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:




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