AWS Vpc Nat Certification Key Topics

NAT Overview

  • Network Address Translation (NAT) devices, launched in the public subnet, enables instances in a private subnet to connect to the Internet, but prevents the Internet from initiating connections with the instances.
  • Instances in private subnets would need internet connection for performing software updates or trying to access external services
  • NAT device performs the function of both address translation and port address translation (PAT)
  • NAT instance prevents instances to be directly exposed to the Internet and having to be launched in Public subnet and assignment of the Elastic IP address to all, which are limited.
  • NAT device routes the traffic, from the private subnet to the Internet, by replacing the source IP address with its address and for the response traffic it translates the address back to the instances' private IP addresses.
  • AWS allows NAT configuration in 2 ways
    • NAT Instance
    • NAT Gateway, managed service by AWS

NAT device Configuration Key Points

  • needs to be launched in the Public Subnet
  • needs to be associated with an Elastic IP address (or public IP address)
  • should have the Source/Destination flag disabled to route traffic from the instances in the private subnet to the Internet and send the response back
  • should have a Security group associated that
    • allows Outbound Internet traffic from instances in the private subnet
    • disallows Inbound Internet traffic from everywhere
  • Instances in the private subnet should have the Route table configured to direct all Internet traffic to the NAT device

NAT Gateway

NAT gateway is a AWS managed NAT service that provides better availability, higher bandwidth, and requires less administrative effort.

  • A NAT gateway supports bursts of up to 10 Gbps of bandwidth.
  • For over 10 Gbps bursts requirement, the workload can be distributed by splitting the resources into multiple subnets, and creating a NAT gateway in each subnet
  • NAT gateway is associated with One Elastic IP address which cannot be disassociated after it's creation.
  • Each NAT gateway is created in a specific Availability Zone and implemented with redundancy in that zone.
  • A NAT gateway supports the following protocols: TCP, UDP, and ICMP.
  • NAT gateway cannot be associated a security group. Security can be configured for the instances in the private subnets to control the traffic
  • Network ACL can be used to control the traffic to and from the subnet. NACL applies to the NAT gateway's traffic, which uses ports 1024-65535
  • NAT gateway when created receives an elastic network interface that's automatically assigned a private IP address from the IP address range of the subnet. Attributes of this network interface cannot be modified
  • NAT gateway cannot send traffic over VPC endpoints, VPN connections, AWS Direct Connect, or VPC peering connections. Private subnet's route table should be modified to route the traffic directly to these devices.

NAT Instance

  • NAT instance can be created by using Amazon Linux AMIs configured to route traffic to Internet.
  • They do not provide the same availability and bandwidth and need to configured as per the application needs.
  • NAT instances must have security groups associated with Inbound traffic enabled from private subnets and Outbound traffic enabled to the Internet
  • NAT instances should have the Source Destination Check attribute disabled, as it is neither the source nor the destination for the traffic and merely acts as a gateway

High Availability NAT Instance

NAT Instance High Availability

  • Create One NAT instance per Availability Zone
  • Configure all Private subnet route tables to the same zone NAT instance
  • Use Auto Scaling for NAT availability
  • Use Auto Scaling group per NAT instance with min and max size set of 1. So if NAT instances fail, Auto Scaling will automatically launch an replacement instance
  • NAT instance is highly available with limited downtime
  • Let Auto Scaling monitor health and availability of the NAT instance
  • Bootstrap scripts with the NAT instance to update the Route tables programmatically
  • Keep a close watch on the Network Metrics and scale vertically the NAT instance type to the one with high network performance

Disabling Source/Destination checks

  • Each EC2 instance performs source/destination checks, by default, and the instance must be the source or destination of any traffic it sends or receives
  • However, as the NAT instance acts as a router between the Internet and the instances in the private subnet it must be able to send and receive traffic when the source or destination is not itself.
  • Therefore, the source/destination checks on the NAT instance should be disabled

NAT Gateway & Instance Comparison

NAT Gateway vs NAT Instance

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