutbound: true,
      description: 'security group for a backend server',
    });
    backendServerSG.connections.allowFrom(
      new ec2.Connections({
        securityGroups: [webserverSG],
      }),
      ec2.Port.tcp(8000),
      'allow traffic on port 8000 from the webserver security group',
    );

    // 👇 Create a SG for a database server
    const dbserverSG = new ec2.SecurityGroup(this, 'database-server-sg', {
      vpc,
      allowAllOutbound: true,
      description: 'security group for a database server',
    });

    dbserverSG.connections.allowFrom(
      new ec2.Connections({
        securityGroups: [backendServerSG],
      }),
      ec2.Port.tcp(3306),
      'allow traffic on port 3306 from the backend server security group',
    );
  }
}

Let's go over what we did in the code snippet.

1. We created a backend server security group
2. We used the allowFrom method on an instance of the Connections class to allow inbound connections on port 8000 from instances in the webserverSG security group. The allowFrom method takes the following 3 props:

• connectable - an object that has connection options, in our case a security group

• connection - the Port, Protocol and Type in a security group
  rule

• description - a short description of the security group rule

3. We created a database server security group and used the allowFrom method to allow traffic on port 3306 from the security group of the backend server

The inbound rules for the backendServerSG look like:

The inbound rules for the dbserverSG look like:

The outbound rules are still the default of all traffic is allowed. We will edit the outbound rules of a security group later in the article.

Let's run the deploy command:

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npx aws-cdk deploy

After a deployment, the inbound rules of the backend server security group show that it only allows traffic on port 8000 from requests made from instances within the webserver security group:

The security group of the database server shows that it only allows traffic on port 3306 from requests made from instances within the backend server security group:

Next, we are going to take a look at how we can edit the default outbound rules of a security group.

Updating the Outbound Rules of Security Groups in AWS CDK #

In order to edit the outbound rules of a security group in CDK, we have to set the allowAllOutbound prop to false, when instantiating the SecurityGroup class.

Let's create a security group and customize its outbound traffic rules.

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import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cdk from 'aws-cdk-lib';

export class CdkStarterStack extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    // ... rest

    // 👇 create a SG with custom Outbound rules
    const customOutboundSG = new ec2.SecurityGroup(this, 'custom-outbound-sg', {
      vpc,
      allowAllOutbound: false,
      description: 'a security group with custom outbound rules',
    });

    customOutboundSG.addEgressRule(
      ec2.Peer.ipv4('10.0.0.0/16'),
      ec2.Port.tcp(3306),
      'allow outgoing traffic on port 3306',
    );

    customOutboundSG.addEgressRule(
      ec2.Peer.anyIpv4(),
      ec2.Port.tcp(80),
      'allow outgoing traffic on port 80',
    );
  }
}

Let's go over the code snippet.

1. We created a security group, but this time we set the allowAllOutbound prop to false. This is necessary if we are going to edit the outbound rules for a security group, otherwise our egress rules would just get ignored
2. We used the addEgressRule method on an instance of the SecurityGroup class

The outbound rules we added to the security group look like:

Let's deploy the changes:

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npx aws-cdk deploy

If we look at the VPC management console, we can see that the outbound rules have been applied to the security group:

Note that we haven't added any inbound rules to the security group, so it has none:

Clean up #

To delete the resources we have provisioned, issue the destroy command:

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npx aws-cdk destroy

Importing an Existing Security Group in AWS CDK #

In order to import an existing security group into a CDK stack, we have to use the fromSecurityGroupId static method on the SecurityGroup class.

Let's look at an example of importing a security group in a CDK stack:

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import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cdk from 'aws-cdk-lib';

export class CdkStarterStack extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    // 👇 import security group by ID
    const importedSecurityGroup = ec2.SecurityGroup.fromSecurityGroupId(
      this,
      'imported-security-group',
      'YOUR-SG-ID',
      {allowAllOutbound: true, mutable: true},
    );

    console.log('security group id 👉', importedSecurityGroup.securityGroupId);
  }
}

Let's go over the code snippet.

1. We imported a security group into our CDK stack by using the fromSecurityGroupId static method on the SecurityGroup class

2. The fromSecurityGroupId method takes the following parameters:

• scope - the scope the method is invoked in

• id - the construct identifier (must be unique in the scope)

• securityGroupId - the id of the security group

• securityGroupImportOptions - a configuration object for the imported security group

  The allowAllOutbound property is set to true by default and specifies that the security group allows all outbound traffic. The fromSecurityGroupId method assumes that the imported security group allows all outbound traffic, so it doesn't modify any of the egress rules. If we wanted to modify the outbound rules of the imported security group, we would have to set allowAllOutbound to false.

  The mutable property is also set to true by default. Setting the property to true allows us to add rules to the imported security group. We can only add inbound rules unless allowAllOutbound is set to false.

Adding Inbound rules to an Imported Security Group in CDK #

In order to add an inbound rule to an imported security group in CDK, we have to:

1. Set the mutable property to true when importing the security group. The mutable prop is set to true by default, so we can omit passing it altogether
2. Use the addIngressRule method on the imported security group

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import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cdk from 'aws-cdk-lib';

export class CdkStarterStack extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    // ... rest

    // 👇 `mutable` is `true`, so we can add ingress rules
    importedSecurityGroup.addIngressRule(
      ec2.Peer.anyIpv4(),
      ec2.Port.tcp(22),
      'allow SSH access from anywhere',
    );
  }
}

We used the addIngressRule method to add the following inbound rule to the imported security group:

If I run the npx aws-cdk deploy command with an existing security group id, we can see that the inbound rule gets applied:

Note that if we were to destroy the CDK stack, the inbound rule would get deleted and removed from the security group.

Adding Outbound rules to an Imported Security Group in CDK #

In order to add an outbound rule to an imported security group in CDK, we have to:

1. Set the allowAllOutbound property to false and the mutable property to true

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const importedSecurityGroup = ec2.SecurityGroup.fromSecurityGroupId(
  this,
  'imported-security-group',
  'sg-0364cc5f9a979e9a6',
  {allowAllOutbound: false, mutable: true},
);

2. Use the addEgressRule method on the imported security group

Let's look at an example, where we add an egress rule to an imported security group:

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import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cdk from 'aws-cdk-lib';

export class CdkStarterStack extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    // ... rest

    // 👇 `mutable` is `true`, so we can add ingress rules
    importedSecurityGroup.addEgressRule(
      ec2.Peer.ipv4('10.0.0.0/16'),
      ec2.Port.tcp(3306),
      'allow outgoing traffic on port 3306',
    );
  }
}

In the code snippet, we used the addEgressRule method on the imported security group to add the following outbound rule:

If I deploy the new egress rule, we can see that the outbound rules of the imported security group get updated:

If we take a look at the resources the CloudFormation stack has provisioned, we can see the ingress and egress security group rules:

Deleting the stack would remove all of the ingress and egress rules we added to the imported security group.