Tech
Troubleshooting Google Cloud Load Balancing backends
You received a ProbeResultText indicating the connection was refused, which indicates that the health check probe SYN packet reached the backend VM, but instead of sending a SYN-ACK, the connection was reset/rejected by the backend.
Possible root causes:
The port specified in the health check configuration (port 80 in this example) is not open on the destination machine.
The port on the destination machine is open, but the backlog of pending connections is full.
A local firewall on the destination machine is blocking access.
Other health check details:
gRPC success criteria
Legacy Health Check Success Criteria
3. Configuring the Backend VM
If your health check logs show that your backends aren't responding to HC probes, see this section to verify whether your backend VMs are correctly configured to handle health check probes.
For a proxy load balancer:
The following checks apply to Application Load Balancers:
Make sure the port specified in the health check configuration (port 80 in this example) is open on the backend machine.
On a Linux machine, use the command netstat -tulpn | grep.
For HTTP health checks, it verifies that the application running on the backend machine is responding on the configured HC path and port.
For a pass-through load balancer:
The following checks apply to passthrough network load balancers: Packets sent to a passthrough network load balancer arrive at the backend VMs with a destination IP of the load balancer itself. This type of load balancer is not a proxy, and this is the expected behavior.
Checking service IP and port bindings:
Software running on backend VMs must:
To test this empirically, connect to the backend VM (via SSH or RDP) and run the following tests (using curl, telnet, etc.):
It tries to reach the service by connecting to it using the backend VM's own internal IP address, 127.0.0.1, or localhost.
It attempts to reach the service by connecting using the IP address of the load balancer's forwarding rule.
If users can't access your service using the backend VM's own internal IP address 127.0.0.1, or localhost, then there is a problem with the software for the service running on the backend VM. If users can access your service using the backend VM's own internal IP address 127.0.0.1, or localhost, but can't access it using the IP address in the load balancer's forwarding rule, then the software is not listening (binding) properly to the load balancer's IP address.
You can verify that the software running on the backend VM is properly bound by examining the output of the following command on a Linux system: netstat -tulpn
If the software is bound to the wrong port, or only to the IP address of the backend VM, the customer will need to reconfigure the software.
Check the local routes:
Each backend VM must be configured to accept packets sent to the load balancer, meaning that the destination for delivered packets is the IP address of the load balancer, most often this is implemented with a local route.
For VMs created from Google Cloud images, the guest agent (formerly known as Windows Guest Environment or Linux Guest Environment) installs a local route for the load balancer's IP address. COS-based GKE nodes implement this with iptables instead. If you're using a custom VM image, the local route is not implemented by default.
On a Linux backend VM, you can check the presence of a local route by running the following command, replacing LOAD_BALANCER_IP with the IP of your load balancer:
sudo ip route list table local | grep [LOAD_BALANCER_IP]
If no local route is found, you can manually add a local route to the VM.
sudo ip route add local [LOAD_BALANCER_IP] development [INTERFACE] Proto 66
Replace LOAD_BALANCER_IP with the IP of your load balancer, [INTERFACE] The interface where the route is missing.
If you're running a Google-provided image, check why the guest agent failed to install local routes: Check to see if the guest agent is running. If it's not running, install the guest agent.
There is a known issue (on systemd based systems including Ubuntu) where registered local routes are deleted when the NIC is reset by restarting systemd-networkd. This can occur if your instance is configured with automatic updates enabled (apt upgrade) and is still using an older version of the guest agent, where the update requires a systemd-networkd restart. To prevent this issue, you should update your guest agent to version 20210408.00 or later. More information: Github Issue and Github PR.
Perform traffic capture and analysis.
You can use a packet capture to pinpoint the exact communication between the health check probe and the backend VM, which can be done using tcpdump as follows:
Install tcpdump on the backend VM.
Start the tcpdump capture.
To identify the problem, analyze the tcpdump output.
4. Read the manual
The troubleshooting steps in the sections above can help you identify why a health check is failing, but for a comprehensive understanding of health checks, including their purpose, mechanisms, operational dynamics, limitations, and supported configurations, be sure to consult the Google Cloud documentation.
Engage with Cloud Support: After you identify and analyze your issue, you may need to contact Cloud Support for further assistance. To ensure a smooth experience, be sure to describe your needs, clearly explain the business impact, and provide sufficient context for all the information you've collected.
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Sources 2/ https://cloud.google.com/blog/products/networking/troubleshooting-google-cloud-load-balancing-backends/ The mention sources can contact us to remove/changing this article |
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