Why do I receive the alert that a suspiciously small data fragment has been blocked on Client Securi

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Customer_Care Posts: 548 F-Secure Employee


  • The F-Secure Client Security reports that a suspiciously small datagram fragment has been blocked
  • How to get rid of the warning if it is a false positive? 


This type of alerts might be related to a DDoS attack. If they appear on a network, they might also be a sign of a broken or wrongly configured router or device in the network, for example a printer. 

Proceed to investigate the issue on a network level before applying the modification below. In practice packet with a size below 128 bytes are normally considered inefficient (ratio data/data+headers).

To get rid of the alert, you can change what the F-Secure firewall considers as the minimum size for a fragment. 

In Policy Manager, this setting has to be changed by using the Advanced view. Follow these steps:

  1. Log into Policy Manager Console.
  2. Select the host or domain from the Domain tree.
  3. Go to the Settings tab and select the Advanced view.
  4. Navigate to F-Secure Internet Shield > Settings > Firewall Engine > Minimum fragment size.
  5. Set the Minimum Fragment Size to 0.
  6. Distribute the policy to the hosts.

Article no: 000001900