One-arm sniffer

You can use a one-arm sniffer to configure a physical interface as a one-arm intrusion detection system (IDS). Traffic sent to the interface is examined for matches to the configured security profile. The matches are logged, and then all received traffic is dropped. Sniffing only reports on attacks; it does not deny or influence traffic.

You can also use the one-arm sniffer to configure the FortiGate to operate as an IDS appliance to sniff network traffic for attacks without actually processing the packets. To configure a one-arm IDS, enable sniffer mode on a physical interface and connect the interface to the SPAN port of a switch or a dedicated network tab that can replicate the traffic to the FortiGate.

If the one-arm sniffer option is not available, this means the interface is in use. Ensure that the interface is not selected in any firewall policies, routes, virtual IPs, or other features where a physical interface is specified. The option also does not appear it the role is set to WAN. Ensure the role is set to LAN, DMZ, or undefined.

The following table lists some of the one-arm sniffer settings you can configure:

Field	Description
Filters	Enable this setting to include filters that define a more granular sniff of network traffic. Select specific hosts, ports, VLANs, and protocols. In all cases, enter a number or range for the filter type. The standard protocols are: UDP: 17 TCP: 6 ICMP: 1
Include IPv6 Packets	If the network is running IPv4 and IPv6 addresses, enable this setting to sniff both types; otherwise, the FortiGate will only sniff IPv4 traffic.
Include Non-IPv6 Packets	Enable this setting for a more intense content scan of the traffic.
Security Profiles	The following profiles are configurable in the GUI and CLI: Antivirus Web filter Application control IPS File filter The following profiles are only configurable in the CLI: Email filter DLP IPS DoS

CPU usage and packet loss

Traffic scanned on the one-arm sniffer interface is processed by the CPU, even if there is an SPU, such as NPU or CP, present. The one-arm sniffer may cause higher CPU usage and perform at a lower level than traditional inline scanning, which uses NTurbo or CP to accelerate traffic when present.

The absence of high CPU usage does not indicate the absence of packet loss. Packet loss may occur due to the capacity of the TAP devices hitting maximum traffic volume during mirroring, or on the FortiGate when the kernel buffer size is exceeded and it is unable to handle bursts of traffic.

Sample configuration

The following example shows how to configure a file filter profile that blocks PDF and RAR files used in a one-arm sniffer policy.

To configure a one-arm sniffer policy in the GUI:

1. Go to Network > Interfaces and double-click a physical interface to edit it.

2. For Role, select either LAN, DMZ, or Undefined.

3. For Addressing Mode, select One-Arm Sniffer.

   

4. In the Security Profiles section, enable File Filter and click Edit. The Edit File Filter Profile pane opens.

5. In the Rules table, click Create New.

   

6. Configure the rule:

   1. For File types, click the + and select pdf and rar.

   2. For Action, select Block.

   3. Click OK to save the rule.

7. Click OK to save the file filter profile.

   

8. Click OK to save the interface settings.

9. Go to Log & Report > File Filter to view the logs.

   

To configure a one-arm sniffer policy in the CLI:

1. Configure the interface:

   config system interface
       edit "s1"
           set vdom "root"
           set ips-sniffer-mode enable
           set type physical
           set role undefined
           set snmp-index 31
       next
   end

2. Configure the file filter profile:

   config file-filter profile
       edit "sniffer-profile"
           set comment "File type inspection."
           config rules
               edit "1"
                   set protocol http ftp smtp imap pop3 cifs
                   set action block
                   set file-type "pdf" "rar"
               next
           end
       next
   end

3. Configure the firewall sniffer policy:

   config firewall sniffer
       edit 1
           set interface "s1"
           set file-filter-profile-status enable
           set file-filter-profile "sniffer-profile"
       next
   end

4. View the log:

   # execute log filter  category  19
   # execute log display
   1 logs found.
   1 logs returned.
   
   1: date=2020-12-29 time=09:14:46 eventtime=1609262086871379250 tz="-0800" logid="1900064000" type="utm" subtype="file-filter" eventtype="file-filter" level="warning" vd="root" policyid=1 sessionid=792 srcip=172.16.200.55 srcport=20 srcintf="s1" srcintfrole="undefined" dstip=10.1.100.11 dstport=56745 dstintf="s1" dstintfrole="undefined" proto=6 service="FTP" profile="sniffer-profile" direction="outgoing" action="blocked" filtername="1" filename="hello.pdf" filesize=9539 filetype="pdf" msg="File was blocked by file filter."