This topic describes possible reasons and resolutions for failing to access a Server Load Balancer (SLB) instance from a client.
|1||SLB cannot be accessed by backend servers. For a layer-4 SLB service, a backend ECS instance cannot directly provide services for clients and function as the backend server of the SLB service at the same time.||None|
|2||Health check exceptions.||For more information, see How do I troubleshoot health check exceptions of a layer-4 (TCP/UDP) listener? and How do I troubleshoot a health check exception of a layer-7 (HTTP/HTTPS) listener?.|
|3||Using FTP, TFTP, H.323, and SIP protocols through SLB is not supported.||
|4||The internal firewall of the server does not allow port 80.||You can run the following command to temporarily disable the firewall to do a test.
|5||Backend port exceptions.||
|6||The rp_filter feature conflicts with the policy-based route of the LVS of SLB.||
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.
|8||No listener is added to the SLB instance.||Configure a listener. For more information, see Listener overview.|
|9||SLB cannot be accessed through the domain name. This may be caused by an error in domain name resolution.||None|
|10||Exceptions of the local network of the client or exceptions of the intermediate link of the service provider.||Perform access tests on the service port of SLB in different regions and network environments.
If the exception only occurs when the SLB instance is accessed from the local network, it can be determined that the problem is caused by a network exception. Then you can do further troubleshooting and analysis through Ping tests or MTR route tracing.
|11||The client IP address is blocked by Alibaba Cloud Security.||
|12||After you switch to Anti-DDoS Basic from Anti-DDoS Pro, the whitelist is not disabled.||Disable the whitelist.|
|If the problem persists, open a ticket and submit the following details: