Elastic Compute Service:Secondary private IP addresses

Introduction

If the primary private IP address on an Elastic Compute Service (ECS) instance's ENI is not enough, you can assign one or more secondary private IP addresses. This allows a single ECS instance to have multiple private IP addresses. The following figure shows the relationship between an ECS instance, its ENIs, and its private IP addresses.

Use cases

You can use secondary private IP addresses in the following scenarios:

• Multi-application hosting: When you deploy multiple applications on a single ECS instance, you can assign an independent secondary private IP address to each application. This lets you host multiple services on the same ECS instance and use it more efficiently.

• Failover: If an instance fails, you can unbind its secondary ENI and bind it to a standby instance. Request traffic directed to its secondary private IP addresses is then routed to the standby instance, achieving high availability.

• Load balancing: By assigning multiple secondary private IP addresses to an instance, you can implement a load balancing mechanism to improve system scalability and performance.

Usage notes

• The maximum number of private IP addresses you can assign to a single ENI depends on its status.

  • For an ENI in the Available state, you can assign up to 10 private IP addresses.

  • For an ENI in the InUse state, the maximum number of private IP addresses you can assign cannot exceed the limit for the instance type. For more information, see the Private IPv4 addresses per ENI column in the instance type table of each instance family in Instance family overview.

• A Virtual Private Cloud (VPC) security group can contain only a limited number of private IP addresses. Plan your secondary private IP addresses accordingly. For more information, see Security groups.

Step 1: Assign one or more secondary private IP addresses to an ENI

You can add or delete secondary private IP addresses on the Manage ENI IP Addresses page for the target ENI, which can be either the primary or a secondary ENI of an ECS instance.

1. Go to ECS console - Elastic Network Interfaces.

2. In the top navigation bar, select the region and resource group of the resource that you want to manage.

3. On the ENIs page, find the ENI to which you want to manage and click Manage ENI IP Addresses in the Operation column.

4. In the Manage ENI IP Addresses dialog box, click Increase in the Secondary Private IPv4 Address or IPv6 section.

   • Auto-assign: The system randomly assigns an IP address from the ENI's IPv4 or IPv6 private CIDR block.

   • Enter manually: Manually enter a secondary private IP address. Make sure the value is within the ENI's IPv4 or IPv6 private CIDR block.

5. Click Confirm to complete the secondary private IP address assignment.

6. Confirm that the secondary private IP address is correctly assigned to the target ENI.

   You can verify this information on the ENI details page of the ECS instance, as shown in the following example.

   

Step 3: Configure the OS to recognize the secondary private IP address

After assigning a secondary private IP address to an instance, you must also configure the address within the instance's operating system (OS) based on the operating system and IP address type. The specific configuration steps vary depending on the OS and IP address type.

sudo vi /etc/sysconfig/network/ifcfg-eth0

#Disable DHCP to allow you to manually configure IP addresses.
BOOTPROTO=static
STARTMODE=auto
USERCONTROL=no

#Suffix serial numbers to multiple IP addresses.
IPADDR_1=192.168.1.201
NETMASK_1=255.255.255.0
LABEL_1=

IPADDR_2=192.168.1.202
NETMASK_2=255.255.255.0
LABEL_2=0

IPADDR_3=192.168.1.203
NETMASK_3=255.255.255.0
LABEL_3=1

cat /etc/sysconfig/network/routes

default 192.168.1.253 - -

Unassign a secondary private IP address

If a secondary private IP address is no longer needed on an ENI, you can unassign it.

Automatic network configuration by cloud-init

cloud-init is an open-source initialization tool that provides initialization configuration capabilities for Linux operating systems, such as initializing passwords, setting hostnames, and executing custom scripts. Alibaba Cloud installs cloud-init on all public images by default. When an ECS instance starts, cloud-init automatically generates the system's network configuration information.

• Explanation of cloud-init in network configuration files.

  You can see an explanation of the automatic generation by cloud-init in the corresponding network configuration file. For example, in the network configuration file /etc/sysconfig/network-scripts/ifcfg-eth0 of Alibaba Cloud Linux 3.2:

  

• Disable the network configuration feature of cloud-init.

  • When to disable: You may need to disable this feature in the following situations. Otherwise, the configuration automatically generated by cloud-init upon system reboot may overwrite your custom network settings.

    • Static network configuration: Manually configuring the network by modifying the network configuration file automatically created by cloud-init.

    • Managing the network through other network management services (such as NetworkManager).

  • How to disable: Create the file /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg and add the configuration network: {config: disabled}.

    Important

    For more detailed information about cloud-init, you can log in to the Linux instance and view the information in /etc/cloud/cloud.cfg.d/README.

Configure IP addresses

IP addresses in an operating system can be configured using either DHCP (the default) or static assignment. When configuring a secondary private IP address, you need to choose the configuration method based on your actual situation.

• Static configuration: Requires manual configuration of necessary network parameters, such as subnet mask, default gateway, and DNS server addresses.

• DHCP (dynamic acquisition): DHCP is a network protocol that lets devices in a network automatically obtain IP addresses, subnet masks, default gateways, DNS server addresses, and other configuration information. You can also configure a secondary private IP address for a Linux instance while maintaining DHCP dynamic acquisition.