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Using AWS Security Groups and NACLs for advanced traffic filtering in the cloud


Amazon Web Services (AWS) is one of the most popular options for organizations looking to migrate their business applications to the cloud. It’s easy to see why:  AWS offers high capacity, scalable and cost-effective storage, and a flexible, shared responsibility approach to security. Essentially, AWS secures the infrastructure, and you secure whatever you run on that infrastructure.

However, this model does throw up some challenges. What exactly do you have control over?  How can you customize your AWS infrastructure so that it isn’t just secure today, but will continue delivering robust, easily managed security in the future?

The basics:  security groups

AWS offers virtual firewalls to organizations, for filtering traffic that crosses their cloud network segments.  The AWS firewalls are managed using a concept called Security Groups.  These are the policies, or lists of security rules, applied to an instance – a virtualized computer in the AWS estate.  AWS Security Groups are not identical to traditional firewalls, and they have some unique characteristics and functionality that you should be aware of, and we’ve discussed them in detail in video lesson 1:  the fundamentals of AWS Security Groups, but the crucial points to be aware of are as follows.

First, security groups do not deny traffic – that is, all the rules in security groups are positive, and allow traffic.  Second, while security group rules can be set to specify a traffic source, or a destination, they cannot specify both on the same rule.  This is because AWS always sets the unspecified side (source or destination) as the instance to which the group is applied.

Finally, single security groups can be applied to multiple instances, or multiple security groups can be applied to a single instance:  AWS is very flexible.  This flexibility is one of the unique benefits of AWS, allowing organizations to build bespoke security policies across different functions and even operating systems, mixing and matching them to suit their needs.

Adding Network ACLs into the mix

To further enhance and enrich its security filtering capabilities AWS also offers a feature called Network Access Control Lists (NACLs).  Like security groups, each NACL is a list of rules, but there are two important differences between NACLs and security groups.

The first difference is that NACLs are not directly tied to instances, but are tied with the subnet within your AWS virtual private cloud that contains the relevant instance.  This means that the rules in a NACL apply to all of the instances within the subnet, in addition to all the rules from the security groups.  So a specific instance inherits all the rules from the security groups associated with it, plus the rules associated with a NACL which is optionally associated with a subnet containing that instance.  As a result NACLs have a broader reach, and affect more instances than a security group does.

The second difference is that NACLs can be written to include an explicit action, so you can write ‘deny’ rules – for example to block traffic from a particular set of IP addresses which are known to be compromised.  The ability to write ‘deny’ actions is a crucial part of NACL functionality.

It’s all about the order

As a consequence, when you have the ability to write both ‘allow’ rules and ‘deny’ rules, the order of the rules now becomes important.  If you switch the order of the rules between a ‘deny’ and ‘allow’ rule, then you’re potentially changing your filtering policy quite dramatically.

To manage this, AWS uses the concept of a ‘rule number’ within each NACL.  By specifying the rule number, you can identify the correct order of the rules for your needs. You can choose which traffic you deny at the outset, and which you then actively allow.

As such, with NACLs you can manage security tasks in a way that you cannot do with security groups alone.  However, we did point out earlier that an instance inherits security rules from both the security groups, and from the NACLs – so how do these interact?

The order by which rules are evaluated is this; For inbound traffic, AWS’s infrastructure first assesses the NACL rules.  If traffic gets through the NACL, then all the security groups that are associated with that specific instance are evaluated, and the order in which this happens within and among the security groups is unimportant because they are all ‘allow’ rules.

For outbound traffic, this order is reversed:  the traffic is first evaluated against the security groups, and then finally against the NACL that is associated with the relevant subnet.

You can see me explain this topic in person in my new whiteboard video:

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