Elastic Compute Service:Secondary private IP addresses

Introduction

If the primary ENI or a secondary ENI on your instance requires more than one IP address, in addition to the primary private IP address, you can assign multiple secondary private IP addresses to the ENI. In this case, a single ECS instance has multiple private IP addresses. The following figure shows the private IP address assignment to the primary and secondary ENIs.

Scenarios

Secondary private IP addresses are suitable for the following scenarios:

• Scenarios that involve multiple applications: If multiple applications are deployed on an ECS instance, you can assign secondary private IP addresses to the applications. This way, each application can use a separate IP address for communication and a single instance can provide multiple services to maximize instance utilization.

• Failover scenarios: If an instance fails, you can unbind secondary ENIs from the instance and bind the ENIs to another instance. Traffic destined for the secondary private IP addresses of the failed instance can be diverted to the normal instance to ensure service continuity.

• SLB scenario: After multiple secondary private IP addresses are assigned to each instance, the SLB mechanism can evenly distribute traffic to different private IP addresses across instances. This improves system scalability and performance.

Considerations

• The maximum number of private IP addresses that can be assigned to an ENI varies based on the status of the ENI.

  • 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 that you can assign varies based on the instance type of the instance to which the ENI is bound. For more information, see the Private IPv4 addresses per ENI column in the instance type table of each instance family in Overview of instance families.

• A limited number of private IP addresses can be contained in a security group that belongs to a virtual private cloud (VPC). For more information, see the Security group limits section in the "Limits" topic.

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

You can add or delete secondary private IP addresses in the Manage ENI IP Addresses dialog box of the primary ENI or a secondary ENI bound to an ECS instance in the ECS console. Perform the following steps.

1. Log on to the ECS console.

2. In the left-side navigation pane, choose Network & Security > Elastic Network Interfaces.

3. In the top navigation bar, select the region and resource group to which the resource belongs.

4. On the Elastic Network Interfaces page, find the ENI to which you want to assign secondary private IP addresses and click Manage ENI IP Addresses in the Operation column.

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

   • If you leave the fields empty, random IP addresses are automatically assigned from within the private IPv4 or IPv6 CIDR blocks of the vSwitch to which the ENI is connected.

   • If you enter IP addresses, make sure that the IP addresses are within the private IPv4 or IPv6 CIDR blocks of the vSwitch to which the ENI is connected.

6. Click Confirm to complete the assignment of the secondary private IP addresses.

7. Check whether the secondary private IP addresses are assigned to the ENI as expected.

   Go to the instance details page of the instance to which the ENI is bound, click the ENIs tab, and then view the address assignment information. The following figure shows an example.

   

Step 3: Configure the operating system of the instance to recognize the secondary private IP addresses

After you assign a secondary private IP address to an instance, you must configure the secondary private IP address for the instance based on the operating system type and IP address type. The operations that you must perform vary based on the operating system type 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 secondary private IP addresses

When an ENI no longer requires secondary private IP addresses, you can unassign the secondary private IP addresses from the ENI. You can perform the following steps to unassign secondary private IP addresses.

cloud-init automatically initializes network configurations

cloud-init is an open source initialization tool that automates initialization operations for Linux operating systems, such as generating an initial password, configuring the hostname, and running user data scripts. cloud-init is pre-installed on all Alibaba Cloud public images. When an ECS instance starts, cloud-init automatically generates network configurations for the instance.

• View the description of the network configuration file to check whether the file is automatically generated by cloud-init.

  You can view information about whether cloud-init automatically generates a network configuration file. In this example, the description of the /etc/sysconfig/network-scripts/ifcfg-eth0 configuration file for Alibaba Cloud Linux 3.2 is viewed.

  

• Disable the network configuration feature of cloud-init.

  • Scenarios in which the network configuration feature of cloud-init can be disabled: In the following scenarios, you may disable the network configuration feature of cloud-init. Otherwise, if the instance restarts, the network configurations automatically generated by cloud-init may overwrite your custom network configurations.

    • Manually configure the network by modifying the network configuration file automatically created by cloud-init.

    • Manage the network by using other network management services, such as NetworkManager.

  • Method used to disable the network configuration feature of cloud-init: Create the /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg file and add the network: {config: disabled} configuration to the file.

    Important

    For more information about cloud-init, log on to the Linux instance operating system and view information in the /etc/cloud/cloud.cfg.d/README file.

Configure IP addresses

The methods used to configure IP addresses in the operating system include DHCP (default) and static. When you configure secondary private IP addresses, select a configuration method based on your business scenario.

• Static configuration: You must manually configure necessary network parameters, such as subnet masks, default gateways, and DNS server addresses.

• DHCP dynamic acquisition: DHCP is a network protocol that allows devices in a network to automatically acquire network configurations, such as IP addresses, subnet masks, default gateways, and DNS server addresses. You can also configure secondary private IP addresses for Linux instances when DHCP dynamic acquisition is enabled.