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This article explains how certain FTP file transfers fail when antivirus is enabled Useful links.
File transfers performed with the REST (Restart) FTP command will not work through the FortiGate when antivirus is enabled.
FortiGate units do not support the use of this command because it allows a file transfer to be restarted at a certain byte-point. This means the entire file could be transferred in many multiple smaller chunks. This technique could be used to thwart the current antivirus scanning feature of the FortiGate unit.
Possible workarounds:
Configure the FTP client to not use the REST command. As an alternative, use the GET command.
Or
Add a firewall policy for FTP with antivirus scanning disabled to and from these particular IP addresses.
