Overview

Security groups have the following characteristics:

• Each ECS instance must belong to at least one security group and can belong to multiple security groups at the same time.
• A security group can manage multiple ECS instances within the same region.
• ECS instances in different security groups cannot communicate with each other over the internal network. However, you can configure security group rules to allow access between the security groups.
• Security groups are stateful. The maximum session timeout period for a security group is 910 seconds. By default, a security group allows traffic in both directions within the same session. For example, if the request traffic during a session is allowed to flow in, the response traffic is also allowed to flow out.

Before a connection for data communication is established, the security group matches access requests against all rules to determine whether to allow the requests. A security group rule is defined by attributes such as rule direction, authorization policy, protocol type, port range, and authorization object. The following table describes these attributes.

Attribute	Description
Rule direction	The rule direction depends on the network type of instances. VPC: inbound and outbound Classic network: Internet ingress, Internet egress, inbound, and outbound
Authorization policy	Both Allow and Forbid policies are supported.
Protocol type	Application layer protocols such as TCP, UDP, ICMP (IPv4), and GRE.
Port range	Ports enabled for applications or protocols. For more information, see Common ports used by applications.
Priority	The priority of a security group rule. The value can range from 1 to 100. A smaller value indicates a higher priority. For security group rules of the same type, the rule that has the highest priority is applied. If an ECS instance belongs to multiple security groups, the security group rules of these security groups are applied to the instance in descending order of priority. Security group rules are applied based on the following principles: If two security group rules are different only in the authorization policy, the Forbid policy is applied and the Allow policy is not. If two security group rules are different only in priorities, the rule with a higher priority is applied.
Authorization object	You can specify CIDR blocks or security group IDs as authorization objects. Note For rules of advanced security groups, you cannot specify security group IDs or authorize mutual access between two security groups.

You must configure different attributes for security group rules based on communication scenarios. For example, when you log on to a Linux instance by using an Xshell client, a security group detects an SSH request from the Internet or internal network. The security group then matches the request against each inbound rule to check whether the rule contains the IP address of the request sender, whether the rule has the highest priority among the rules of the same type, whether the authorization policy is Allow, and whether SSH port 22 is enabled. The connection for data communication is not established until an inbound rule that allows the request is matched. The following figure shows how the security group matches its rules to control the access request for a Linux instance by using an Xshell client.

When you create an ECS instance in a region by using the ECS console, a Default Security Group is created if no security group has been created within the current account in this region. The default security group is a basic security group and has the same network type as the ECS instance.

By default, the default security group has the following security group rules:

• Inbound:
  • ICMP traffic and traffic over SSH port 22 and RDP port 3389 is allowed. The authorization object is 0.0.0.0/0.
  • You can also allow traffic over HTTP port 80 and HTTPS port 443.
  • The rule priority is 100.
    Note By default, the priority of security group rules created before May 27, 2020 is 110.
• Outbound: All access requests are allowed.

ECS instances and ENIs have the following requirements for their security group types:

• The primary ENI of an ECS instance cannot belong to both a basic and an advanced security group at the same time.
• A secondary ENI cannot belong to both a basic and an advanced security group at the same time.

When you use security groups, we recommend that you adhere to the following best practices:

• Use security groups as a whitelist only when few requests are allowed to access ECS instances in the security groups. Configure Forbid rules for all security groups and then add Allow rules one by one to allow access requests.
• Do not use a security group to manage all applications because isolation requirements are different at different layers.
• Add instances that have the same security requirements to the same security group. Do not create a separate security group for each instance.

When you add security group rules, we recommend that you adhere to the following best practices:

• Configure simple security group rules. If you add an ECS instance to multiple security groups, hundreds of rules may apply to the instance. Changes to these rules may cause connection errors.
• If you want to modify the rules of a security group in the production environment, we recommend that you first test the modification on a cloned security group to avoid potential impacts on online applications. For more information, see Clone a security group.
• Follow the principle of least privilege when you configure security group rules for applications. For example, we recommend that you adhere to the following best practices:
  • Select a specific port over which to allow traffic, such as 80/80. Do not set a range of ports, such as 1/80.
  • When you add security group rules, do not grant access permissions to the 0.0.0.0/0 CIDR block unless necessary.