terraform-aws-vpc

The AWS documentation on VPCs has a number of scenarios, which explore issues around routing and privacy that are quite fundamental. One of these scenarios is the "VPC with public and private subnet".

This set of scripts is used to set up an example of this scenario, launching an EC2 instance in each of the subnets, and deploying SSH keys so that you can SSH to the "public" host, and from there to the "private" host, but the private host itself does not have a public IP. Both hosts are able to reach out to the internet for things like patching and software installation, but are behind a NAT router. The "private" instances send outgoing traffic through the NAT gateway.

Usage

Using these scripts assume you have Terraform installed and are familiar with it, and that you are running on some sort of Unix. It also assumes that you have an AWS account to target the scripts against, and a suitably empowered user set up to create VPCs, networking stuff, and EC2 instances.

To begin with, create the bootstrap/env.rc file from the template, then execute the bootstrap/bootstrap.sh. If all goes well, you should wind up with some PEM files in the data directory, and some new key pairs in the AWS account. You should also verify that the S3 bucket and DynamoDB table got created.

Next, from within the platform-scripts directory, create the terraform.tfvars from the template, and update the backend.tf from the values setup by the bootstrap operation. (Annoyingly the Terraform backend does not support interpolation, so we need to copy the values manually from place to place.) Finally do a terraform init (if you have not already) followed by terraform apply.

After a certain amount of grinding, you should see some output from the scripts, e.g.:

bastion_private_dns = ip-172-21-10-59.eu-west-2.compute.internal
bastion_public_dns = ec2-35-178-47-224.eu-west-2.compute.amazonaws.com
connect_string = ssh -i vpc_test_bastion.pem [email protected]
nat_ip = 35.177.79.24
protected_private_dns = ip-172-21-20-45.eu-west-2.compute.internal

The gobastion.sh script should allow you to ssh to the bastion host. From there, you should be able to ssh to the protected host:

ec2-user@ip-172-21-10-59 ~]$ ssh -i .ssh/vpc_test_protected.pem [email protected]
ec2-user@ip-172-21-20-45 ~]$

And there you have it! A host in a subnet which can reach the internet, but not be accessed from outside, and both hosts protected by a NAT gateway.

A Note on NACL

The best way of thinking of the NACL is as a stateless firewall "in front of" the subnet to which they are attached. This can lead to some surprising requirements. The two in particular that should be noted here are:

• we need to allow port 22 out from the bastion to the protected subnet (and the ephemeral range in the opposite direction) so that we can SSH from the bastion to the protected subnet;
• we need to allow port 80 from the protected subnet (and ephemeral in the opposite direction), so that outgoing HTTP requests from the protected subnet can be routed through the bastion subnet.

Cleanup

To cleanup, execute terraform destroy from within the platform-scripts directory, and then from within the bootstrap-scripts/terraform directory. You may also want to remove the data/*.pem files.