An external appliance fails to connect to the vCenter Server SDK endpoint

Products

• VMware vCenter Server

Issue/Introduction

A third-party appliance fails its initial connection/discovery workflow and generating the following error message:

"The appliance is unable to reach FQDN/IP address specified for vCenter http://[vCenter_FQDN_or_IP]:443/sdk due to the error: Unable to connect to remote server."

Environment

vCenter 8.0U3

Cause

The appliance's connection workflow fails because it requires direct SSH access to the ESXi hosts within the cluster, which was disabled.

While the primary error pointed to a vCenter SDK connection failure (port 443), many third-party integrations (such as backup, security, or monitoring appliances) utilize a multi-tiered connection process. They query the vCenter SDK for inventory information but concurrently require SSH (port 22) access to the underlying ESXi hosts to deploy agents, mount storage, or complete the initial handshake. Because the ESXi SSH service is disabled by default for security, the appliance's overall connection routine timed out and failed.

Resolution

Enabling the SSH service on the ESXi hosts provided the necessary communication path, allowing the external appliance to complete its host-level requirements and successfully establish the connection.

Note / Best Practice Advisory: While enabling SSH resolved the immediate connection issue, leaving SSH enabled on ESXi hosts indefinitely triggers a persistent security warning in vCenter and violates standard VMware hardening guidelines. If the appliance only required SSH for initial deployment or agent injection, it is highly recommended to disable the SSH service again to maintain a secure posture.