AWS - VPC & Networking Basic Information

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AWS Networking in a Nutshell

A VPC contains a network CIDR like (with its routing table and network ACL).

This VPC network is divided in subnetworks, so a subnetwork is directly related with the VPC, routing table and network ACL.

Then, Network Interfaces attached to services (like EC2 instances) are connected to the subnetworks with security group(s).

Therefore, a security group will limit the exposed ports of the network interfaces using it, independently of the subnetwork. And a network ACL will limit the exposed ports to to the whole network.

Moreover, in order to access Internet, there are some interesting configurations to check:

  • A subnetwork can auto-assign public IPv4 addresses

  • An instance created in the network that auto-assign IPv4 addresses can get one

  • An Internet gateway need to be attached to the VPC

    • You could also use Egress-only internet gateways

  • You could also have a NAT gateway in a private subnet so it's possible to connect to external services from that private subnet, but it's not possible to reach them from the outside.

    • The NAT gateway can be public (access to the internet) or private (access to other VPCs)


Amazon Virtual Private Cloud (Amazon VPC) enables you to launch AWS resources into a virtual network that you've defined. This virtual network will have several subnets, Internet Gateways to access Internet, ACLs, Security groups, IPs...


Subnets helps to enforce a greater level of security. Logical grouping of similar resources also helps you to maintain an ease of management across your infrastructure.

  • Valid CIDR are from a /16 netmask to a /28 netmask.

  • A subnet cannot be in different availability zones at the same time.

  • AWS reserves the first three host IP addresses of each subnet for internal AWS usage: he first host address used is for the VPC router. The second address is reserved for AWS DNS and the third address is reserved for future use.

  • It's called public subnets to those that have direct access to the Internet, whereas private subnets do not.

Route Tables

Route tables determine the traffic routing for a subnet within a VPC. They determine which network traffic is forwarded to the internet or to a VPN connection. You will usually find access to the:

  • Local VPC

  • NAT

  • Internet Gateways / Egress-only Internet gateways (needed to give a VPC access to the Internet).

    • In order to make a subnet public you need to create and attach an Internet gateway to your VPC.

  • VPC endpoints (to access S3 from private networks)

In the following images you can check the differences in a default public network and a private one:


Network Access Control Lists (ACLs): Network ACLs are firewall rules that control incoming and outgoing network traffic to a subnet. They can be used to allow or deny traffic to specific IP addresses or ranges.

  • It’s most frequent to allow/deny access using security groups, but this is only way to completely cut established reverse shells. A modified rule in a security groups doesn’t stop already established connections

  • However, this apply to the whole subnetwork be careful when forbidding stuff because needed functionality might be disturbed

Security Groups

Security groups are a virtual firewall that control inbound and outbound network traffic to instances in a VPC. Relation 1 SG to M instances (usually 1 to 1). Usually this is used to open dangerous ports in instances, such as port 22 for example:

Elastic IP Addresses

An Elastic IP address is a static IPv4 address designed for dynamic cloud computing. An Elastic IP address is allocated to your AWS account, and is yours until you release it. By using an Elastic IP address, you can mask the failure of an instance or software by rapidly remapping the address to another instance in your account.

Connection between subnets

By default, all subnets have the automatic assigned of public IP addresses turned off but it can be turned on.

A local route within a route table enables communication between VPC subnets.

If you are connection a subnet with a different subnet you cannot access the subnets connected with the other subnet, you need to create connection with them directly. This also applies to internet gateways. You cannot go through a subnet connection to access internet, you need to assign the internet gateway to your subnet.

VPC Peering

VPC peering allows you to connect two or more VPCs together, using IPV4 or IPV6, as if they were a part of the same network.

Once the peer connectivity is established, resources in one VPC can access resources in the other. The connectivity between the VPCs is implemented through the existing AWS network infrastructure, and so it is highly available with no bandwidth bottleneck. As peered connections operate as if they were part of the same network, there are restrictions when it comes to your CIDR block ranges that can be used. If you have overlapping or duplicate CIDR ranges for your VPC, then you'll not be able to peer the VPCs together. Each AWS VPC will only communicate with its peer. As an example, if you have a peering connection between VPC 1 and VPC 2, and another connection between VPC 2 and VPC 3 as shown, then VPC 1 and 2 could communicate with each other directly, as can VPC 2 and VPC 3, however, VPC 1 and VPC 3 could not. You can't route through one VPC to get to another.

VPC Flow Logs

Within your VPC, you could potentially have hundreds or even thousands of resources all communicating between different subnets both public and private and also between different VPCs through VPC peering connections. VPC Flow Logs allow you to capture IP traffic information that flows between your network interfaces of your resources within your VPC.

Unlike S3 access logs and CloudFront access logs, the log data generated by VPC Flow Logs is not stored in S3. Instead, the log data captured is sent to CloudWatch logs.


  • If you are running a VPC peered connection, then you'll only be able to see flow logs of peered VPCs that are within the same account.

  • If you are still running resources within the EC2-Classic environment, then unfortunately you are not able to retrieve information from their interfaces

  • Once a VPC Flow Log has been created, it cannot be changed. To alter the VPC Flow Log configuration, you need to delete it and then recreate a new one.

  • The following traffic is not monitored and captured by the logs. DHCP traffic within the VPC, traffic from instances destined for the Amazon DNS Server.

  • Any traffic destined to the IP address for the VPC default router and traffic to and from the following addresses, which is used for gathering instance metadata, and which is used for the Amazon Time Sync Service.

  • Traffic relating to an Amazon Windows activation license from a Windows instance

  • Traffic between a network load balancer interface and an endpoint network interface

For every network interface that publishes data to the CloudWatch log group, it will use a different log stream. And within each of these streams, there will be the flow log event data that shows the content of the log entries. Each of these logs captures data during a window of approximately 10 to 15 minutes.


Basic AWS VPN Components

  1. Customer Gateway:

    • A Customer Gateway is a resource that you create in AWS to represent your side of a VPN connection.

    • It is essentially a physical device or software application on your side of the Site-to-Site VPN connection.

    • You provide routing information and the public IP address of your network device (such as a router or a firewall) to AWS to create a Customer Gateway.

    • It serves as a reference point for setting up the VPN connection and doesn't incur additional charges.

  2. Virtual Private Gateway:

    • A Virtual Private Gateway (VPG) is the VPN concentrator on the Amazon side of the Site-to-Site VPN connection.

    • It is attached to your VPC and serves as the target for your VPN connection.

    • VPG is the AWS side endpoint for the VPN connection.

    • It handles the secure communication between your VPC and your on-premises network.

  3. Site-to-Site VPN Connection:

    • A Site-to-Site VPN connection connects your on-premises network to a VPC through a secure, IPsec VPN tunnel.

    • This type of connection requires a Customer Gateway and a Virtual Private Gateway.

    • It's used for secure, stable, and consistent communication between your data center or network and your AWS environment.

    • Typically used for regular, long-term connections and is billed based on the amount of data transferred over the connection.

  4. Client VPN Endpoint:

    • A Client VPN endpoint is a resource that you create in AWS to enable and manage client VPN sessions.

    • It is used for allowing individual devices (like laptops, smartphones, etc.) to securely connect to AWS resources or your on-premises network.

    • It differs from Site-to-Site VPN in that it is designed for individual clients rather than connecting entire networks.

    • With Client VPN, each client device uses a VPN client software to establish a secure connection.

Site-to-Site VPN

Connect your on premisses network with your VPC.

  • VPN connection: A secure connection between your on-premises equipment and your VPCs.

  • VPN tunnel: An encrypted link where data can pass from the customer network to or from AWS.

    Each VPN connection includes two VPN tunnels which you can simultaneously use for high availability.

  • Customer gateway: An AWS resource which provides information to AWS about your customer gateway device.

  • Customer gateway device: A physical device or software application on your side of the Site-to-Site VPN connection.

  • Virtual private gateway: The VPN concentrator on the Amazon side of the Site-to-Site VPN connection. You use a virtual private gateway or a transit gateway as the gateway for the Amazon side of the Site-to-Site VPN connection.

  • Transit gateway: A transit hub that can be used to interconnect your VPCs and on-premises networks. You use a transit gateway or virtual private gateway as the gateway for the Amazon side of the Site-to-Site VPN connection.


  • IPv6 traffic is not supported for VPN connections on a virtual private gateway.

  • An AWS VPN connection does not support Path MTU Discovery.

In addition, take the following into consideration when you use Site-to-Site VPN.

  • When connecting your VPCs to a common on-premises network, we recommend that you use non-overlapping CIDR blocks for your networks.

Client VPN

Connect from your machine to your VPC


  • Client VPN endpoint: The resource that you create and configure to enable and manage client VPN sessions. It is the resource where all client VPN sessions are terminated.

  • Target network: A target network is the network that you associate with a Client VPN endpoint. A subnet from a VPC is a target network. Associating a subnet with a Client VPN endpoint enables you to establish VPN sessions. You can associate multiple subnets with a Client VPN endpoint for high availability. All subnets must be from the same VPC. Each subnet must belong to a different Availability Zone.

  • Route: Each Client VPN endpoint has a route table that describes the available destination network routes. Each route in the route table specifies the path for traffic to specific resources or networks.

  • Authorization rules: An authorization rule restricts the users who can access a network. For a specified network, you configure the Active Directory or identity provider (IdP) group that is allowed access. Only users belonging to this group can access the specified network. By default, there are no authorization rules and you must configure authorization rules to enable users to access resources and networks.

  • Client: The end user connecting to the Client VPN endpoint to establish a VPN session. End users need to download an OpenVPN client and use the Client VPN configuration file that you created to establish a VPN session.

  • Client CIDR range: An IP address range from which to assign client IP addresses. Each connection to the Client VPN endpoint is assigned a unique IP address from the client CIDR range. You choose the client CIDR range, for example,

  • Client VPN ports: AWS Client VPN supports ports 443 and 1194 for both TCP and UDP. The default is port 443.

  • Client VPN network interfaces: When you associate a subnet with your Client VPN endpoint, we create Client VPN network interfaces in that subnet. Traffic that's sent to the VPC from the Client VPN endpoint is sent through a Client VPN network interface. Source network address translation (SNAT) is then applied, where the source IP address from the client CIDR range is translated to the Client VPN network interface IP address.

  • Connection logging: You can enable connection logging for your Client VPN endpoint to log connection events. You can use this information to run forensics, analyze how your Client VPN endpoint is being used, or debug connection issues.

  • Self-service portal: You can enable a self-service portal for your Client VPN endpoint. Clients can log into the web-based portal using their credentials and download the latest version of the Client VPN endpoint configuration file, or the latest version of the AWS provided client.


  • Client CIDR ranges cannot overlap with the local CIDR of the VPC in which the associated subnet is located, or any routes manually added to the Client VPN endpoint's route table.

  • Client CIDR ranges must have a block size of at least /22 and must not be greater than /12.

  • A portion of the addresses in the client CIDR range are used to support the availability model of the Client VPN endpoint, and cannot be assigned to clients. Therefore, we recommend that you assign a CIDR block that contains twice the number of IP addresses that are required to enable the maximum number of concurrent connections that you plan to support on the Client VPN endpoint.

  • The client CIDR range cannot be changed after you create the Client VPN endpoint.

  • The subnets associated with a Client VPN endpoint must be in the same VPC.

  • You cannot associate multiple subnets from the same Availability Zone with a Client VPN endpoint.

  • A Client VPN endpoint does not support subnet associations in a dedicated tenancy VPC.

  • Client VPN supports IPv4 traffic only.

  • Client VPN is not Federal Information Processing Standards (FIPS) compliant.

  • If multi-factor authentication (MFA) is disabled for your Active Directory, a user password cannot be in the following format.

  • The self-service portal is not available for clients that authenticate using mutual authentication.

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