Unable to start backup because agent is unreachable

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Scope

This article provides troubleshooting steps to resolve the backup error Unable to start backup because agent is unreachable on a ShadowSnap or Datto Windows Agent-protected machine

Prerequisites

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

Causes

When this error occurs, it can indicate the following:

  • ShadowSnap or Datto Windows Agent services are present, but not running on the target machine.
  • Port 25566, the port used by ShadowSnap to communicate with the Datto device, is blocked or filtered. If the ShadowSnap agent is running, this port will be listening. Review  Getting Started With The ShadowSnap Agent for more information.
  • Port 25568, the port used by the Datto Windows Agent to communicate with the Datto device, is blocked or filtered. If the Datto Windows agent is running, this port will be listening. Review  Getting Started with the Datto Windows Agent for more information.
  • Port 3260, the port used by iSCSI for encrypted backup transfers for the agent, and sometimes backups for the Datto Windows Agent, is blocked or filtered.
  • Port 3262, the primary port used by the Datto Windows Agent for backup transfers, is blocked or filtered.
  • The Datto device cannot contact the protected machine when requesting a snapshot.
  • The Windows firewall is stopped or disabled.

Solutions

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

1. Connect to the protected machine reporting the issue and attempt to ping the Datto appliance's IP address through the Windows Command prompt.

  • If the ping request fails, or if you observe high packet loss, this indicates a networking issue beyond the scope of the Datto appliance. Investigate your network infrastructure for contributing factors.
  • Make sure that the environment of the protected machine and the Datto appliance meets the networking and bandwidth requirements described in this article.

2. Attempt to ping the protected machine from another machine on the network. Verify that the protected machine's IP address, hostname, or domain name has not changed since it was originally paired with the Datto device. If it has changed, rename the agent on the Configure Agent Settings page of the Datto appliance GUI.

3. Verify that the Windows Firewall is enabled and running. Completely disabling the firewall prevents agent communication.

4. Check that your firewall includes the required exceptions defined here, and that it is not blocking or filtering the following required ports:

  • ShadowSnap: Port 25566 and port 139.
  • Encrypted ShadowSnap: Port 25566 and port 3260.
  • Datto Windows Agent: Port 25568, ports 3260, 3262.

4. Check that all the following services are in a working, stable state. Restart the services if they are not.

For the ShadowSnap Agent:

  • StorageCraft Raw Agent
  • StorageCraft Shadow Copy Provider
  • ShadowProtect Service

For the Datto Windows Agent:

  • Datto Backup Agent Service
  • DattoProvider
If you try all of the above steps, and still cannot resolve the issue, contact Datto Technical Support for further assistance.

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