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Azure API Management: Error occurred while calling backend service

Azure API Management is a powerful tool for managing APIs. However, sometimes we will run into the “Error occurred while calling backend service” issue.

Understanding the key causes and how to fix them helps with the smooth operation of our API services. Here are the main reasons this error occurs and how to resolve them:

Cause 1: Incorrect Backend Hostname Configuration

If the backend hostname is incorrectly configured in APIM, it may not resolve the IP address and fail to connect to the backend service.

Solution: Verify that the backend hostname is correctly configured in APIM. Double-check the hostname settings to ensure they match the backend service’s actual hostname.

Cause 2: Connectivity Issues Between APIM and the Backend

Firewall or network security group rules can block the connection between APIM and the backend on port 443.

Solution: Check the firewall and NSG rules to make sure that they allow connectivity between APIM and the backend service on port 443. Also, make sure no rules are blocking the necessary traffic.

Cause 3: Use of a Self-Signed Certificate on the Backend

APIM expects a publicly trusted SSL/TLS certificate on the backend. If a self-signed certificate is used, APIM will fail to connect.

Solution: We can replace the self-signed certificate with a publicly trusted SSL/TLS certificate on the backend or configure APIM to skip certificate chain validation if it meets our security needs.

Cause 4: Abrupt Connection Termination Between APIM and the Backend

If the backend service unexpectedly closes the connection during data transfer, APIM will log this error.

Solution: Implement retry logic in the APIM policy to handle such connection failures gracefully. This can help mitigate the impact of temporary disruptions.

Cause 5: Exhaustion of SNAT Ports

APIM might exhaust its pre-allocated Source Network Address Translation (SNAT) ports when making outbound connections to the backend, resulting in connection failures.

Solution: Place APIM in a virtual network and route outbound calls through Azure Firewall, which has a much higher SNAT port allocation. This setup can help prevent SNAT port exhaustion.

Let us know in the comments if you need further help with this error.

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Conclusion

In brief, our Support Experts demonstrated how to fix “Azure API Management: Error occurred while calling backend service”.