Chapter 1 What’s New for FortiOS 5.2.1 : System Features : Disable Honoring the Don't-Fragment Flag
  
Disable Honoring the Don't-Fragment Flag
Honoring the Don't-Fragment flag can now be disabled through the CLI. This allows the FortiGate unit to fragment the packet as required, even when the Don't-Fragment flag is set.
Syntax
config system global
set honor-df disable
end