Connecting your AWS infrastructure to your internal network

Introduction

In some cases you will want to keep your infrastructure in AWS completely separate from your internal network.  I needed to integrate the infrastructure with the rest of our infrastructure, including connecting Windows server to our domain.

There are three obvious ways to connect the networks:

If you have an MPLS WAN, you probably want to extend this to your AWS VPC (Virtual Private Cloud).  For this you would use DirectConnect.

Another option is to use the Amazon Virtual Private Gateway (VPG) to set up a VPN connection to your edge device.

I opted for the third option, which is to create a server to provide the VPN connections.  The main drivers behind this decision were that we are planning on changing our edge device soon, we already use OpenSWAN to connect some of our networks, and we need to connect three different networks to AWS. I'll share a template and a script that will connect to a suitably configured OpenSWAN installation on your network, or connect to another instance in a different VPC (created with the same template).

Once the network connection is in place, add DHCP options to your VPC and you're ready to add Windows servers to your domain.  I added a Read Only Domain Controller to cache DNS and authentication.

Creating the VPN

This article walks you through connecting two VPCs using OpenSWAN.  You can read that for a lot of the background.  I have created a template that you can get here that will set up an OpenSWAN instance for you, using parameters you provide.  You can use this to connect to a VPN endpoint in your internal network, or, of course, to another instance in another VPC.

In my previous post, I shared a template to create a NAT instance.  There I used "UserData" to simply script the whole installation.  In this template I follow a different approach (although it would have been just as easy to do it the same way).  Using cfn-init and "Metadata" allows a more involved configuration, e.g. where reboots are required, such as adding a Windows server to a domain, but here, on linux, it's just another way of doing the same thing.

The template shows how to install a linux package openswan, create a number of configuration files (assembled from parameters provided), set the services to run (to make it persistent in case of a reboot) and run all the commands required to start the services.

The only resource created other than the single ec2 instance is a security group that allows access from private IP addresses (both internal and in AWS) only.

An alternative is to use "sources" instead of files.  This allows you to have zipped (or tarred) files downloaded from an S3 bucket and extracted to a location you specify.  This is ideal if you want to set up multiple VPN connections.

This snippet, inserted before the "files" section, will retrieve and unzip files into the /etc/ipsec.d folder

"sources" : {
  "/etc/ipsec.d" : { "Fn::Join" : ["", ["https://s3-eu-west-1.amazonaws.com/",
    { "Ref" : "SourceBucket" },
    "/",
    { "Ref" : "SourceFilesKey" }
    ]]} 
},

In my github account I have a "work in progress" template that uses this.

Although I'm using this blog mostly to record, for my own purposes, things I pick up, I also find it a lot of work - none of the scripts or templates I use here end up having much in common with what I actually use in anger - all part of trying to make it work with less things already there.

Scripting the creation

As in my previous post, I use a PowerShell script to actually create the VPN using the CloudFormation template above.

This should be simple, shouldn't it?  Actually, if you have a look at the script, you'll notice that there's 60 lines to guess the parameters if you haven't provided them, 30 lines of "help" and really only a little bit at the end that actually creates the VPN, assign the external IP and delete the previous VPN instance if there is one.

The idea of the script is to replace an existing instance, either to update settings or because something is not working as expected, with minimal disruption of the tunnel.

Preparing for extending you AD

Once you have your VPN up and running and you have created routes on your internal network to get to your VPC, you can start putting private servers in your private subnets.  This includes extending your Active Directory into your Virtual Private Cloud, should you have a reason to do so.

Once you know that instances in your VPC can happily access your on site resources, you can potentially add Windows instances to your domain.

Before it's possible to add an instance to your domain, you need to make one more change.  In the AWS Console, go to the VPC Management Console and DHCP Option Sets.  Create a new option set with a domain name and domain name servers set to your internal DNS servers (Domain Controllers).

Once this is saved, go to "Your VPCs", select your VPC, under Actions, Edit DHCP Option set and select the newly created DHCP Option set.

You probably want to create one or two read only domain controllers (RODC) in your VPC.  I've found that an RODC runs quite happily on a micro instance.  Once this is running, you need to create a new DHCP option set with your RODC(s) set as the first DNS server(s) (leave your internal ones later down the list).

You're ready to start building domain joined Windows servers in AWS now.

AWS Cloudformation - creating a NAT instance

Introduction

Having got the basic networking in place with public and private subnets, there is one more networking ingredient that is required.  If you want to be able to use many of the AWS services in your private subnets, instances in these subnets need internet access.

In this post, I'll create a NAT instances to give internet access to private subnets. 

To build the NAT instance I'll use Cloudformation, and then I'll create a Powershell script to create or replace the NAT instance.

NAT instance

You can create an EC2 NAT instance by using one of the community NAT AMIs - I'm sure that's OK, but the one I happened to pick didn't work, and using this approach is a lot more flexible, and just as easy.

Amazon provide a very good article on how to set up high availability NAT.  What I'm doing here is quite a bit simpler - it will not give the high availability of the Amazon solution, but will allow a fairly rapid replacement of a malfunctioning NAT instance (probably 10 minutes rather than 10 seconds). I also needed to NAT an incoming port, forwarding it to another instance, meaning I could only have one working instance.

Cloudformation

Cloudformation is a fantastic service.  There is no cost associated with Cloudformation - you pay for what you create.  In this case I'm creating an EC2 instance and will pay the hourly rate as soon as it starts.  With Cloudformation you use a template to create a Cloudformation "stack".

The stack I'll create here contains an EC2 instance doing NAT and a security group.

The template I use is quite simple, and is easy to deconstruct and see how templates work.  If you want to, feel free to download the template from here, modify it, and use Cloudformation to build it.  If you are familiar with linux and bash, you will see how "UserData" in the instance properties can be used to do virtually any configuration of a linux instance.  A note here - the template is in JSON format, which can be very easy to get wrong.  I use Notepad ++ in a Windows environment, and add the JSON viewer plug-in.  This allows you to select your whole template and quickly verify if the JSON is OK.

The instance you create with this template may be included in the free tier, costing nothing.  However, it's a pretty useless instance on it's own - without modification you won't be able to access it except from other instances in your VPC.  To allow external access, make a second copy of the line:

{ "IpProtocol" : "tcp", "FromPort" : {"Ref" : "ForwardPort" }, "ToPort" : {"Ref" : "ForwardPort" }, "CidrIp" : "0.0.0.0/0" } ,

replacing  "ForwardPort" with "SSHPort", and preferably locking down the CidrIp to your own IP address.

Once the stack has been created, I need to make it work.  In my previous post I mentioned the two routing tables.  Once I have the NAT instance in place, I create a routing table with the NAT instance as default route, and I associate this routing table with the Private subnets.

PowerShell

I use a Powershell script to create or replace the NAT instance.  This script can be called by a monitoring server, or manually.

PowerShell may seem an odd choice - while the PowerShell module for AWS is very good, I'm pretty sure it's development is behind the normal AWS CLI (Command Line Interface), that can be used from any platform.  The script would have been very easy to create in bash, python or whatever.  There are two reasons I'm using PowerShell - the first is that I quite like PowerShell, but the real reason is that I'm currently building a Windows environment.  In a future post I hope to share my script for creating a full mirrored SQL server environment.  For this you really do need PowerShell, so I'd rather give my clients a consistent tool set.

The script can be used to create an initial NAT instance, as well as to replace it, either after updating the template, to change the parameters, or if the instance stops working as expected.

To use the script, ensure you have initialised your AWS settings in Powershell as explained in a previous post.  Change to the directory where the script is saved. Upload the template to an S3 bucket (if you have a local copy, you can just use the following PowerShell commands):

New-S3Bucket <uniquebucketname>
Write-S3Object -File NATInstance.template -BucketName <uniquebucketname> -Key NatInstance.template

After this, you can use the template you have uploaded with the script.  You can get the url of the file from the AWS console, or use https://s3-eu-west-1.amazonaws.com/uniquebucketname/NATInstance.template (or similar, depending on your region).

To create an initial NAT instance that serves purely to allow outgoing internet access, make sure at least one subnet has "auto-assign Public IP" enabled, and you have created a second routing table (you don't need to have anything in the routing table). Then simply run:
./Replace-NAT.ps1 -NatTemplateURL https://.....
This should create a NAT instance.  For private subnets to start using it, associate your private subnets with this routing table.

If you need to use incoming NAT on a specific port, you can create and assign an EIP to this instance (that allows you to retain the same IP address for future use).

You can replace the instance at any time, whether because of issues, or because you want to make changes (e.g. to implement incoming NAT you can simply add the -ForwardHost and -ForwardPort parameters).  The script will, by default, replace the route and delete the previous stack (after confirming with you, or without confirmation if you specify -Force).

For more information on the script (help is very limited, but at least it will list available parameters if you don't feel like editing the script), run
Get-Help ./Replace-Nat.ps1

That's all for this post - next post I'm planning on showing an alternative way to customise a linux instance.