The error message "Connection could not be established to the host" occurs when the Vembu BDR Backup Server is unable to establish a connection with the target agent on the host. This results in the failure of communication between the BDR Backup Server and the proxy agent installed on the target host. Several reasons can lead to this proxy connection failure:

1. The target host is unreachable.

2. Vembu Integration Service is not available on the target host.

3. Vembu Integration Service/ BDRShield Delegation Service is not running on the target host.

4. The Vembu Integration Service/ BDRShield Delegation Service on the target host is of a lower version compared to the BDR backup server.

5. Identical Disk UUIDs.

Note: BDRShield Backup Server versions earlier than v5.3.0, the service name is "Vembu Integration Service," while for versions v5.3.0 and later, it is "BDRShield Delegation Service."

Solution:

To resolve the "Connection could not be established to the host" error and related backup issues, follow these steps:

General Connection Issues:

1. Check Host Reachability:

1. Verify whether the target host is operational and reachable. Ensure that the host settings are correctly configured.
   

2. Verify Vembu Integration Service:

1. Open the 'Control Panel' from the Windows Menu.
2. Search for 'Programs' and select 'Programs and Features.'
3. Check for the presence of the "Vembu Integration Service agent" application.
4. If it's not available, install the Vembu Integration Service (VIS) agent on the target host.
   

3. Check the Service Status:

1. Open the 'Run' dialog and type "Services.msc."
2. In the list of services, look for "Vembu Integration Service"/ "BDRShield Delegation Service" and check its status.
3. If the service is in a stopped state, manually start it.
4. If the service is disabled, change the startup type from "Disabled" to "Automatic."
   

4. Verify Firewall Settings:

1. Open the 'Control Panel' from the Windows Menu.
2. Search for 'System and Security' and select 'Windows Firewall.'
3. Click 'Advanced settings,' then navigate to 'Inbound rules.'
4. Check for the "VembuIntegrationService"/ "BDRShieldDelegationService" rule.
5. If port 42005 is blocked, allow the connection by right-clicking the rule and selecting "Allow the connection."
6. If the port is not available, manually add it to the firewall rules.
   

5. Identical Disk UUIDs:

1. Execute the following command to check if the VMs being backed up have the same disk UUID: 
   

vmkfstools -J getuuid /vmfs/volumes/datastore/VM/vm.vmdk

Example:

vmkfstools -J getuuid /vmfs/volumes/6262f3e4-d79d776c-6567-386077bd1d14/1UUID/1000_Merge.vmdk

UUID is 60 00 C2 95 4c 37 b9 35-9f 11 05 fb e5 93 f4 ca

1. If the UUIDs are the same, you can set a new UUID for the disk using the following command:
   

vmkfstools -J setuuid /vmfs/volumes/datastore/VM/vm.vmdk

Example: 

vmkfstools -J setuuid /vmfs/volumes/6262f3e4-d79d776c-6567-386077bd1d14/1UUID/1000_Merge.vmdk

UUID is 60 00 C2 9c d7 4e d9 70-de 1f 59 d2 f0 e0 20 ac

A new UUID will be generated after the command execution. Please refer to KB 1021189 for further information. Note that this issue has been addressed in BDRShield v7.0 release.

Note: BDRShield Backup Server versions earlier than v5.3.0, the service name is "Vembu Integration Service," while for versions v5.3.0 and later, it is "BDRShield Delegation Service."

By following these steps, you can troubleshoot and resolve connection issues to the host and related backup problems in BDRShield environments.