What can I do if my ECC instance is declared unhealthy after I enable health checks for Server Load Balancer?

For layer-7 ELB service, when an ECC instance is declared as unhealthy, you can troubleshoot problems from the following aspects:

1. Make sure you can directly access your service through the ECC instance.
2. Make sure the backend port you configured in the listener is opened on the backend server.
3. Check whether the backend ECC instance has installed a firewall or other security protection software. This type of software may block the local IP address of the ELB service, and thus disable the communication between the ELB service and the backend server.
4. Check whether the ELB health check parameters are correctly set. We recommend that you use default health check settings.
5. We recommend that you use a static page for health checks. If the static page you use is not the default health check page of the backend ECC instance, you must set this page as the health check page in health check configurations. We recommend that you use a simple HTML page for health checks and use the page only for checking health check responses. We do not recommend that you use dynamic scripting languages such as php.
6. Check whether the backend ECC instance has high loads, which can slow the response speed of the ECC instance.

Besides, because the layer-7 ELB service communicates with the backend ECC instance through intranet, the ECC instance must listen the intranet or all-network ports. You can check the ECC instance with the following methods:

1. Check whether the listening function is normal.
Assume that the frontend port of ELB and backend port of the ECC instance are both 80. The ECC intranet IP address is 10.11.192.1. Run the following command on the server. If you can see the monitoring information of 10.1.1.192.1: 80, or the monitoring information of 0.0.0.0: 80, the listening function of the ports is normal.

• Windows server: netstat -ano | findstr :80
• Linux server: netstat -anp | grep :80

2. Check whether the intranet firewall of the server allows port 80. You can disable the firewall temporarily to do a test. Enter the following command to disable the firewall.

• Windows: firewall.cpl
• Linux: /etc/init.d/iptables stop

3. Check whether the backend port is normal.

1. For layer-4 ELB service, you can perform a telnet test. If you receive responses, the backend port is normal. Example: Use
      telnet 10.11.192.1 80 to test.
2. For layer-7 ELB service, you can determine whether the port is normal by checking the HTTP status code received. The HTTP status code must be a status code that indicates a normal condition, such as 200. The test methods are as follows:
       - Windows: Access the intranet IP address of the ECC instance. In this example, access http://10.11.192.1.
       - Linux: Run the curl -I command and check whether the status is HTTP/1.1 200 OK. In this example, run
       curl -I 10.11.192.1.