ESXi Hosts Intermittently Showing "Not Responding" in vCenter Due to DNS Connectivity Issues

Products

• VMware vCenter Server

Issue/Introduction

ESXi hosts may intermittently appear in a "Not Responding" state within vCenter Server. During the occurrence of this issue, vCenter may generate repeated "Cannot synchronize host agent" alerts for the affected hosts.

From /var/log/vmware/dnsmasq.log on the vCenter Server, it can be observed that name resolution fails during the period of disconnection.

Month DD HH:MM:SS dnsmasq [2481] : query [AAAA] example.esxihost.com from 127.0.0.1
Month DD HH:MM:SS dnsmasq[2481]: cached example.esxihost.com is NXDOMAIN
Month DD HH:MM:SS dnsmasq[2481] : query[A] example.esxihost.com from 127.0.0.1
Month DD HH:MM:SS dnsmasq[2481]: cached example.esxihost.com is NXDOMAIN
Month DD HH:MM:SS dnsmasq [2481] : query [AAAA]
Month DD HH:MM:SS dnsmasq[2481] : cached example.esxihost.com is NXDOMAIN
Month DD HH:MM:SS dnsmasq[2481] : query[A] example.esxihost.com from 127.0.0.1

Environment

vCenter Server 7.x
vCenter Server 8.x

Cause

This behavior is commonly associated with hostname resolution failures between vCenter Server and the ESXi hosts.

A common scenario that can cause this issue includes the following:

• One of the configured DNS servers being unreachable from the vCenter Server in an environment with multiple DNS servers.
• Network-level restrictions or policies blocking DNS communication.

Resolution

Verify connectivity from the vCenter Server to all configured DNS servers with the assistance of the internal networking team.

Additional Information

Refer to the following KB article to troubleshoot DNS failures and NXDOMAIN errors: Troubleshooting DNS Resolution Failures: Understanding and Resolving "NXDOMAIN" Errors