FortiOS 6 – Data leak prevention

Blocking emails larger than 15 MB and logging emails from 5 MB to 15 MB

Multiple filters will have to be used in this case and the order that they are used is important. Because there is no mechanism to move the filters within the sensor the order that they are added to the sensor is important.

  1. Go to Security Profiles > Data Leak Prevention.
  2. Use the Create New icon to add a new sensor. Give it a descriptive Name, such as block_large_emails. Optionally, enter a descriptive comment.

Once the sensor has been created, a new filter will need to be added.

  1. Create the filter to block the emails over 15 MB. In the filters table select Create New.
  2. Set Type to Messages and enter 15360 in the field next to File size over. (1MB = 1024KB, 15 MB = 15 x 1024KB = 15360KB)
  3. Go to Examine the Following Services and select all Email services .
  4. Set Action to Block.
  5. Select OK.
  6. Create the filter to log emails between 5 MB and 10 MB. In the filters table select Create New.
  7. Set Type to Files.
  8. Enter 5120 in the field next to File size over. (1MB = 1024KB, 5 MB = 5 x 1024KB = 5124KB)
  9. Go to Examine the Following Services and select all the email services .
  10. Set action to Log Only.
  11. Select

The reason that the block filter is placed first is because the filters are applied in sequence and once the traffic triggers a filter, the action is applied and then the traffic is passed on to the next test. If the Log Only filter which checks for anything over 1MB is triggered this would include traffic over 15MB, so a 16 MB file would only be logged. In the described order, the 16 MB file will be blocked and the 3 MB file will be logged.

Blocking selectively based on a fingerprint

The following is a fairly complex example but shows what can be done by combining various components in the correct configuration.

The company has a number of copyrighted documents that it does not want “escaping” to the Internet but it does want to be able to send those documents to the printers for turning into hardcopy.

The policies and procedures regarding this issue state that:

  • Only members of the group Senior_Editors can send copyrighted material to the printers.
  • Every member of the company by default is included in the group employees.
  • Even permitted transmission of copyrighted material should be recorded. l All of the printers IP addresses are in a group called approved_printers.
  • There is a file share called copyrighted where any file that is copyrighted is required to have a copy stored.
  • It doesn’t happen often but for legal reasons sometimes these files can be changed, but all versions of a file in this directory need to be secured.
  • All network connections to the Internet must have AntiVirus enabled using at least the default profile. l The SSL/SSH Inspection profile used will be default.

It is assumed for the purposes of this example that:

  • Any addresses or address groups have been created. l User accounts and groups have been created. l The account used by the FortiGate is fgtaccess. l The copyrighted sensitivity level needs to be created. l The copyrighted material is stored at \\\books\copyrighted\
  1. Add a new Sensitivity Level by running the following commands in the CLI:

config dlp fp-sensitivity edit copyrighted


  1. Apply files to the fingerprint database by running these commands in the CLI:

config dlp fp-doc-source edit “copyrighted_material” set server-type smb set server set username fgtaccess


set password ****** set file-path books/copyrighted/ set file-pattern *.pdf set sensitivity copyrighted set period daily set tod-hour 2 set tod-min 0 set scan-subdirectories enable set remove-deleted disable set keep-modified enable



Two Sensors need to be created. One for blocking the transmission of copyrighted material and a second for allowing the passing of copyrighted material under specific circumstances.

  1. Create the first DLP Sensor with the following commands in CLI:

config dlp sensor edit block_copyrighted config filter edit 1 set proto smtp pop3 imap http-get http-post ftp nntp mapi

set filter-by fingerprint set fp-sensitivity copyrighted

set action block




  1. Create the second DLP Sensor config dlp sensor edit allow_copyrighted config filter edit 2 set proto smtp pop3 imap http-get http-post ftp nntp mapi

set filter-by fingerprint set fp-sensitivity copyrighted set action log-only




  1. Create a policy to allow transmission of copyrighted material.
    1. Go to Policy & Objects > IPv4 Policy.
    2. Select Create New.
    3. Use the following values in the policy:
Incoming Interface LAN
Source Address all
Outgoing Interface wan1
Destination Address all
Schedule always
Service all
Enable NAT enabled — Use Destination Interface Address
AntiVirus <ON> default
DLP <ON> Copyrighted
SSL/SSH Inspection <ON> default
Enable this policy <ON>

This policy should be placed as close to the beginning of the list of policies so the it is among the first tested against.

  1. Create a policy to block transmission of copyrighted material.

This will in effect be the default template for all following policies in that they will have to use the DLP profile that blocks the transmission of the copyrighted material. a. Go to Policy & Objects > IPv4 Policy.

  1. Select Create New or Edit an existing policy.
  2. Use the following values in the Policy:

The fields should include what ever values you need to accomplish your requirements are but each policy should include the DLP sensor block_copyrighted. Alternatively, if a different DLP configuration is required it should include a filter that blocks copyrighted fingerprinted file.

If you need to create a policy that is identity based make sure that there is an Authentication rule for the group employees that uses the DLP sensor that blocks copyrighted material.


This entry was posted in Administration Guides, FortiOS 6 and tagged , on by .

About Mike

Michael Pruett, CISSP has a wide range of cyber-security and network engineering expertise. The plethora of vendors that resell hardware but have zero engineering knowledge resulting in the wrong hardware or configuration being deployed is a major pet peeve of Michael's. This site was started in an effort to spread information while providing the option of quality consulting services at a much lower price than Fortinet Professional Services. Owns PacketLlama.Com (Fortinet Hardware Sales) and Office Of The CISO, LLC (Cybersecurity consulting firm).

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