Setting up NAT / WAF in Amazon AWS - amazon-web-services

Question

I have the following issue:
I want to setup a NAT or WAF in the following scenario:
Internet->VPC->IGW->Rotuer->Subnet1->EB1
->Subnet2->EB2
->Subnet3->EB3
in short words, my scenario is: VPC, Internet Gateway, Route table (Default from VPC) and subnets with EB into.
I tried some steps where I have to create another route table, set NAT instanc as target to 0.0.0.0/0 in this second one and attach the EB subnets to this one; attach the NAT subnet to the main route table and this seems not work.
Tried to Amazon AWS scneario 1 example that is my current configuration without results.
What can be missing to setup correctly it?
Thanks in advance.

Answer 1

I'm not entirely clear about what you are wanting to achieve, but I think you want your subnets to access the Internet, and prevent the Internet from having a direct access to the instances the reside in your private subnet(s).
It's hard to visualize your workflow that way. But i think I follow:
(AWS VPC Internet Gateway) --> Internet
AWS VPC CIDR range contains:
a) public/private subnet #1
b) private subnet #2. private subnet #3.. etc.
A typical set up of achieving requires the following Amazon entities:
1) IGW - internet Gateway
2) VPC with CIDR range of how big or small you want.
for the VMs that you want to protect, while still having internet access, you need them to be assign subnets. The subnets have to be WITHIN the VPC CIDR range. But do not make the subnet to be the same CIDR of the VPC, if you do this, then it essentially makes your private subnet a public accessible one since you have consumed all of the IP range and in which case you must assign it to the IGW which is not what you want.
3) A bastion server - what is a bastion server? This is a server that you purposely put to take internet incoming requests, which protect your private subnets. This can be in the form of a server that acts as a router or NAT. This can be either
3a) a homebrew install of Ubuntu (my fave distro) with iptables and ipv4 packets forwarded, or you can do
3b) a Nat gateway in your amazon VPC, or
3c) a vpc-nat AMI that you can find in the AMI Community. So just chose what method you wish.
4)Create Security group #1 - create a security group for the Bastion Server. I usually call it "NAT SG" and have inbound 0.0.0.0/0 and outbound to TCP 80, 443, 22 and etc, according to your outbound port needs determined by whatever you are running.
5) Security group #2 - create a security group for the private subnet. This is for traffic that will allow all within the subnet(s) and to funnel all outbound non private data to 0.0.0.0/0 to the bastion server.
To implement this is a simplified conceptual order of operations you need to do:
1) Create a VPC with the CIDR range (let's say 10.0.0.0/16)
1.5) create an Internet Gateway and assign it to the VPC (10.0.0.0/16)
2) Create private subnets with CIDR range that's in the VPC CIDR (10.0.1.0/24, 10.0.2.0/24, 10.0.2.0/24, etc).
3) Create a public subnet that will contain the bastion server, or whatever you want the Internet to see. (10.0.254.0/24 (this is huge, allowing 256 servers in the subnet. You will most likely only have the bastion server consuming only 1 server, so it will probably auto assign 10.0.254.2)
4) create 2 routing tables - one called private and one called public. For the private routing table, edit, then add a new Route for 0.0.0.0/0 goes to the bastion server ID. For the public routing table, edit , then add a new route for 0.0.0.0/0 to the Internet Gateway ID (see step 1.5)
5) Either launch your own server and set it up to become a NAT router (if you know how to do that), or start up the vpc-nat AMI (amzn-ami-vpc-nat-hvm-2016.03.0.x86_64-ebs - ami-311a1a5b) is the latest one, so use that. When you create the instance, select the public subnet, the VPC, and assign it a public IP (don't wait and give it an Elastic IP, you will want the public IP),select the "NAT SG" security group, and then Launch it. After it is launched you will select it and select Network --> src/dest. check and DISABLE it. This allows all traffic to funnel in. You only need 1 bastion server, I'm not stating to do all 3.
6) Launch your AMI or custom VM that you want private, and be within the private subnets (10.0.1.0/24, 10.0.2.0/24, 10.0.3.0/24... etc). And assign it a private security group, and the same VPC. If it is linux or windows you may want to generate a new private key (pem) for future login. Convert the pem to a ppk using puttygen.
Testing, in order to ensure you have internet connectivity:
the bastion server must be able to reach the world
the VM instance in the private subnet (if the routing was done accurately), should reach the internet by flowing packets to the 0.0.0.0/0 rule to pass through the bastion server, and the bastion server will allow resolution and flow to the internet for outbound requests.
I usually do a ping to google since they do not block ICMP requests and are quite reliable. So ssh into the bastion server, ping google.com. If you get the IP but it cannot get a reply then it means your Internet gateway or the Security group did not all all outbound traffic 0.0.0.0/0, so double-check that.
Now SSH into the private server (assuming linux) and ping the bastion server (10.0.254.X) and you should be able to get a response. That's a good sign. now ping google.com and see if you get a reply. I hope you do. If you get DNS resolution but it is stuck at get any responses, then you forgot to disable Src/Dest checking on the bastion server. Then go to your ec2 console, locate the bastion server and select, then actions, networking, and select src/dest check and confirm DISABLE and save.
That should make your private network resolve and hit the world, while still being protected from the internet.
I hope this helps.
Richard.