vCenter Server Cannot Connect to ESXi Hosts Due to Missing DNS Configuration After Cluster Restart

Products

• VMware vSphere ESXi

Issue/Introduction

• Symptoms:
  • Following a planned HCI cluster restart, the entire datacenter management plane is inaccessible.
  • vCenter Server reports ESXi hosts as Deregistered or Not Responding.
  • The vCenter Server VM may be inaccessible via FQDN but reachable via IP address.
  • Manual attempts to reconnect hosts in the vSphere Client fail with name resolution errors.

Environment

• VMware vCenter Server 
• VMware ESXi 

Cause

The vCenter Server Appliance (VCSA) is unable to resolve the Management FQDNs of the ESXi hosts. Investigation shows that the /etc/resolv.conf file on the vCenter Server is missing the required DNS server entries. Without valid DNS servers, vCenter cannot resolve the host IP addresses during the service startup following a cluster reboot, leading to a management disconnect.

Resolution

1. Log in to the vCenter Server Appliance (VCSA) via SSH or the Virtual Machine Console as the root user.
   • NOTE: Take a snapshot of the vCenter Server Appliance before making configuration changes.
     
     
2. Verify current DNS resolution by running the following command:

   nslookup <esxi_host_fqdn>

   If this fails with "connection timed out" or "no servers could be reached," proceed to the next step.
3. Check the contents of the resolver configuration file:

   cat /etc/resolv.conf

4. If the required DNS server is missing, edit the file using a text editor (e.g., vi):

   vi /etc/resolv.conf

5. Add the active DNS server IP address to the file:

   nameserver <dns_server_ip>

6. Save and exit the editor (:wq).
7. Verify resolution again:

   nslookup <esxi_host_fqdn>

8. Log in to the vSphere Client.
9. Navigate to Inventory > Hosts and Clusters.
10. Right-click the disconnected/deregistered ESXi hosts and select Connection > Connect.

Additional Information

• If the DNS settings revert after a reboot, ensure that the networking configuration is also updated via the vCenter Server Appliance Management Interface (VAMI) at https://<vcenter_fqdn>:5480.
• Refer to the links below for standard DNS requirements in a vSphere environment.
  • Configure the DNS, IP Address, and Proxy Settings
  • Configuring Network Settings