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VPC Internet Access

Links: 101 AWS SAA Index


Internet Gateway

  • An internet gateway allows EC2 instances in a VPC to connect to the internet.
  • It scales horizontally and is highly available and redundant.
  • It must be created separately from a VPC.
  • One VPC can only be attached to one IGW and vice versa
  • IGW on their own do not allow internet access the route tables must also be edited.
  • When you create and IGW it will be in detached state attach it to the VPC.
  • For instances in the public subnet with access to internet NAT is performed by Internet Gateway.

Route Tables

  • For each VPC we create we get a default route table.
  • In the beginning we since we haven't created any route tables for subnets all the subnets are implicitly attached to the default/main route table.
  • So we create our own route tables and then explicitly associate them with their respective subnets. I have created two, one for private subnet and another one for public subnet.
  • Now to make public subnet available to the internet we go to the public route table and edit its route. We say for any route other than 10.0.0.0/16 send it to the internet gateway.

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  • A subnet is deemed to be a Public Subnet if it has a Route Table that directs traffic to the Internet Gateway. Otherwise there is no difference between a public and private subnet.

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  • In cases where your EC2 instance cannot be accessed from the Internet (or vice versa), you usually have to check two things:

    • Does it have an EIP or public IP address?
    • Is the route table properly configured?

Bastion Hosts

  • If we create an EC2 instance in the private subnet we won’t be able to SSH into it since no internet access it allowed.
  • If we give the private subnet access to the internet gateway then it would make the EC2 instances in the private subnet public thereby defeating the purpose of a private subnet.
  • To connect to the instances in the private subnet we use a bastion host in the public subnet.

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  • The security group of private EC2 instances should only allow SSH from the bastion host Security Group.

  • A bastion host is an EC2 instance in the public subnet which is then connected to all the private subnets.
  • Bastion host security group must be tightened. Only open ssh port that to from a specific IP.
  • We can SSH into the private EC2 instance from the bastion host since traffic can move between private and public subnets. In the route tables it is defined 10.0.0.0/16 to local.

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  • If you ssh into the private EC2 instance from the bastion host and then try ping google.com it won’t work since the private instances don’t have access to the internet. So both outbound and inbound is blocked.

  • For outbound internet access in private subnet we need a NAT gateway.
  • These are small instances since you don’t have to do a lot of processing.

NAT Instance

  • It is outdated and replaced by NAT Gateway but still in the exam.
  • NAT stands for Network Address Translation.
  • NAT instances must be launched in a public subnet.
  • They allow EC2 instances in the private subnet to connect to the internet.
  • They should have an elastic IP attached to them.
  • Route tables must be configured to route traffic from the private subnets to the NAT instance.
  • We have to provision and manage an EC2 instance for NAT Instance.
  • We must disable source and destination check in EC2.
  • We have preconfigured AMIs for NAT Instances.
  • NAT instances are not highly available and reliant.
  • A lot of configuration is needed to make them work.
Always prefer NAT Gateway over NAT instance. Extra things NAT Instances provide over NAT Gateway
  • NAT instance can be used as a bastion host.
  • Security Groups can be associated with a NAT Instance.
  • NAT instance supports port forwarding

All these things cannot be done with a NAT Gateway.

NAT Gateway

  • They are AWS managed NAT, higher availability, higher bandwidth and no administration.
  • A NAT Gateway is an AWS service that allows a private subnet to have access to the Internet, but prevents the Internet from initiating a connection directly to the instances.

    • Traffic destined for the Internet will flow from the private subnet to the NAT Gateway in the public subnet, and then out to the Internet through the Internet Gateway.
  • We pay per hour of usage and bandwidth so it is not a free service like VPC or IGW.

  • NAT Gateway is created in a specific AZ and uses an Elastic IP.

  • NAT gateway is resilient only within a single AZ.
    • You must create multiple NAT gateways in multiple AZs for fault tolerance.
    • Single NAT Gateway in an AZ is enough. No need to have multiple NAT Gateways in an AZ.
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How would you make sure that NAT Gateway is not a single point of failure in your architecture?

Create a NAT Gateway in each availability zone. Configure the route table in each private subnet to ensure that instances use the NAT Gateway in the same availability zone.

  • It can’t be used by the EC2 instance in the same subnet.

  • For internet access to in private subnet a NAT Gateway requires an IGW to be present in VPC also the public subnet must have a route to IGW.

    • The route is Private Subnet → NATG (Public Subnet) → IGW
  • There is no need to manage any security groups. For NAT Instances we need security Groups.

  • 5 Gbps bandwidth with automatic scaling upto 45 Gbps.
  • It must be in a public subnet.

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  • After creating the NAT gateway in the public subnet we need to modify the route tables of the private subnet to use the NAT gateway.

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VPC Reachability Analyser

  • It doesn’t send packets just analyses the configuration.
  • It is a network diagnostics tool that troubleshoots network connectivity between two endpoints in your VPC(s)
  • It builds a model of the network configuration, then checks the reachability based on these configurations (it doesn't send packets)
  • When the destination is reachable it produces hop-by-hop details of the virtual network path
  • When not reachable it identifies the blocking component(s) (e.g., configuration issues in SGs, NACLs, Route Tables, ...)
  • Use cases: troubleshoot connectivity issues, ensure network configuration is as intended etc.

Last updated: 2022-05-06