AWS Security: Bastion Host, NAT instances and VPC Peering

What is a bastion host, and do I need one?

Bastion hosts are instances that sit within your public subnet and are typically accessed using SSH or RDP. Once remote connectivity has been established with the bastion host, it then acts as a ‘jump’ server, allowing you to use SSH or RDP to log in to other instances (within private subnets) deeper within your VPC. When properly configured through the use of security groups and Network ACLs (NACLs), the bastion essentially acts as a bridge to your private instances via the internet.

You may ask yourself, do I need a bastion host in my environment? If you require remote connectivity with your private instances over the public internet, the answer is yes!

This diagram shows connectivity flowing from an end user to resources on a private subnet through a bastion host:

When designing the bastion host for your AWS infrastructure, you shouldn’t use it for any other purpose, as this could open unnecessary security holes. Instead, I would suggest that you look into hardening your chosen operating system for even tighter security.

Here are the basic steps for creating a bastion host for your AWS infrastructure:

1. Launch an EC2 instance as you normally would for any other instance.
2. Apply OS hardening as required.
3. Set up the appropriate security groups (SG).
4. Implement either SSH-agent forwarding (Linux connectivity) or Remote Desktop Gateway (Windows connectivity).
5. Deploy an AWS bastion host in each of the Availability Zones you’re using.

Security groups are essential for maintaining tight security and play a big part in making this solution work (you can read more about AWS security groups here). First, create a SG that will be used to allow bastion connectivity for your existing private instances. This SG should only accept SSH or RDP inbound requests from your bastion hosts across your Availability Zones (AZ). Apply this group to all of your private instances that require connectivity.

Next, create a security group to be applied to your bastion host. Inbound and outbound traffic must be restricted at the protocol level as much as possible. The inbound rule base should accept SSH or RDP connections only from the specific IP addresses (usually those of your administrators). You definitely want to avoid allowing wide open access (0.0.0.0/0). Your outbound connection should again be restricted to SSH or RDP access to the private instances of your AWS infrastructure. An easy way to do this is to populate the ‘Destination’ field with the ID of the security group you’re using for your private instances.

SSH and RDP connections require private and public key access to authenticate. This does not pose a problem when you are trying to connect to your bastion host from your local machine, as you can easily store the private key locally. However, once you have connected to your bastion host, logging in to your private instances from the bastion would require having their private keys on the bastion. As you will probably already know (and if not, then take careful note now), storing private keys on remote instances is not a good security practice.

As a result, AWS suggests that you implement either Remote Desktop Gateway (for connecting to Windows instances) or SSH-agent forwarding (for Linux instances). Both of these solutions eliminate the need for storing private keys on the bastion host. AWS provides great documentation on how to implement Windows Remote Desktop Gateway and SSH-agent forwarding.

As with all cloud deployments, you should always consider the resiliency and high availability of your services. With this in mind, I recommend deploying a bastion within each public Availability Zone that you are using. Remember: if the AZ hosting your only AWS bastion host goes down, you will lose connectivity to your private instances in other AZs.

NAT Instances & NAT Gateways

A NAT (Network Address Translation) instance is, like an bastion host, an EC2 instance that lives in your public subnet. A NAT instance, however, allows your private instances outgoing connectivity to the internet while at the same time blocking inbound traffic from the internet.

Many people configure their NAT instances to allow private instances to access the internet for important operating system updates. As I’ve discussed previously, patching your OS is an important part of maintaining instance level security.

NAT Gateways provide the same functionality as a NAT instance, however a NAT Gateway is an AWS managed NAT service. As a result, these NAT Gateways offer greater availability and bandwidth and require less configuration and administration.

Launching a NAT Instance Inside Your VPC

You can create and launch a NAT instance in three steps:

1. Create a Security Group, which will be applied to your NAT.
2. Select a pre-defined AMI and configure it as with any other EC2 instance.
3. Set up correct routing.

Once your NAT has been launched, it’s important to disable source/destination checks. To do this, right click on your NAT Instance within the AWS Console and select ‘Networking > Change Source/Dest. Check > Yes, Disable’.

When creating a security group for your NAT, make sure that you allow inbound traffic from your private instances through the HTTP (80) and HTTPS (443) ports to allow for OS and software updates. Your outbound rule set should have an open destination of 0.0.0.0/0 for port 80 and 443 as well. If your instances will require you to open any other ports, this is where to do it.

AWS provides some Amazon Machine Images (AMIs) that are already pre-configured as NAT instances—I recommend that you consider using one. NAT AMIs have names that include the string ‘amzn-ami-vpc-nat’. This makes them easy to find by searching from the Community AMI tab in step one when launching an EC2 instance. These AMIs are a good idea, as they’re configured right out of the box for IPv4 forwarding and iptables IP masquerading. ICMP redirects are disabled.

You will now have to modify the route table used by your private subnets. Make sure you have a route ‘Destination’ that points to the outside world of ‘0.0.0.0/0’ with a ‘Target’ of ‘your new NAT instance.’ Your NAT-instance must be launched within your public subnet and it must have a public IP address. The route table of your public subnet where your NAT resides must have a route to the internet via your Internet Gateway. This will ensure that any request from your private instance will first go to the NAT, and the NAT will forward that traffic out via the IGW to the internet.

Your NAT is now set up and your private instances should be able to communicate with the outside world for updates etc. using ports 80 and 443. However, it’s important to note that connections initiated from the internet will not reach your private instances,as this configuration protects them.

Launching a NAT Gateway inside your VPC

Creating a NAT Gateway requires less configuration compared to a NAT instance:

1. From within the VPC dashboard in the AWS Management Console, select NAT Gateways > Create NAT Gateway.
2. Select the subnet to deploy your NAT Gateway.
3. Apply an available Elastic IP Address (EIP) to your NAT Gateway and click ‘Create.’
4. Set up correct routing for your private and public subnets as per the explanation above for NAT instances.

NAT Instance vs. NAT Gateway

The following table from AWS Documentation highlights the main differences between NAT instances and NAT Gateways.

Introduction to AWS VPC Peering (Virtual Private Cloud)

With AWS VPC Peering, you can connect two VPCs as a single network. Instead of routing data through the internet or a VPN connection, AWS VPC Peering uses the internal AWS network infrastructure to share resources between AWS VPCs. AWS VPC peering provides a tight and secure shared environment that minimizes external exposure.

Because peered VPCs communicate across their private CIDR blocks, it’s important to ensure that the two VPCs do not have overlapping CIDR address ranges. It’s also important to note that you cannot directly reference a security group from one VPC to the other. Instead, you’ll need to enter a CIDR block or specific IP address in the Source/Destination section of your SG rules.

AWS VPC Peering Design

When setting up a peered connection, one VPC acts as the requester (the VPC initiating the connection) while the other acts as a peer. Before a connection can be established, the owner of the peer VPC has to acknowledge the request and accept the Peering connection. Once a connection has been established, routing between the CIDR blocks of each VPC must be added to a route table to enable resources within the networks to communicate via the private IP address range.

From a design perspective, you cannot daisy chain VPCs together and expect them to communicate across one large network. Each AWS VPC will only communicate with its ‘requester’ or ‘peer.’ For example, if you have a peering connection between VPC 1 and VPC 2, and another connection between VPC 2 and VPC 3 as below:

Then VPC 1 and VPC 2 could communicate with each other directly, as can VPC 2 and VPC 3. However, because you cannot route through one VPC to get to another, VPC 1 and VPC 3 could not communicate directly.

To enable direct communication between VPC 1 and VPC 3, you would have to implement a separate peering connection between the two, as shown below:

AWS VPC peering provides an excellent secure and trusted connection between VPCs for enhanced management and resource sharing. Depending on how you have configured your VPCs, you may want to incorporate such an architecture into your environment. AWS offers some scenarios that are definitely worth exploring.

For advanced configuration of Subnets and Routing for VPC Peering and other VPC solutions including various gateways, see our course, AWS Virtual Private Cloud: Subnets & Routing.

Let’s summarize what we have covered this week:

• Bastion host: An AWS bastion host can provide a secure primary connection point as a ‘jump’ server for accessing your private instances via the internet.
• NAT instance. For your private instances, a NAT instance can provide access to the internet for essential software updates while blocking incoming traffic from the outside world.
• VPC Peering. VPC peering can be used to create secure connectivity and resource sharing between two VPCs.