PROBLEM

In a VMware backup, after a snapshot of the virtual machine is taken, VM Backup requests access to the datastore containing the VM data through VDDK. Failure by the machine where VM Backup resides to resolve the DNS name of the host exposing the snapshot will result in a backup failure.

VDDK log will also be detailing the issue as follows:

2017/09/21 21:50:31.672: CnxAuthdConnect: Returning false because CnxAuthdConnectTCP failed
2017/09/21 21:50:31.672: CnxConnectAuthd: Returning false because CnxAuthdConnect failed
2017/09/21 21:50:31.672: Cnx_Connect: Returning false because CnxConnectAuthd failed
2017/09/21 21:50:31.672: Cnx_Connect: Error message: Host address lookup for server ESXserver.service.local failed: Unknown error 11001 (0×2af9)
2017/09/21 21:50:31.672: NBD_ClientOpen: Couldn't connect to ESXserver.service.local:902 Host address lookup for server ESXserver.service.local failed: Unknown error 11001 (0×2af9)
2017/09/21 21:50:31.672: DISKLIB-DSCPTR: : "vpxa-nfc://[ESXserver] vCenter05/vCenter05.vmdk@ESXserver.service.local:902" : Failed to open NBD extent.
2017/09/21 21:50:31.672: DISKLIB-LINK : "vpxa-nfc://[ESXserver] vCenter05/vCenter05.vmdk@ESXserver.service.local:902" : failed to open (NBD_ERR_NETWORK_CONNECT).
2017/09/21 21:50:31.672: DISKLIB-CHAIN : "vpxa-nfc://[ESXserver] vCenter05/vCenter05.vmdk@ESXserver.service.local:902" : failed to open (NBD_ERR_NETWORK_CONNECT).
2017/09/21 21:50:31.672: DISKLIB-LIB : Failed to open 'vpxa-nfc://[ESXserver] vCenter05/vCenter05.vmdk@ESXserver.service.local:902' with flags 0×1e NBD_ERR_NETWORK_CONNECT (2338).
2017/09/21 21:50:31.672: VixDiskLib: Detected DiskLib error 2338 (NBD_ERR_NETWORK_CONNECT).

CAUSE

VM Backup is unable to resolve DNS name of ESXi Host

SOLUTION

The VM Backup machine needs to be able of DNS resolution of the ESX host name to IP in order to be able to reach and access the snapshot.

NSlookup can be used to ensure that the correct IP is being obtained. If that is not the case, adding a DNS A record of the ESXi host and IP on the domain's DNS server should then yield the correct result.

Once the correct IP is resolved, make sure that the port 902 can also be telnetted to using the DNS name. eg: Telnet ESXserver 902

The session should be greeted by the below server response.