[AWS] VPC – Security

VPCs can be protected with 2 layers of firewalls; One for a subnet (NACL) and another for an instance (Security Group).

Network Access Control List (NACL)

• NACL controls the traffic across the boundary of subnets.
  • Any traffic inside subnet is not affected by NACL.
• NACL operates at Layer 4 (Transport – TCP/UDP) – Stateless (state is a session-layer concept).
  • For a given inbound rules, the corresponding outbound rules should be allowed.
• A subnet must be associated with a NACL, which only impacts traffic crossing the boundary of a subnet.

• NACL is a collection of rules that can explicitly allow or deny traffic based on its protocol, port range, and source/destination.
  • Rules are processed in numbered order, lowest first.
  • When a match is found, the processing stops. The “*” rule is processed last.
  • The default is an implicit deny. (catch-all deny)
• NACL has two sets of rules:
  • Inbound: From outside of a subnet to inside
  • Outbound: From inside of a subnet to outside

• A default VPC has a default NACL, which allows all inbound and outbound traffics.

• Custom network ACL denies all inbound and outbound traffics.
  • Custom NACL does not have Rule #100 that allows all traffic.

• Ephemeral Ports: When a client initiates communication with a server, it uses the well-known port numbers, but the response might use the ephemeral port on the client. NACL outbound rules should allow this.

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Security Groups

A Security Group is a software firewall that can be attached to an EC2 instance and AWS network interfaces.

• Each group has inbound rules and outbound rules.
• A security group has a hidden (implicit/default) Deny rule.
• Only allow rules can be specified, and it cannot explicitly deny traffic.
  • You cannot block specific IPs using security group.
• Security groups are stateful, which means that for any allowed traffic, the return traffic is automatically enabled. (OSI Layer 5 – Session)

• Security group can be shared across two VPCs in the same region.
• Changes to security groups take effect immediately.
• You can attach multiple security groups to an EC2 instance.
  • Each ENI is allowed up to 5 security groups.
• You can have any number of EC2 instances within a security group.

• A default VPC creates a default security group.
  • It disallows all inbound traffic except from the same security group.
    • Allows all traffic between instances in the same security group
  • It allows all outbound traffic.

• A new Security Group has different rules.
  • It denies all inbound traffic.
  • It allows all outbound traffic.