Connect to a service on an VM in backend-pool behind a load balancer through Private Link Service

The "connection refused" error you're encountering could be due to several potential issues in your configuration. Here are a few things to check:

Kafka Broker Binding Configuration:

• Ensure that Kafka is properly configured to listen on both its public and private IPs. Kafka typically binds to either specific IP addresses or all interfaces (0.0.0.0). If it is only listening on the public IP, requests from your private network (via the Private Endpoint) would be refused. Verify the listeners and advertised.listeners settings in your server.properties file.

Network Security Group (NSG) Rules:

  - Check the Network Security Groups (NSGs) associated with both the VM and the Private Endpoint. Ensure that there are inbound and outbound rules allowing traffic on the Kafka broker's ports (usually 9092). For Private Endpoint traffic, make sure the required ports are open.
  
  **Load Balancer Health Probe**:
  
     - Verify that the Load Balancer health probes are correctly configured for the Kafka broker’s listening ports. If the health probe is failing, the Load Balancer may not route traffic properly to your VM.
     
     **Private Link Configuration**:
     
        - Make sure that the Private Link Service and Private Endpoint are configured correctly. Double-check the IP range assigned to the Private Endpoint and ensure that it matches the private IP of the Load Balancer. Also, ensure that the Private Link Service is properly associated with the Load Balancer and that the required ports are exposed.
        
        **Kafka’s `advertised.listeners` Setting**:
        
           - For Kafka, the `advertised.listeners` must be set to the correct IP (the private IP through the Load Balancer, in your case). This helps clients (from the BTP subaccount in this case) resolve the broker's IP correctly when attempting to connect. Make sure this setting is properly configured.
           
           **DNS Resolution**:
           
              - Verify that the Private Endpoint is resolving the correct IP address and DNS name. The DNS name should resolve to the Load Balancer's private IP, not the public IP.
              
              **Private Link Service Binding**:
              
                 - Ensure that the Private Link Service is properly bound to your application in the SAP BTP subaccount. Sometimes, after creating or approving the Private Endpoint, a restart or re-binding may be needed to refresh the connection.
                 
                 **Firewall or ACLs**:
                 
                    - If there are any firewalls or access control lists (ACLs) in your environment, ensure they are not blocking traffic between the Private Endpoint and the Load Balancer or Kafka broker.
                    

Next Steps:

• Start by confirming Kafka’s listeners and advertised.listeners settings, making sure both the VM’s private IP and the Load Balancer IP are included.
• Ensure the Load Balancer’s health probes and traffic routing are correctly set up.
• Review the NSG and security rules to allow traffic through the necessary ports.
• Confirm DNS resolution for the Private Endpoint to ensure it resolves correctly to the Load Balancer’s IP.

If these configurations are correct, your Kafka client should be able to connect via the Private Endpoint to the broker through the Load Balancer. The "connection refused" error you're encountering could be due to several potential issues in your configuration. Here are a few things to check:

Kafka Broker Binding Configuration:

• Ensure that Kafka is properly configured to listen on both its public and private IPs. Kafka typically binds to either specific IP addresses or all interfaces (0.0.0.0). If it is only listening on the public IP, requests from your private network (via the Private Endpoint) would be refused. Verify the listeners and advertised.listeners settings in your server.properties file.

Network Security Group (NSG) Rules:

  - Check the Network Security Groups (NSGs) associated with both the VM and the Private Endpoint. Ensure that there are inbound and outbound rules allowing traffic on the Kafka broker's ports (usually 9092). For Private Endpoint traffic, make sure the required ports are open.
  
  **Load Balancer Health Probe**:
  
     - Verify that the Load Balancer health probes are correctly configured for the Kafka broker’s listening ports. If the health probe is failing, the Load Balancer may not route traffic properly to your VM.
     
     **Private Link Configuration**:
     
        - Make sure that the Private Link Service and Private Endpoint are configured correctly. Double-check the IP range assigned to the Private Endpoint and ensure that it matches the private IP of the Load Balancer. Also, ensure that the Private Link Service is properly associated with the Load Balancer and that the required ports are exposed.
        
        **Kafka’s `advertised.listeners` Setting**:
        
           - For Kafka, the `advertised.listeners` must be set to the correct IP (the private IP through the Load Balancer, in your case). This helps clients (from the BTP subaccount in this case) resolve the broker's IP correctly when attempting to connect. Make sure this setting is properly configured.
           
           **DNS Resolution**:
           
              - Verify that the Private Endpoint is resolving the correct IP address and DNS name. The DNS name should resolve to the Load Balancer's private IP, not the public IP.
              
              **Private Link Service Binding**:
              
                 - Ensure that the Private Link Service is properly bound to your application in the SAP BTP subaccount. Sometimes, after creating or approving the Private Endpoint, a restart or re-binding may be needed to refresh the connection.
                 
                 **Firewall or ACLs**:
                 
                    - If there are any firewalls or access control lists (ACLs) in your environment, ensure they are not blocking traffic between the Private Endpoint and the Load Balancer or Kafka broker.
                    

Next Steps:

• Start by confirming Kafka’s listeners and advertised.listeners settings, making sure both the VM’s private IP and the Load Balancer IP are included.
• Ensure the Load Balancer’s health probes and traffic routing are correctly set up.
• Review the NSG and security rules to allow traffic through the necessary ports.
• Confirm DNS resolution for the Private Endpoint to ensure it resolves correctly to the Load Balancer’s IP.

If these configurations are correct, your Kafka client should be able to connect via the Private Endpoint to the broker through the Load Balancer.