Unable to start backup because agent is unreachable



This article describes the possible cause and solution(s) when the backup error, "Unable to start backup because agent is unreachable" occurs.

If agents paired by hostname are reporting as unreachable, make sure that a DNS Search Domain has been specified on the Network Configuration User Interface. Your Datto appliance will be unable to resolve hostnames without this information.


Prior to investigating the cause behind this, please see the following knowledge base for more information about troubleshooting backup errors:

This knowledge base reviews the usage of ShadowProtect and ShadowSnap in Datto's backup process and how adamant it is to have that software. It also includes the necessary services for agent communication.  Once the steps are followed in the above knowledge base, please see the next section.

Possible Causes

When this error occurs, it can indicate the following:

  • The ShadowSnap services are present, but are not running on the target machine.
  • Port 25566, the port used by ShadowSnap to communicate with the Datto device is blocked or filtered. This port will be listening if the ShadowSnap agent is running.
  • Port 3260, the port used by iSCSI to facilitate encrypted backup transfers for the ShadowSnap agent, is blocked or filtered.
  • The Datto device cannot contact the protected machine when requesting a snapshot.
  • The Windows firewall is stopped / disabled

Possible Solutions

Here are some of the possible solutions to remedy the error:

1. Connect to the Protected Machine having the issue and attempt to ping the Datto device's IP address through the cmd prompt

  • If the ping request fails or high packet loss is observed, this indicates a networking issue beyond the scope of the Datto device.
  • Ensure all networking requirements are met for both the Datto device and the Protected Machine as listed in the following article: Datto Networking Requirements.

2. Check that the Protected Machine's IP address, host, or domain name has not changed from what it originally was when paired with the Datto device

  • If it has changed, please change it back to what it originally was or contact Datto Technical Support to perfect an agent rename

3. Ensure that the Windows Firewall is running and enabled properly

  • By having it completely disabled, it prevents agent communication. Check that the proper provisions are made within the firewall settings to allow the agent communication with the StorageCraft activation servers.
  • Ensure that the local firewall settings are not blocking or filtering port 25566 or port 139.

4. Check that all the following services are in a working, stable state:

  • StorageCraft Raw Agent
  • StorageCraft Shadow Copy Provider
  • ShadowProtect Service
  • If one or more are not turning off/starting properly, this may require an uninstall/reinstall of the ShadowSnap agent
If none of these fix the issue, please contact Datto Technical Support for further assistance.

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