Building a Dynamic DNS for Route 53 using CloudWatch Events and Lambda

Jeremy Cowan, AWS Solutions Architect

Efrain Fuentes, Enterprise Solutions Architect

Introduction

Dynamic registration of resource records is useful when you have instances that are not behind a load balancer that you would like to address by a host name and domain suffix of your choosing, rather than the default <region>.compute.internal or ec2.internal.

In this post, we explore how you can use CloudWatch Events and Lambda to create a Dynamic DNS for Route 53. Besides creating A records, this solution allows you to create alias, i.e. CNAME records, for when you want to address a server by a “friendly” or alternate name. Although this is antithetical to treating instances as disposable resources, there are still a lot of shops that find this useful.

Using CloudWatch and Lambda to respond to infrastructure changes in real-time

With the advent of CloudWatch Events in January 2016, you can now get near real-time information when an AWS resource changes its state, including when instances are launched or terminated.  When you combine this with the power of Amazon Route 53 and AWS Lambda, you can create a system that closely mimics the behavior of Dynamic DNS.

For example, when a newly-launched instance changes its state from pending to running, an event can be sent to a Lambda function that creates a resource record in the appropriate Route 53 hosted zone.  Similarly, when instances are stopped or terminated, Lambda can automatically remove resource records from Route 53.

The example provided in this post works precisely this way.  It uses information from a CloudWatch event to gather information about the instance, such as its public and private DNS name, its public and private IP address, the VPC ID of the VPC that the instance was launched in, its tags, and so on.  It then uses this information to create A, PTR, and CNAME records in the appropriate Route 53 public or private hosted zone.  The solution persists data about the instances in an Amazon DynamoDB table so it can remove resource records when instances are stopped or terminated.

Route 53 Hosted zones

Route 53 offers the convenience of domain name services without having to build a globally distributed highly reliable DNS infrastructure.  It allows instances within your VPC to resolve the names of resources that run within your AWS environment.  It also lets clients on the Internet resolve names of your public-facing resources.  This is accomplished by querying resource record sets that reside within a Route 53 public or private hosted zone.

A private hosted zone is basically a container that holds information about how you want to route traffic for a domain and its subdomains within one or more VPCs, whereas a public hosted zone is a container that holds information about how you want to route traffic from the Internet.

Choosing between VPC DNS or Route 53 Private Hosted Zones

Admittedly, you can use VPC DNS for internal name resolution instead of Route 53 private hosted zones.  Although it doesn’t dynamically create resource records, VPC DNS will provide name resolution for all the hosts within a VPC’s CIDR range.

Unless you create a DHCP option set with a custom domain name and disable hostnames at the VPC, you can’t change the domain suffix; all instances are either assigned the ec2.internal or <region>.compute.internal domain suffix.  You can’t create aliases or other resource record types with VPC DNS either.

Private hosted zones help you overcome these challenges by allowing you to create different resource record types with a custom domain suffix.  Moreover, with Route 53 you can create a subdomain for your current DNS namespace or you can migrate an existing subdomain to Route 53.  By using these options, you can create a contiguous DNS namespace between your on-premises environment and AWS.

So, while VPC DNS can provide basic name resolution for your VPC, Route 53 private hosted zones offer richer functionality by comparison.  It also has a programmable API that can be used to automate the creation/removal of records sets and hosted zones which we’re going leverage later in this post.

Route 53 doesn’t offer support for dynamic registration of resource record sets for public or private hosted zones.  This can pose challenges when an Auto Scaling event occurs and the instances are not behind a load balancer.  A common workaround is to use an automation framework like Chef, Puppet, Ansible, or Salt to create resource records, or by adding instance user data to the launch profile of the Auto Scaling group.  The drawbacks to these approaches are that:

1)   automation frameworks typically require you to manage additional infrastructure.

2)   instance user data doesn’t handle the removal of resource records when the instance is terminated.

This was the motivation for creating a serverless architecture that dynamically creates and removes resource records from Route 53 as EC2 instances are created and destroyed.

DDNS/Lambda example

Make sure that you have the latest version of the AWS CLI installed locally.   For more information, see Getting Set Up with the AWS Command Line Interface.

For this example, create a new VPC configured with a private and public subnet, using Scenario 2: VPC with Public and Private Subnets (NAT) from the Amazon VPC User Guide.  Ensure that the VPC has the DNS resolution and DNS hostnames options set to yes.

After the VPC is created, you can proceed to the next steps.

Step 1 – Create an IAM role for the Lambda function

In this step, you use the AWS Command Line Interface (AWS CLI) to create the Identity and Access Management (IAM) role that the Lambda function assumes when the function is invoked.  You need to create an IAM policy with the required permissions and then attach this policy to the role.

Download the ddns-policy.json and ddns-trust.json files from the AWS Labs GitHub repo.

ddns-policy.json

The policy includes ec2:Describe permission, required for the function to obtain the EC2 instance’s attributes, including the private IP address, public IP address, and DNS hostname.   The policy also includes DynamoDB and Route 53 full access, required for the function to create the DynamoDB table and to update the Route 53 DNS records.  The policy also allows the function to create log groups and log events.

{
  "Version": "2012-10-17",
  "Statement": [{
    "Effect": "Allow",
    "Action": "ec2:Describe*",
    "Resource": "*"
  }, {
    "Effect": "Allow",
    "Action": [
      "dynamodb:*"
    ],
    "Resource": "*"
  }, {
    "Effect": "Allow",
    "Action": [
      "logs:CreateLogGroup",
      "logs:CreateLogStream",
      "logs:PutLogEvents"
    ],
    "Resource": "*"
  }, {
    "Effect": "Allow",
    "Action": [
      "route53:*"
    ],
    "Resource": [
      "*"
    ]
  }]
}

ddns-trust.json

The ddns-trust.json file contains the trust policy that grants the Lambda service permission to assume the role.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "",
      "Effect": "Allow",
      "Principal": {
        "Service": "lambda.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

Create the policy using the policy document in the ddns-pol.json file.  You need to replace <LOCAL PATH> with your local path to the ddns-pol.json file.   The output of the aws iam create-policy command includes the Amazon Resource Locator (ARN).  Save the ARN, since you will need it for future steps.

aws iam create-policy --policy-name ddns-lambda-policy --policy-document file://<LOCAL PATH>/ddns-pol.json

Create the ddns-lambda-role IAM role using the trust policy in the ddns-trust.json file.  You need to replace <LOCAL PATH> with your local path to the ddns-trust.json file.  The output of the aws iam create-role command includes the ARN associated with the role that you created.  Save this ARN,  since you will need it when you create the Lambda function in the next section.

aws iam create-role --role-name ddns-lambda-role --assume-role-policy-document file://<LOCAL PATH>/ddns-trust.json

Attach the policy to the role.  Use the ARN returned in step 2 for the –policy-arn input parameter.

aws iam attach-role-policy --role-name ddns-lambda-role --policy-arn <enter-your-policy-arn-here>

Step 2 – Create the Lambda function

The Lambda function uses modules included in the Python 2.7 Standard Library and the AWS SDK for Python module (boto3), which is preinstalled as part of the Lambda service.  As such, you do not need to create a deployment package for this example.

The function code performs the following:

• Checks to see whether the “DDNS” table exists in DynamoDB and creates the table if it does not. This table is used to keep a record of instances that have been created along with their attributes. It’s necessary to persist the instance attributes in a table because once an EC2 instance is terminated, its attributes are no longer available to be queried via the EC2 API. Instead, they must be fetched from the table.

• Queries the event data to determine the instance’s state. If the state is “running”, the function queries the EC2 API for the data it will need to update DNS. If the state is anything else, e.g. “stopped” or “terminated”, it will retrieve the necessary information from the “DDNS” DynamoDB table.

• Verifies that “DNS resolution” and “DNS hostnames” are enabled for the VPC, as these are required in order to use Route 53 for private name resolution. The function then checks whether a reverse lookup zone for the instance already exists. If it does, it checks to see whether the reverse lookup zone is associated with the instance’s VPC. If it isn’t, it creates the association. This association is necessary in order for the VPC to use Route 53 zone for private name resolution.

• Checks the EC2 instance’s tags for the CNAME and ZONE tags. If the ZONE tag is found, the function creates A and PTR records in the specified zone. If the CNAME tag is found, the function creates a CNAME record in the specified zone.

• Verifies whether there’s a DHCP option set assigned to the VPC. If there is, it uses the value of the domain name to create resource records in the appropriate Route 53 private hosted zone. The function also checks to see whether there’s an association between the instance’s VPC and the private hosted zone. If there isn’t, it creates it.

• Deletes the required DNS resource records if the state of the EC2 instance changes to “shutting down” or “stopped”.

Use the AWS CLI to create the Lambda function:

• Download the union.py.zip file from the AWS Labs GitHub repo.
• Execute the following command to create the function.  Note that you need to update the command to use the ARN of the role that you created earlier, as well as the local path to the union.py.zip file containing the Python code for the Lambda function.

aws lambda create-function --function-name ddns_lambda --runtime python2.7 --role <enter-your-role-arn-here> --handler union.lambda_handler --timeout 30 --zip-file fileb://<LOCAL PATH>/union.py.zip

• The output of the command returns the ARN of the newly-created function. Save this ARN, as you will need it in the next section

Step 3 – Create the CloudWatch Events Rule

In this step, you create the CloudWatch Events rule that triggers the Lambda function whenever CloudWatch detects a change to the state of an EC2 instance.  You configure the rule to fire when any EC2 instance state changes to “running”, “shutting down”, or “stopped”.  Use the aws events put-rule command to create the rule and set the Lambda function as the execution target:

aws events put-rule --event-pattern "{\"source\":[\"aws.ec2\"],\"detail-type\":[\"EC2 Instance State-change Notification\"],\"detail\":{\"state\":[\"running\",\"shutting-down\",\"stopped\"]}}" --state ENABLED --name ec2_lambda_ddns_rule

The output of the command returns the ARN to the newly created CloudWatch Events rule, named ec2_lambda_ddns_rule. Save the ARN, as you will need it to associate the rule with the Lambda function and to set the appropriate Lambda permissions.

Next, set the target of the rule to be the Lambda function.  Note that the —targets input parameter requires that you include a unique identifier for the Id target.  You also need to update the command to use the ARN of the Lambda function that you created previously.

aws events put-targets --rule ec2_lambda_ddns_rule --targets Id=id123456789012,Arn=<enter-your-lambda-function-arn-here>

Next, you add the permissions required for the CloudWatch Events rule to execute the Lambda function.   Note that you need to provide a unique value for the –statement-id input parameter.  You also need to provide the ARN of the CloudWatch Events rule you created earlier.

aws lambda add-permission --function-name ddns_lambda --statement-id 45 --action lambda:InvokeFunction --principal events.amazonaws.com --source-arn <enter-your-cloudwatch-events-rule-arn-here>

Step 4 – Create the private hosted zone in Route 53

To create the private hosted zone in Route 53, follow the steps outlined in Creating a Private Hosted Zone.

Step 5 – Create a DHCP options set and associate it with the VPC

In this step, you create a new DHCP options set and set the domain to be that of your private hosted zone.

1. Follow the steps outlined in Creating a DHCP Options Set to create a new set of DHCP options.
2. In the Create DHCP options set dialog box, give the new options set a name, set Domain name to the name of the private hosted zone that you created in Route 53, and set Domain name servers to “AmazonProvidedDNS”.  Choose Yes, Create.
3. Next, follow the steps outlined in Changing the Set of DHCP Options a VPC Uses to update the VPC to use the newly-created DHCP options set.

Step 6 – Launching the EC2 instance and validating results

In this step, you launch an EC2 instance and verify that the function executed successfully.

As mentioned previously, the Lambda function looks for the ZONE or CNAME tags associated with the EC2 instance.  If you specify these tags when you launch the instance, you should include the trailing dot.  In this example, the ZONE tag would be set to “ddnslambda.com.” and the CNAME tag could be set to “test.ddnslambda.com.”.

Because you updated the DHCP options set in this example, the Lambda function uses the specified zone when it creates the Route 53 DNS resource records.  You can use the ZONE tag to override this behavior if you want the function to update a different hosted zone.

In this example, you launch an EC2 instance into the private subnet of the VPC.  Because you updated the domain value of the DHCP options set to be that of the private hosted zone, the Lambda function creates the DNS resource records in the Route 53 zone file.

Launching the EC2 instance

1. Follow the steps to launch an EC2 instance outlined in Launching an Instance.
2. In Step 3: Configure Instance Details, for Network, select the VPC.  For Subnet, select the private subnet.  Choose Review and Launch.
3. (Optional) If you would like to update a different private hosted zone than the one you associated with the VPC, specify the ZONE tag in this step.  You can also specify the CNAME tag if you would like the function to create a CNAME resource record in the associated zone.

Choose Edit tags in the Step 7: Review Instance Launch.

Enter the key and value for Step 5: Tag Instance then choose Review and Launch.

Complete the launch of the instance and wait until the instance state changes to “running”.  Then, continue to the next step.

Validating results

In this step, you verify that your Lambda function successfully updated the Route 53 resource records.

1. Log in to the Route 53 console.
2. In the left navigation pane, choose Hosted Zones to view the list of private and public zones currently configured in Route 53.
3. Select the hosted zone that you created in step 4 to view the zone file.
4. Verify that the resource records were created.
5. Now that you’ve verified that the Lambda function successfully updated the Route 53 resource records in the zone file, stop the EC2 instance and verify that the records are removed by the function.
6. Log in to the EC2 console.
7. Choose Instances in the left navigation pane.
8. Select the EC2 instance you launched earlier and choose Stop.
9. Follow Steps 1 – 3 to view the DNS resource records in the Route 53 zone.
10. Verify that the records have been removed from the zone file by the Lambda function.

Conclusion

Now that you’ve seen how you can combine various AWS services to automate the creation and removal of Route 53 resource records, we hope you are inspired to create your own solutions.  CloudWatch Events is a powerful tool because it allows you to respond to events in real-time, such as when an instance changes its state.  When used with Lambda, you can create highly scalable serverless infrastructures that react instantly to infrastructure changes.

To learn more about CloudWatch Events, see Using CloudWatch Events in the Amazon CloudWatch Developer Guide.  To learn more about Lambda and serverless infrastructures, see the AWS Lambda Developer Guide and the “Microservices without the Servers” blog post.

We’ve open-sourced the code in this example in the AWS Labs GitHub repo and can’t wait to see your feedback and your ideas about how to improve the solution