Security Groups and Network Access Control Lists (NACLs) are
both used to manage access to resources in a Virtual Private Cloud (VPC), but
they serve different purposes and have distinct characteristics. Here’s a
breakdown of the key differences:
1. Purpose
- Security Group: Acts as a virtual firewall for Amazon EC2
instances to control inbound and outbound traffic. It is associated with
instances and governs their access.
- Network ACL (NACL): Provides an additional layer of
security at the subnet level, controlling traffic entering and leaving one or
more subnets in a VPC.
2. Statefulness
- Security Group: Stateful. If you allow incoming traffic
from a specific IP address, the response traffic is automatically allowed,
regardless of outbound rules.
- Network ACL: Stateless. If you allow incoming traffic, you
must also explicitly allow the outgoing response traffic in the rules.
3. Rules
- Security Group: Can have multiple rules for inbound and
outbound traffic. All rules are evaluated to determine whether to allow or deny
traffic. By default, security groups deny all inbound traffic and allow all
outbound traffic.
- Network ACL: Also supports multiple rules, but these rules
are evaluated in order, starting from the lowest numbered rule. The first rule
that matches the traffic is applied. By default, NACLs allow all inbound and
outbound traffic.
4. Association
- Security Group: Can be associated with EC2 instances and
other AWS resources (like RDS or Lambda functions).
- Network ACL: Associated with subnets. Each subnet in a VPC
can be associated with only one NACL, but a NACL can be associated with
multiple subnets.
5. Rule Limits
- Security Group: Typically has a limit on the number of
inbound and outbound rules, depending on the cloud provider.
- Network ACL: Has a different limit on the number of rules
compared to security groups. Each NACL can have a maximum of 20 inbound and 20
outbound rules (this limit can sometimes be increased).
6. Default Behavior
- Security Group: By default, all inbound traffic is denied,
while all outbound traffic is allowed.
- Network ACL: By default, both inbound and outbound traffic
is allowed.
Summary
In summary, Security Groups are more instance-specific and
stateful, allowing more granular control over traffic for individual instances.
Network ACLs operate at the subnet level and are stateless, providing an
additional layer of security for controlling traffic flow across multiple
resources within a subnet. Both are important tools in managing security within
a VPC.
