This topic describes why Cloud Monitor fails to monitor an ECS instance after you
disable access to the internal network for the ECS instance.
If you want to use Cloud Monitor to monitor an ECS instance, make sure that the ECS
instance can access the internal network of Alibaba Cloud.
An ECS instance communicates with Cloud Monitor by using the domain name open.cms.aliyun.com.
This domain name can be resolved only on the internal network. Cloud Monitor collects
monitoring data from an ECS instance by using the internal network. To allow Cloud
Monitor to collect monitoring data from an ECS instance, make sure that the ECS instance
can communicate with open.cms.aliyun.com by using port 80.