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> Chapter 8 - Firewall > Inside FortiOS DoS Protection

Inside FortiOS: Denial of Service (DoS) Protection

FortiOS DoS protection maintains network integrity and performance by identifying and blocking harmful IPv4 and IPv6-based denial of service (DoS) attacks.

About DoS and DDoS attacks

A denial of service (DoS) occurs when an attacker overwhelms server resources by flooding a target system with anomalous data packets, rendering it unable to service genuine users. A distributed denial of service (DDoS) occurs when an attacker uses a master computer to control a network of compromised systems, otherwise known as a ‘botnet’, which collectively inundates the target system with excessive anomalous data packets.

FortiOS DoS and DDoS protection

FortiOS DoS protection identifies potentially harmful traffic that could be part of a DoS or a DDoS attack by looking for specific traffic anomalies. Traffic anomalies that become DoS attacks include: TCP SYN floods, UDP floods, ICMP floods, TCP port scans, TCP session attacks, UDP session attacks, ICMP session attacks, and ICMP sweep attacks. Only traffic identified as part of a DoS attack is blocked; connections from legitimate users are processed normally.

FortiOS applies DoS protection very early in its traffic processing sequence to minimize the effect of a DoS attack on FortiOS system performance. DoS protection is the first step for packets after they are received by a FortiGate interface. Potential DoS attacks are detected and blocked before the packets are sent to other FortiOS systems.

FortiOS also includes an access control list feature that is implemented next. This accelerated ACL technology uses NP6 processors to block traffic (including DoS attacks) by source and destination address and service again before the packets are sent to the FortiGate CPU.

FortiOS DoS protection can operate in a standard configuration or operate out of band in sniffer mode, also known as one-arm mode, similar to intrusion detection systems. When operating in sniffer mode the FortiGate unit detects attacks and logs them without blocking them.

FortiOS DoS policies determine the course of action to take when anomalous traffic reaches a configured packet rate threshold. You can block an attacker, block an interface, block an attacker and interface, or allow traffic to pass through for monitoring purposes. This allows you to maintain network security by gathering information about attacks, monitor potentially offending traffic, or block offenders for the most protection.

FortiGates with NP6 processors also support synproxy DoS protection. An NP6-accelerated TCP SYN proxy offloads the three-way TCP handshake TCP SYN anomaly checking DoS protection to NP6 processors.

FortiOS DDoS Prevention

In addition to using DoS protection for protection against DoS attacks, FortiOS includes a number of features that prevent the spread of Botnet and C&C activity. Mobile Malware or Botnet and C&C protection keeps Botnet and C&C code from entering a protected network and compromising protected systems. As a result, systems on the protected network cannot become Botnet clients.

In addition, FortiOS can monitor and block outgoing Botnet connection attempts. Monitoring allows you to find and remove Botnet clients from your network and blocking prevents infected systems from communicating with Botnet sites.

Configuration options

Choose the standard configuration for maximum protection or configure sniffer mode to gather information.

Standard configuration

DoS protection is commonly configured on a FortiGate unit that connects a private or DMZ network to the Internet or on a FortiWiFi unit that connects a wireless LAN to an internal network and to the Internet. All Internet traffic or wireless LAN traffic passes through DoS protection in the FortiGate unit or the FortiWiFi unit.

Out of band configuration (sniffer mode)

A FortiGate unit in sniffer mode operates out of band as a one-armed Intrusion Detection System by detecting and reporting attacks. It does not process network traffic nor does it take action against threats. The FortiGate interface operating in sniffer mode is connected to a Test Access Point (TAP) or a Switch Port Analyzer (SPAN) port that processes all of the traffic to be analyzed. The TAP or SPAN sends a copy of the switch traffic to the out of band FortiGate for analysis.

FortiOS records log messages and sends alerts to system administrators when a DoS attack is detected. IDS scanning does not affect network performance or network traffic if the IDS fails or goes offline.

DoS policies

DoS policies provide effective and early DoS detection while remaining light on system resources. They are configured to monitor and to stop traffic with abnormal patterns or attributes. The DoS policy recognizes traffic as a threat when the traffic reaches a user-configured packet rate threshold. The policy then determines the appropriate action. In addition to choosing whether or not to log each type of anomaly, you can choose to pass or block threats.

DoS policy anomaly protection is applied to all incoming traffic to a single FortiGate interface, but you can narrow policies by specifying service, source address, and destination address. The FortiGate unit processes DoS policies in their own respective order first, followed by all other firewall policies.

Hardware acceleration

Hardware acceleration enhances protection and increases the efficiency of your network. FortiOS integrated Content Processors (CPs), Network Processors (NPs), and Security Processors (SPs) accelerate specialized security processing. DoS SYN proxy protection is built in to NP6 processors and many Fortinet Security Processors, like the CE4, XE2, and FE8, to guard against TCP SYN floods. TCP packets with the SYN flag are the most efficient DoS attack tool because of how communication sessions are initiated between systems. NP6 and SP processors can offload TCP SYN flood attack detection and blocking. The SP module increases a FortiGate unit’s capacity to protect against TCP SYN flood attacks while minimizing the effect of attacks on the FortiGate unit’s overall performance and the network performance. The result is improved capacity and overall system performance.

The FortiGuard Center

The FortiGuard Center shows information on all the most recent FortiGuard news, including information concerning zero-day research and hot intrusion detections. Research papers are also available that concern a variety of current security issues.

To view recent developments, go to