ESXi hosts intermittently show "Not Responding" in vCenter when TCP port 53 is blocked

Products

• VMware vCenter Server

Issue/Introduction

DNS resolution is successful from the vCenter Server command line for the ESXi host FQDNs, as well as for the vCenter Server FQDN and IP address.

Running the vCenter Diagnostic Tool (VDT) via the command line reports a [FAIL] status for DNS with TCP, and the following message is observed:

Environment

VMware vCenter Server 8.0.x

Cause

vCenter Server uses TCP 53 for DNS queries when UDP fails, or if the size is too large for a single UDP packet. 

Resolution

Coordinate with the network security team to ensure that TCP port 53 traffic is permitted from the vCenter Server to the DNS servers.

Additional Information

Using the VCF Diagnostic Tool for vSphere (VDT)

Required ports for configuring an external firewall to allow ESX/ESXi and vCenter Server traffic