Security Group

A security group is a collection of access control rules for cloud resources, such as cloud servers, containers, and databases, that have the same security protection requirements and that are mutually trusted. After a security group is created, you can configure access rules that will apply to all cloud resources added to this security group.

Each security group can have both inbound and outbound rules. You need to specify the source, port, and protocol for each inbound rule and specify the destination, port, and protocol for each outbound rule to control the inbound and outbound traffic to and from the instances in the security group. As shown in Figure 1, you have a VPC (VPC-A) with a subnet (Subnet-A) in region A. An ECS (ECS-A) is running in Subnet-A and associated with security group Sg-A.

Security group Sg-A has a custom inbound rule to allow ICMP traffic to ECS-A from your PC over all ports. However, the security group does not have rules that allow SSH traffic to ECS-A so you cannot remotely log in to ECS-A from your PC.
If ECS-A needs to access the Internet through an EIP, the outbound rule of Sg-A must allow all traffic from ECS-A to the Internet.