AWS VPC - Internet Gateway vs NAT

Question

What is an Internet Gateway? What is a NAT Instance? What services do they offer?

Reading AWS VPC documentation, I gather they both map private IP addresses to internet route-able addresses for the outgoing requests and route the incoming responses from the internet to the requester on the subnet.

So what are the differences between them? What scenarios do I use a NAT Instance instead of (or beside) an Internet Gateway? Are they essentially EC2 instances running some network applications or are they special hardware like a router?

Instead of simply pointing to AWS documentation links, can you please explain these by adding some background on what is public and private subnets so any beginner with limited knowledge of networking can understand these easily? Also, when should I use a NAT Gateway instead of a NAT instance?

Answer 1

An Internet Gateway is a logical connection between an Amazon VPC and the Internet. It is not a physical device. Only one can be associated with each VPC. It does not limit the bandwidth of Internet connectivity. (The only limitation on bandwidth is the size of the Amazon EC2 instance, and it applies to all traffic -- internal to the VPC and out to the Internet.)

If a VPC does not have an Internet Gateway, then the resources in the VPC cannot be accessed from the Internet (unless the traffic flows via a corporate network and VPN/Direct Connect).

A subnet is deemed to be a Public Subnet if it has a Route Table that directs traffic to the Internet Gateway.

NAT Instance

A NAT Instance is an Amazon EC2 instance configured to forward traffic to the Internet. It can be launched from an existing AMI, or can be configured via User Data like this:

#!/bin/sh echo 1 > /proc/sys/net/ipv4/ip_forward echo 0 > /proc/sys/net/ipv4/conf/eth0/send_redirects /sbin/iptables -t nat -A POSTROUTING -o eth0 -s 0.0.0.0/0 -j MASQUERADE /sbin/iptables-save > /etc/sysconfig/iptables mkdir -p /etc/sysctl.d/ cat <<EOF > /etc/sysctl.d/nat.conf net.ipv4.ip_forward = 1 net.ipv4.conf.eth0.send_redirects = 0 EOF

Instances in a private subnet that want to access the Internet can have their Internet-bound traffic forwarded to the NAT Instance via a Route Table configuration. The NAT Instance will then request the Internet (since it is in a Public Subnet) and the response will be forwarded back to the private instance.

Traffic sent to a NAT Instance will typically be sent to an IP address that is not associated with the NAT Instance itself (it will be destined for a server on the Internet). Therefore, it is important to turn off the Source/Destination Check option on the NAT Instance otherwise the traffic will be blocked.