ALB uses the private CIDR block 100.64.0.0/10 to communicate with backend servers. All requests sent from ALB instances to backend servers come from this CIDR block. These requests include client requests forwarded by ALB instances and health check requests initiated by ALB.
You need to configure security group rules for your backend servers to allow traffic from the 100.64.0.0/10 CIDR block.

Configuration instructions #

Configure an inbound rule in the target security group to allow traffic with the following characteristics: source addresses are from 100.64.0.0/10, the protocol being used is TCP, and the destination ports are the service ports of your backend servers.

Configuration example #

Configuration steps #

1. Go to the Security Group page.
2. In the top navigation bar, select your project and region.
3. In the security group list, find the target security group, and click Edit rule in the Actions column.
4. On the Inbound rules tab, click Add rule.
5. In the dialog box that appears, configure the relevant parameters. For detailed parameter explanations, see Adding a security group rule.