Configure ECS instances that are associated with EIPs to use the same NAT IP address to access the Internet - NAT Gateway

This topic describes how to configure ECS instances that are associated with elastic IP addresses (EIPs) to use the same NAT IP address to access the Internet. This facilitates network management.

Prerequisites

SNAT entries are configured for the virtual private cloud (VPC) in which the ECS instances are deployed. For more information, see Create and manage SNAT entries.

Background information

NAT gateways support the SNAT feature. SNAT enables ECS instances in a VPC to access the Internet when the ECS instances are not assigned public IP addresses. In a VPC, ECS instances that are associated with EIPs preferably use the EIPs to access the Internet. ECS instances that are not associated with EIPs access the Internet through the SNAT service provided by a NAT gateway. Consequently, the ECS instances in the VPC use different public IP addresses to access the Internet, which complicates management operations.

You can configure ECS instances in the VPC to use the same NAT IP address to access the Internet by associating elastic network interfaces (ENIs) with the ECS instances.

The following steps describe how to create an ENI for the ECS instance that is associated with an EIP, associate the ENI with the ECS instance, and then associate the EIP with the ENI. This way, the ECS instance uses the ENI to receive requests from the Internet and accesses the Internet through the NAT gateway.

Step 1: Create an ENI

Log on to the ECS console.
In the left-side navigation pane, choose Network & Security > ENIs.
In the upper-left corner, select the region where you want to create an ENI.
Note
The ENI and the ECS instance must belong to the same region.
On the Network Interfaces page, click Create ENI.

In the Create ENI dialog box, set the following parameters and click OK:

The following table lists only the key parameters. For more information, see Create a secondary ENI.

Parameter	Description
ENI Name	Enter a name for the ENI.
VPC	Select the VPC to which the ECS instance belongs.
vSwitch	Select a vSwitch in the zone of the ECS instance.
Security Group	Select a security group of the VPC.
Primary Private IP Address (optional)	Enter the primary private IPv4 address of the ENI. The IPv4 address must be an idle IP address within the CIDR block of the vSwitch. If you do not specify an IPv4 address, an idle private IPv4 address is automatically assigned to the ENI after the ENI is created. In this example, the primary private IP address is not specified.
Secondary Private IPv4 Address (optional)	Select an option. In this example, Not Assign is selected.

Step 2: Associate the ENI with the ECS instance

Log on to the ECS console.
In the left-side navigation pane, choose Network & Security > ENIs.
In the upper-left corner, select the region where the secondary ENI is deployed.
On the Network Interfaces page, find the ENI that you want to manage and click Bind to Instance in the Actions column.
In the dialog box that appears, select the ECS instance with which you want to associate the ENI and click OK.

Step 3: Disassociate the EIP from the ECS instance

Log on to the EIP console.
In the top navigation bar, select the region of the EIP.
On the Elastic IP Addresses page, find the EIP and click Disassociate from Resource in the Actions column.
In the message that appears, click OK.

Step 4: Associate the EIP with the ENI

Log on to the EIP console.
Select the region of the EIP.
On the Elastic IP Addresses page, find the EIP that you created and click Associate with Resource in the Actions column.

In the Associate EIP with Resource dialog box, set the following parameters and click OK:

Parameter	Description
Instance Type	Select ENI.
Resource Group (optional)	Select the resource group to which the EIP belongs. In this example, the default resource group is selected.
Mode (optional)	Select an association mode for the EIP. In this example, NAT Mode is selected.
Select an instance to associate	Select the secondary ENI with which you want to associate the EIP.

Step 5: Configure routes for the ENIs of the ECS instance

Log on to the ECS console.
In the left-side navigation pane, choose Network & Security > ENIs.
View the details of the ENIs.
Log on to the ECS instance. For more information, see Methods for connecting to an ECS instance.
Run the following command to view the information about the ENIs with which the ECS instance is associated:
```
ip a
```
The following result is returned:
eth0 is the primary ENI. The private IP address of eth0 is 192.168.3.10.
eth1 is a secondary ENI. The private IP address of eth1 is 192.168.3.11, and the public IP address is 118.190.XX.XX.
Set the metric value of the default route for each ENI in the route table based on your business requirements.
Run the following command to view the Gateway and metric values:
```
route -n
```
Note
In this example, one secondary ENI is used. The metric value of the secondary ENI is greater than the metric value of the primary ENI, which indicates that the routing priority of the secondary ENI is lower than the routing priority of the primary ENI. Therefore, you can use the default metric values. If you use multiple secondary ENIs, you must configure metric values based on your business requirements. For more information, see Configure a secondary ENI.
Create a route table and configure policy-based routing.
1. If you want to configure temporary policy-based routes for the ENI of the ECS instance, perform the following steps:
  Note
  After the ECS instance is restarted, the policy-based routes configured for the ENI become invalid.
  1. Run the following command to create a route table:
```
ip -4 route add default via 192.168.3.13 dev eth1 table 101
```
    Note
    We recommend that you keep the name of the route table the same as the metric value of the default route of the ENI. In this example, 101 is used.
  2. Run the following command to check whether the route table is created:
```
ip route list table 101
```
    The following result is returned:
  3. Run the following command to add a policy-based route:
```
ip -4 rule add from 192.168.3.11 lookup 101
```
  4. Run the following command to query the route map:
```
ip rule list
```
    The following result is returned:
2. If you want to add multiple routes for the ENI of the ECS instance, perform the following steps:
  Note
  After the ECS instance is restarted, the policy-based routes for the ENI remain valid.
  1. Run the following command to open the /etc/rc.local script.
```
vi /etc/rc.local
```
  2. At the end of the configuration file, press the i key to enter the edit mode.
  3. Add the following information to the end of the script:
```
ip -4 route add default via 192.168.3.13 dev eth1 table 101
ip -4 rule add from 192.168.3.11 lookup 101
```
    Note
    In this example, one secondary ENI is used. The metric value of the secondary ENI is greater than the metric value of the primary ENI, which indicates that the routing priority of the secondary ENI is lower than the routing priority of the primary ENI. Therefore, you can use the default metric values. If multiple secondary ENIs are used, you must add the command that sets the metric value to the script. For more information about the command that sets the metric value, see Configure a secondary ENI.
  4. Press the Esc key to exit the edit mode. Enter :wq and press the Enter key to save and close the script.
  5. Run the following command to make the /etc/rc.d/rc.local script executable:
```
chmod +x /etc/rc.d/rc.local
```
    Note
    The /etc/rc.local script is a symbolic link to the /etc/rc.d/rc.local script. Therefore, you must run the preceding command to make the /etc/rc.d/rc.local script executable. ou can run the ls -l /etc/rc.local command to check whether the /etc/rc.local script is a symbolic link to the /etc/rc.d/rc.local script.

Step 6: Test the network connectivity

Perform the following operations to check whether the ECS instance can use the EIP that is associated with the ENI to receive requests from the Internet. In this example, an on-premises Linux machine is used to remotely connect to the ECS instance.

Note

To remotely connect to the ECS instance, make sure that the security group rules of the ECS instance allow network traffic on SSH port 22. For more information, see Add security group rules.

Log on to an on-premises Linux machine.
Run the ssh <your username>@public IP command and enter the password of the ECS instance to check whether you can remotely connect to the ECS instance. If the following message appears, the ECS instance is connected.
```
Welcome to Alibaba Cloud Elastic Compute Service!
```

Perform the following operations to check whether the ECS instance can access the Internet through the SNAT. The following example shows how to view the IP address used by the ECS instance to access the Internet.

Log on to the ECS instance.
Run the curl https://myip.ipip.net command to check the EIP that the ECS instance uses to access the Internet. If the NAT IP address is the same as the one in the SNAT entry that is created for the ECS instance, it indicates that the ECS instance preferably uses the SNAT service to access the Internet.