Hands-on walkthrough of the AWS Network Firewall flexible rules engine – Part 2

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Hands-on walkthrough of the AWS Network Firewall flexible rules engine – Part 2

This blog post is Part 2 of Hands-on walkthrough of the AWS Network Firewall flexible rules engine – Part 1. To recap, AWS Network Firewall is a managed service that offers a flexible rules engine that gives you the ability to write firewall rules for granular policy enforcement. In Part 1, we shared how to write a rule and how the rule engine processes rules differently depending on whether you are performing stateless or stateful inspection using the action order method.

In this blog, we focus on how stateful rules are evaluated using a recently added feature—the strict rule order method. This feature gives you the ability to set one or more default actions. We demonstrate how you can use this feature to create or update your rule groups and share scenarios where this feature can be useful.

In addition, after reading this post, you’ll be able to deploy an automated serverless solution that retrieves the latest Suricata-specific rules from the community, such as from Proofpoint’s Emerging Threats OPEN ruleset. By deploying such solutions into your Amazon Web Services (AWS) environment, you can seamlessly enhance your overall security posture as the solutions fetch the latest set of intrusion detection system (IDS) rules from Proofpoint (formerly Emerging Threats) and optionally using them as intrusion prevention system (IPS) thereby keeping the rule groups updated on your Network Firewall. You can select the refresh interval to update these rulesets—the default refresh interval is 6 hours. You can also convert the set of rule groups to intrusion prevention system (IPS) mode. Finally, you have granular visibility of the various categories of rules for your Network Firewall on the AWS Management Console.

How does Network Firewall evaluate your stateful rule group?

There are two ways that Network Firewall can evaluate your stateful rule groups: the action ordering method or the strict ordering method. The settings of your rule groups must match the settings of the firewall policy that they belong to.

With the action order evaluation method for stateless inspection, all individual packets in a flow are evaluated against each rule in the policy. The rules are processed in order based on the priority assigned to them with lowest numbered rules evaluated first. For stateful inspection using the action order evaluation method, the rule engine evaluates based on the order of their action setting with pass rules processed first, then drop, then alert. The engine stops processing rules when it finds a match. The firewall also takes into consideration the order that the rules appear in the rule group, and the priority assigned to the rule, if any. Part 1 provides more details on action order evaluation.

If your firewall policy is set up to use strict ordering, Network Firewall now allows you the option to manually set a strict rule group order for stateful rule groups. Using this optional setting, the rule groups are evaluated in order of priority, starting from the lowest numbered rule, and the rules in each rule group are processed in the order in which they’re defined. You can also select which of the default actions—drop all, drop established, alert all, or alert established—Network Firewall will take when following strict rule ordering.

A customer scenario where strict rule order is beneficial

Configuring rule groups by action order is appropriate for IDS use cases, but can be an obstacle for use cases where you deploy firewalls that follow security best practice, which is to allow only what’s required and deny everything else (default deny). You can’t achieve this best practice by using the default action order behavior. However, with strict order functionality, you can create a firewall policy that allows prioritization of stateful rules, or that can run 5-tuple and Suricata rules simultaneously. Strict rule order allows you to have a block of fine-grain rules with specific actions at the beginning followed by a coarse set of rules with specific actions and finally a default drop action. An example is shown in Figure 1 that follows.

Figure 1: An example snippet of a Network Firewall firewall policy with strict rule order

Figure 1 shows that there are two different default drop actions that you can choose: drop established and drop all. If you choose drop established, Network Firewall drops only the packets that are in established connections. This allows the layer 3 and 4 connection establishment packets that are needed for the upper-layer connections to be established, while dropping the packets for connections that are already established. This allows application-layer pass rules to be written in a default-deny setup without the need to write additional rules to allow the lower-layer handshaking parts of the underlying protocols.

The drop all action drops all packets. In this scenario, you need additional rules to explicitly allow lower-level handshakes for protocols to succeed. Evaluation order for stateful rule groups provides details of how Network Firewall evaluates the different actions. In order to set the additional environment variables that are shown in the snippet, follow the instructions outlined in Examples of stateful rules for Network Firewall and the Suricata rule variables.

An example walkthrough to set up a Network Firewall policy with a stateful rule group with strict rule order and default drop action

In this section, you’ll start by creating a firewall policy with strict rule order. From there, you’ll build on it by adding a stateful rule group with strict rule order and modifying the priority order of the rules within a stateful rule group.

Step 1: Create a firewall policy with strict rule order

You can configure the default actions on policies using strict rule order, which is a property that can only be set at creation time as described below.

1. Log in to the console and select the AWS Region where you have Network Firewall.
2. Select VPC service on the search bar.
3. On the left pane, under the Network Firewall section, select Firewall policies.
4. Choose Create Firewall policy. In Describe firewall policy, enter an appropriate name and (optional) description. Choose Next.
5. In the Add rule groups section.
   1. Select the Stateless default actions:
      1. Under Choose how to treat fragmented packets choose one of the options.
      2. Choose one of the actions for stateless default actions.
   2. Under Stateful rule order and default action
      1. Under Rule order choose Strict.
      2. Under Default actions choose the default actions for strict rule order. You can select one drop action and one or both of the alert actions from the list.
6. Next, add an optional tag (for example, for Key enter Name, and for Value enter Firewall-Policy-Non-Production). Review and choose Create to create the firewall policy.

Step 2: Create a stateful rule group with strict rule order

1. Log in to the console and select the AWS Region where you have Network Firewall.
2. Select VPC service on the search bar.
3. On the left pane, under the Network Firewall section, select Network Firewall rule groups.
4. In the center pane, select Create Network Firewall rule group on the top right.
   1. In the rule group type, select Stateful rule group.
   2. Enter a name, description, and capacity.
   3. In the stateful rule group options select either 5-tuple or Suricata compatible IPS rules. These allow rule order to be strict.
   4. In the Stateful rule order, choose Strict.
   5. In the Add rule section, add the stateful rules that you require. Detailed instructions on creating a rule can be found at Creating a stateful rule group.
   6. Finally, Select Create stateful rule group.

Step 3: Add the stateful rule group with strict rule order to a Network Firewall policy

1. Log in to the console and select the AWS Region where you have Network Firewall.
2. Select VPC service on the search bar.
3. On the left pane, under the Network Firewall section, select Firewall policies.
4. Chose the network firewall policy you created in step 1.
5. In the center pane, in the Stateful rule groups section, select Add rule group.
6. Select the stateful rule group you created in step 2. Next, choose Add stateful rule group. This is explained in detail in Updating a firewall policy.

Step 4: Modify the priority of existing rules in a stateful rule group

1. Log in to the console and select the AWS Region where you have Network Firewall.
2. Select VPC service on the search bar.
3. On the left pane, under the Network Firewall section, choose Network Firewall rule groups.
4. Select the rule group that you want to edit the priority of the rules.
5. Select the Edit rules tab. Select the rule you want to change the priority of and select the Move up and Move down buttons to reorder the rule. This is shown in Figure 2.

Figure 2: Modify the order of the rules within a stateful rule groups

Note:

• Rule order can be set to strict order only when network firewall policies or rule groups are created. The rule order can’t be changed to strict order evaluation on existing objects.
• You can only associate strict-order rule groups with strict-order policies, and default-order rule groups with default-order policies. If you try to associate an incompatible rule group, you will get a validation exception.
• Today, creating domain list-type rule groups using strict order isn’t supported. So, you won’t be able to associate domain lists with strict order policies. However, 5-tuple and Suricata compatible rules are supported.

Automated serverless solution to retrieve Suricata rules

To help simplify and maintain your more advanced Network Firewall rules, let’s look at an automated serverless solution. This solution uses an Amazon CloudWatch Events rule that’s run on a schedule. The rule invokes an AWS Lambda function that fetches the latest Suricata rules from Proofpoint’s Emerging Threats OPEN ruleset and extracts them to an Amazon Simple Storage Service (Amazon S3) bucket. Once the files lands in the S3 bucket another Lambda function is invoked that parses the Suricata rules and creates rule groups that are compatible with Network Firewall. This is shown in Figure 3 that follows. This solution was developed as an AWS Serverless Application Model (AWS SAM) package to make it less complicated to deploy. AWS SAM is an open-source framework that you can use to build serverless applications on AWS. The deployment instructions for this solution can be found in this code repository on GitHub. 

Figure 3: Network Firewall Suricata rule ingestion workflow

Multiple rule groups are created based on the Suricata IDS categories. This solution enables you to selectively change certain rule groups to IPS mode as required by your use case. It achieves this by modifying the default action from alert to drop in the ruleset. The modified stateful rule group can be associated to the active Network Firewall firewall policy. The quota for rule groups might need to be increased to incorporate all categories from Proofpoint’s Emerging Threats OPEN ruleset to meet your security requirements. An example screenshot of various IPS categories of rule groups created by the solution is shown in Figure 4. Setting up rule groups by categories is the preferred way to define an IPS rule, because the underlying signatures have already been grouped and maintained by Proofpoint.   

Figure 4: Rule groups created by the solution based on Suricata IPS categories

The solution provides a way to use logs in CloudWatch to troubleshoot the Suricata rulesets that weren’t successfully transformed into Network Firewall rule groups.
The final rulesets and discarded rules are stored in an S3 bucket for further analysis. This is shown in Figure 5. 

Figure 5: Amazon S3 folder structure for storing final applied and discarded rulesets

Conclusion

AWS Network Firewall lets you inspect traffic at scale in a variety of use cases. AWS handles the heavy lifting of deploying the resources, patch management, and ensuring performance at scale so that your security teams can focus less on operational burdens and more on strategic initiatives. In this post, we covered a sample Network Firewall configuration with strict rule order and default drop. We showed you how the rule engine evaluates stateful rule groups with strict rule order and default drop. We then provided an automated serverless solution from Proofpoint’s Emerging Threats OPEN ruleset that can aid you in establishing a baseline for your rule groups. We hope this post is helpful and we look forward to hearing about how you use these latest features that are being added to Network Firewall.