Security groups can manage the access to CVMs. 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.
The 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 forbids remote access from external networks. To enable public access to your CVM, you need to open the corresponding port to the Internet in the inbound rule.
• 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 the ICMP protocol, the outbound rule for the security group opens all ports to the Internet. If you select a custom security group rule, the outbound rule blocks all ports by default, and you need to configure the outbound rule to open the corresponding port to the Internet.

Common Use Cases

This document provides several common use cases of security groups. You can directly use its recommended security group configurations if a use case meets your requirements.

Scenario 1: remotely connecting to a Linux CVM via SSH

Case: you have created a Linux CVM and want to remotely connect to it via SSH.
Solution: when adding a security group rule, set Type to Login Linux CVMs(22), enter WebShell proxy IP address for Source, and open TCP port 22 to the Internet to enable Linux login via SSH.
You can open all IP addresses or a specified IP address (or IP range) to the Internet as required. This allows you to configure the source IP addresses of the CVMs that can be remotely connected to 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 it by using Remote Desktop (RDP).
Solution: when adding a security group rule, set Type to Login Windows CVMs(3389), enter the WebRDP proxy IP addresses for Source, 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 range) to the Internet as required. This enables you to configure the source IP addresses of the CVMs that can be remotely connected to via RDP.

Scenario 3: pinging a CVM on the Internet

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

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

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

• Allow all public IP addresses 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 public IP addresses to visit this website.

  Direction	Type	Source	Protocol Port	Policy
  Inbound	HTTP (80)	IP address or IP range that is allowed to access your website	TCP: 80	Allow
  Inbound	HTTPS (443)	IP address or IP 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 externally accessible.
Solution: when adding a security group rule, set Type to Custom and open TCP port 1101 to the Internet to allow external access to the specified service port.
You can open all IP addresses or a specified IP address (or IP range) to the Internet as required. This allows the source IP address to access the specified service port.

Scenario 7: rejecting an external IP address to access a specified port

Case: you have deployed a service and want to prevent external access to a specified service port (such as port 1102).
Solution: when adding a security group rule, set Type to Custom, configure the TCP port 1102, and set Policy to Reject, so that external services cannot access 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 as follows.

• Allow the CVM instance to access a specified external IP address.
• Forbid the CVM instance from accessing any public IP addresses via any protocol.

Note：

The first rule takes priority over the second.

Scenario 9: prohibiting 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 as follows.

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

Case: you want to allow uploads and downloads over FTP.
Solution: add a security group rule as follows.

Multi-scenario Configurations

You can configure multiple security group rules to meet your business requirements. For example, both inbound and outbound runes can be simultaneously configured. A CVM instance can be bound to one or multiple security groups. When it is bound to multiple security groups, the security group rules will be matched sequentially from top to bottom. You can adjust the priorities of security groups at any time. For more information about the priorities, see Rule Priorities.