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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.

To assign an interface as a sniffer interface in the GUI, go to Network > Interfaces and edit the interface. For Addressing mode, select One-Arm Sniffer.

If the option is not available, 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 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

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.

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.

To assign an interface as a sniffer interface in the GUI, go to Network > Interfaces and edit the interface. For Addressing mode, select One-Arm Sniffer.

If the option is not available, 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 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

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.