A Terraform module for creating resilient bastion host using auto-scaling group (min=max=desired=1) and populate its ~/.ssh/authorized_keys with public keys fetched from S3 bucket.

This module can append public keys, setup cron to update them and run additional commands at the end of setup. Note that if it is set up to update the keys, removing a key from the bucket will also remove it from the bastion host.

Only SSH access is allowed to the bastion host.

Input variables:

• name - Name (default, bastion)
• instance_type - Instance type (default, t2.micro)
• ami - AMI ID of Ubuntu (see samples/ami.tf)
• region - Region (default, eu-west-1)
• iam_instance_profile - IAM instance profile which is allowed to access S3 bucket (see samples/iam_s3_readonly.tf)
• enable_monitoring - Whether to enable detailed monitoring (default, true)
• s3_bucket_name - S3 bucket name which contains public keys (see samples/s3_ssh_public_keys.tf)
• s3_bucket_uri – S3 URI which contains the public keys. If specified, s3_bucket_name will be ignored
• vpc_id - VPC where bastion host should be created
• subnet_ids - List of subnet IDs where auto-scaling should create instances
• keys_update_frequency - How often to update keys. A cron timespec or an empty string to turn off (default)
• additional_user_data_script - Additional user-data script to run at the end
• user_data_file - Override whold user-data script to add some custom security policies or to add support for OS you prefer. If not specified (by default) standard script will be used.
• associate_public_ip_address - Whether to auto-assign public IP to the instance (by default - false)
• eip - EIP to put into EC2 tag (can be used with scripts like https://github.com/skymill/aws-ec2-assign-elastic-ip, default - empty value)
• key_name - Launch configuration key name to be applied to created instance(s).
• allowed_cidr - A list of CIDR Networks to allow ssh access to. Defaults to "0.0.0.0/0"
• allowed_ipv6_cidr - A list of IPv6 CIDR Networks to allow ssh access to. Defaults to "::/0"
• allowed_security_groups - A list of Security Group ID's to allow access to the bastion host (useful if bastion is deployed internally) Defaults to empty list
• extra_tags - Optional a list of Key/Values Tags to be associated to the bastion host (see Interpolated Tags)

Outputs:

• ssh_user - SSH user to login to bastion
• security_group_id - ID of the security group the bastion host is launched in.

Terraform versions

For Terraform 0.12, use the version from master:

source  = "github.com/terraform-community-modules/tf_aws_bastion_s3_keys"

For Terraform 0.11, pin the module version to match v1.*. For e.g.:

source  = "github.com/terraform-community-modules/tf_aws_bastion_s3_keys?ref=v1.10.0"

Example

Basic example - In your terraform code add something like this:

module "bastion" {
  source                      = "github.com/terraform-community-modules/tf_aws_bastion_s3_keys"
  instance_type               = "t2.micro"
  ami                         = "ami-123456"
  region                      = "eu-west-1"
  iam_instance_profile        = "s3_readonly"
  s3_bucket_name              = "public-keys-demo-bucket"
  vpc_id                      = "vpc-123456"
  subnet_ids                  = ["subnet-123456", "subnet-6789123", "subnet-321321"]
  keys_update_frequency       = "5,20,35,50 * * * *"
  additional_user_data_script = "date"
}

If you want to assign EIP to instance launched by an auto-scaling group you can provide the desired eip as module input and then execute additional_user_data_script which sets EIP. This way you can use Route53 with EIP, which will always point to existing bastion instance. You will also need to add ec2:AssociateAddress permission to iam_instance_profile (see samples/iam_allow_associateaddress.tf):

module "bastion" {
  // see above
  eip = "${aws_eip.bastion.public_ip}"
  iam_instance_profile        = "s3_readonly-allow_associateaddress"
  additional_user_data_script = <<EOF
REGION=$(curl -s http://169.254.169.254/latest/dynamic/instance-identity/document | grep region | awk -F\" '{print \$4}')
INSTANCE_ID=$(curl -s http://169.254.169.254/latest/meta-data/instance-id)
aws ec2 associate-address --region $REGION --instance-id $INSTANCE_ID --allocation-id ${aws_eip.bastion.id}
EOF
}

resource "aws_eip" "bastion" {
  vpc = true
}

resource "aws_route53_record" "bastion" {
  zone_id = "..."
  name    = "bastion.example.com"
  type    = "A"
  ttl     = "3600"
  records = [aws_eip.bastion.public_ip]
}

After you run terraform apply you should be able to login to your bastion host like:

$ ssh ${module.bastion.ssh_user}@${aws_eip.bastion.public_ip}

or even like this:

$ ssh ubuntu@bastion.example.com

PS: In some cases you may consider adding flag -A to ssh command to enable forwarding of the authentication agent connection.

Inputs

Name	Description	Type	Default	Required
additional_user_data_script		string	""	no
allowed_cidr	A list of CIDR Networks to allow ssh access to.	list(string)	[ "0.0.0.0/0" ]	no
allowed_ipv6_cidr	A list of IPv6 CIDR Networks to allow ssh access to.	list(string)	[ "::/0" ]	no
allowed_security_groups	A list of Security Group ID's to allow access to.	list(string)	[]	no
ami		string	n/a	yes
apply_changes_immediately	Whether to apply the changes at once and recreate auto-scaling group	string	"false"	no
associate_public_ip_address		string	"false"	no
eip		string	""	no
enable_hourly_cron_updates		string	"false"	no
enable_monitoring		string	"true"	no
extra_tags	A list of tags to associate to the bastion instance.	object	[]	no
iam_instance_profile		string	n/a	yes
instance_type		string	"t2.micro"	no
instance_volume_size_gb	The root volume size, in gigabytes	string	"8"	no
key_name		string	""	no
keys_update_frequency		string	""	no
name		string	"bastion"	no
region		string	"eu-west-1"	no
s3_bucket_name		string	n/a	yes
s3_bucket_uri		string	""	no
security_group_ids	Comma seperated list of security groups to apply to the bastion.	string	""	no
ssh_user		string	"ubuntu"	no
subnet_ids	A list of subnet ids	list	[]	no
user_data_file		string	"user_data.sh"	no
vpc_id		string	n/a	yes

Outputs

Name	Description
asg_id
security_group_id
ssh_user

Authors

Created and maintained by Anton Babenko.

License

Apache 2 Licensed. See LICENSE for full details.