Deploying a Bastion Host in AWS using CloudFormation.

Although toil is highly discouraged, sometimes we need to ssh into an instance in order to do some kind of debugging. As a result, we need to expose that instance to the internet and that is no bueno. One way to prevent this from happening, is to implement a Bastion Host.

The idea of implementing this is being able to reduce the attack surface of our infrastructure by doing 2 things:

1. Remove the application instances (could also be database instances) or other servers that are not meant to be open to the world.

2. Be able to harden one machine (the bastion) and not each and every other server in our infrastructure. So in this case the more less the merrier.

Other benefit that the a Bastion Host can have is logging in order to prevent repudiation. This work because every engineer has their own key pair. So you can keep track of what Alice and Bob did during their last session.

Make sure you create a key pair in us-east-1 availability zone. We will use it to connect to our instance.

# deploy.sh

STACK_NAME=bastion-poc
REGION=us-east-1
CLI_PROFILE=<your-aws-profile-with-an-appropiate-role>
EC2_INSTANCE_TYPE=t2.micro
KEY_NAME=<your-key-pair-name>

./deploy.sh

After a couple of minutes you should see 2 IPs. When this happens we are golden ;)

[
    "http://10.0.0.x",
    "http://x.x.x.x"
]

Note: If you want to debug or see what happened go to the respective CloudFormation stack in the AWS console.

Go to ~/.ssh/config and add the following hosts:

...

### The Bastion Host
Host bastion-host-poc
    HostName <public-ip-from-output>
    User ec2-user
    Port 22
    IdentityFile ~/.ssh/<your-key-pair-private-key>

### The App Host
Host app-host-poc
    HostName <private-ip-from-output>
    User ec2-user
    IdentityFile ~/.ssh/<your-key-pair-private-key>
    ProxyJump bastion-host-poc

Go to your terminal and ssh into your app-host-poc

ssh app-host-poc

Voilà. You are inside a machine that is running in a private subnet. Isn't it cool?

Remember, this is just a PoC. For example, the Application Instance can still send traffic to the whole world (do you really want that?). Similarly, the Bastion Instance has yet to be hardened.

Implementing a Bastion can be useful for your current processes, especially if you have some instances exposed to the world and / or you want to control who can ssh into an specific instance.

Although you probably have a more sophisticated setup, a Bastion Host might be the right solution for you, and this could be the kickstart of your implementation.