When you back up VMs or restore data using the vSphere Agent, the vCenter sends ESX and ESXi server host names to the Agent. If the vCenter provides a host name to the Agent and the Agent cannot resolve the host name, connections fail and “Host address lookup” errors occur.
If you are not using a DNS server, ensure that the vSphere Agent can resolve host names by adding a static entry on the Agent for the vCenter and each ESX/ESXi server.
To add a static entry, enter the following command in the Agent CLI:
net hosts add <ipaddress> <hostname>
Where <ipaddress> is the IP address that is mapped to the <hostname>.