Security groups function as virtual firewalls that provide Stateful Packet Inspection (SPI) and packet filtering functions to isolate security domains on the cloud. You can configure security group rules to control the inbound and outbound traffic of ECS instances in security groups.

Definition and characteristics

A security group is a logically isolated, mutually accessible group of instances within the same region that all share the same security requirements.

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 all directions within the same session. For example, if request traffic during a session is allowed to flow in, response traffic is also allowed to flow out.

Security group types

Security groups are classified into basic security groups and advanced security groups. The following table compares the features of the two types.

Feature Basic security group Advanced security group
Support all instance types Yes No. Only VPC-type instances are supported.
Network type Supports VPCs and the classic network. Supports VPCs only.
Access policy when no rules are added
  • Inbound: denies all access requests.
  • Outbound: allows all access requests.
  • Inbound: denies all access requests.
  • Outbound: denies all access requests.
Manually add rules Supports both allow and forbid policies. Supports only allow policies.
Set rule priorities The default value is 1 and can be modified. The value is fixed to 1 and cannot be modified.
Allow access to or from other security groups Supports access to or from other security groups. Does not support access to or from other security groups.
Bind ENIs to instances of any instance type No. The instance must be of the VPC type. No. The instance must be of the VPC type.
Maximum number of private IP addresses 2,000 65,536
Mutual access between instances within the same security group Allows mutual access between instances over the internal network by default. By default, instances are isolated from each other over the internal network. You must manually add security group rules to allow access between the instances.
Scenario Scenarios that require fine-grained network control, multiple ECS instance types, and moderate network connections. Scenarios that have high requirements on O&M efficiency, ECS instance types, and compute nodes.

Security group rules

Before a connection for data communication is established, the security group matches access requests against all the rules. A security group rule is defined by attributes such as rule direction, action, protocol type, port range, and authorization object. The following table describes these attributes.
Attribute Description
NIC type Depending on the network type of the instance:
  • VPC: Not required
  • Classic network: Internal network and Internet
Rule direction Inbound and outbound.
Action Both allow and forbid rules are supported.
Note For advanced security groups, you can set only allow rules.
Protocol type Application layer protocols such as SSH, ICMP, and RDP.
Port range Ports enabled for applications or protocols. For more information, see Common ports used by applications.
Priority

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 with the highest priority takes effect. When an ECS instance is added to multiple security groups, the rules of those groups are matched in descending order of priority. In one security group or between different security groups, if two security group rules have the same protocol type, port range, action, and authorization object, which rule takes effect depends on the priority and action settings of each rule.
  • If a forbid rule and an allow rule have the same priority, the forbid rule takes precedence.
  • If two rules have different priorities, the rule with a higher priority takes effect.
Note For advanced security groups, the priority is fixed to 1 and cannot be modified.
Authorization type Security group access and CIDR block access.
Authorization object CIDR blocks and security group IDs.
Note For advanced security groups, you cannot set security group IDs or authorize mutual access between two security groups.
Different attributes of security group rules are required for different communication scenarios. For example, when you log on to a Linux ECS 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 rule action 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 ECS instance.Match security group rules

For information about examples of rule configuration, see Scenarios for security groups.

Default security group

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 under the current account in this region. The default security group is a basic security group and is of the same network type as the ECS instance.Default security group
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 The priority of default security group rules created before May 27, 2020 is 110.
  • Outbound: All access requests are allowed.

Limits

ECS instances and ENIs have the following requirements for their security group types:
  • An ECS instance cannot belong to both a basic and an advanced security group at the same time.
  • An ENI cannot belong to both a basic and an advanced security group at the same time.

For information about the limits and quotas of security groups, see the "Security group limits" section in Security group limits.

Best practices

For information about the workflow of a security group, see Manage security groups. The following section provides some practical suggestions.

When you use security groups, we recommend that you adhere to the following best practices:
  • Use security groups as a whitelist when only a few requests are allowed to access ECS instances of security groups. Set deny 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. Any 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 about how to clone a security group, see Clone a security group.
  • Follow the least privilege principle when you configure inbound or outbound 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.