Security Groups

• A security group acts as a firewall that controls the traffic allowed to and from the resources in your virtual private cloud (VPC).

• One can choose the ports and protocols to allow for inbound traffic and outbound traffic.

• They contain only allow rules.

Responsibilities

• They regulate access to ports.

• They regulate authorized IP ranges for IPv4 and IPv6.

• Control of inbound network traffic.

• Control of outbound network traffic.

Basics

• A good basic introduction can be obtained here.

• One security groups can be attached to multiple instances.

• They apply to a region/VPC combination. One SG group is applicable only to one region only.

• By default, all inbound traffics are blocked to AWS resource and all outbound traffics are allowed from AWS resource.

• One or more security group can be attached to other security group/s.

• One AWS resource can have multiple security groups attached to it.

• Some commonly used service name and port can be found as follows,

  Service name	Port
  SSH, SFTP	22
  FTP	21
  HTTP	80
  HTTPS	443
  RDP	3389

• If connection refused error occurs, then it is an application issue as it may have errored out and did not launch.

• ssh command to log into an Linux ec2-instance looks as follows for Linux or mac,

    ssh -i <path-to-pem-file> ec2-user@<public-ip-address> 

By default amazon linux 2 AMI has ec2-user has default user setup. Ensure the pem/ppk file has correct user permission.

References

• What is SG