Security groups are used to manage whether a Cloud Virtual Machine (CVM) is accessible. You can configure inbound and outbound rules for security groups to specify whether your server can be accessed by or can access other network resources.
Default inbound and outbound rules for security groups are as follows:

• To ensure data security, the inbound rule for a security group is a rejection policy that denies remote access from external networks. To make your CVM accessible to external resources, you need to allow the inbound rule for the corresponding port.
• The outbound rule for a security group specifies whether your CVM can access external network resources. If you select Open All Ports or Open Ports 22, 80, 443, and 3389 and ICMP, the outbound rule for the security group opens the ports to the Internet. If you select a custom security group rule, the outbound rule blocks all ports by default, and you need to set the outbound rule to allow the corresponding port to access external network resources.

Common Use Cases

This document describes several common use cases for security groups. If any of the following cases meet your requirements, you can set your security groups according to the configuration recommended for the corresponding use case.

Scenario 1: remotely connecting to a Linux CVM through SSH

Case: you have created a Linux CVM and want to remotely connect to the CVM through SSH.
Solution: when adding an inbound rule, set Type to Linux Login and open TCP port 22 to the Internet to allow Linux login through SSH.
You can open all IP addresses or a specified IP address (or IP address range) to the Internet as required. This allows you to configure the source IP addresses that can remotely access the CVMs through SSH.

Scenario 2: remotely connecting to a Windows CVM through RDP

Case: you have created a Windows CVM and want to remotely connect to the CVM through Remote Desktop Connection (RDP).
Solution: when adding an inbound rule, set Type to Windows Login and open TCP port 3389 to the Internet to enable remote login to Windows.
You can open all IP addresses or a specified IP address (or IP address range) to the Internet as required. This enables you to configure the source IP addresses that can remotely access the CVMs through RDP.

Scenario 3: pinging a CVM from the Internet

Case: you have created a CVM and want to check whether the communication between the CVM and other CVMs is normal.
Solution: test the connection by using the ping program. Specifically, when adding an inbound rule, set Type to Ping and open Internet Control Message Protocol (ICMP) ports to the Internet to enable other CVMs to gain access to this CVM through ICMP.
You can open all IP addresses or a specified IP address (or IP address range) to the Internet as required. This allows you to configure the source IP addresses that can access this CVM through ICMP.

Scenario 4: remotely logging in to a CVM through Telnet

Case: you want to remotely log in to a CVM through Telnet.
Solution: when adding an inbound rule, configure the following security group rule:

Scenario 5: authorizing access to a web service through HTTP or HTTPS

Case: you have built a website and want to allow users to access your website through HTTP or HTTPS.
Solution: when adding an inbound rule, configure the following security group rules as required:

• Allow all IP addresses on the Internet to access this website

  Direction	Type	Source	Protocol Port	Policy
  Inbound	HTTP (80)	0.0.0.0/0	TCP: 80	Allow
  Inbound	HTTPS (443)	0.0.0.0/0	TCP: 443	Allow

• Allow some IP addresses on the Internet to access this website

  Direction	Type	Source	Protocol Port	Policy
  Inbound	HTTP (80)	The IP address or IP address range that is allowed to access your website	TCP: 80	Allow
  Inbound	HTTPS (443)	The IP address or IP address range that is allowed to access your website	TCP: 443	Allow

Scenario 6: allowing an external IP address to access a specified port

Case: you have deployed a service and want the specified service port (such as port 1101) to be accessible externally.
Solution: when adding an inbound rule, set Type to Custom and open TCP port 1101 to the Internet to allow external resources to access the specified service port.
You can open all IP addresses or a specified IP address (or IP address range) to the Internet as required. This allows the source IP address to access the specified service port.

Scenario 7: denying access to a specified port from external IP addresses

Case: you have deployed a service and want to block external access to a specified service port (such as port 1102).
Solution: when adding an inbound rule, set Type to Custom, configure TCP port 1102, and set Policy to Reject to deny external access to the specified service port.

Scenario 8: allowing a CVM to access only a specified external IP address

Case: you want your CVM to access only a specified external IP address.
Solution: add two outbound security group rules by referring to the following configurations:

• Allow the CVM instance to access a specified public IP address
• Disallow the CVM instance to access any public IP addresses through any protocol

The rule that permits access should have a higher priority than the rule that denies access.

Scenario 9: denying a CVM from accessing a specified external IP address

Case: you do not want your CVM to access a specified external IP address.
Solution: add a security group rule by referring to the following configuration:

Scenario 10: uploading a file to or downloading a file from a CVM through FTP

Case: you want to upload a file to or download a file from a CVM by using an FTP program.
Solution: add a security group rule by referring to the following configuration:

Combination of Multiple Scenarios

In an actual scenario, you may want to configure multiple security group rules based on service requirements, for example, configuring inbound or outbound rules at the same time. One CVM may be bound to one or more security groups. When a CVM is bound to multiple security groups, these security groups are matched and executed in descending order of priorities. You can adjust the priorities of these security groups whenever needed.