For small networks, and those that do not have accessible from the outside servers, it may be a nice to have feature. You would need then to fix the thresholds, then again. The false positives, especially for TCP SYN and alike protections, would block legitimate clients to the internal servers available from the Internet due to sudden surge of the client requests. From my personal experience, to protect large networks with this DoS feature of Fortigate is more hassle than help.In FortiOS 6.x and newer it is called DoS Policy. Note: in previous versions of FortiOS the feature was called DoS sensor, so I mention it for easier reference only.This means, though, that even if some security rule allows traffic, if such traffic exceeds DoS thresholds it may be blocked. The system tries unsuccessfully to determine which applications are waiting for the transferred data, and then, as a result, sends an ICMP packet back to the sender along with the message destination unreachable. Fortigate applies Dos protection early in the policy matching, before the Security policy is checked, so it consumes less resources than blocking the same traffic in Security rules. In regards to DoS and DDoS attacks, UDP packets are sent to random ports on the target system.For smarter anti-DDoS solution Fortinet have FortiDDoS physical appliance.Dos sensor/policy protects against INCOMING traffic for the specified interface.To block the sender IP completely, you can use set qurantine parameter under the specific anomaly. By default, only exceeding the threshold packets get blocked.Thresholds for anomalies are configurable and do what they say - once traffic matched by this policy exceeds the threshold, it gets blocked.You can (actually must) specify: source/destination IPs to match the DoS policy ( all can be used), service ( ALL can be used), and incoming interface to apply the DoS policy to.On Fortigates with hardware NP modules, you also have Proxy as an action in tcp_syn_flood protection to enable, which makes Fortigate to proxy SYN connections. All anomalies are set by default to Pass the offending traffic and are disabled, so make sure under the given anomaly to set status enable and action to block.You only have the choice which ones to enable and which ones not to. Customers protecting their virtual networks against DDoS attacks have detailed visibility into attack traffic and actions taken to mitigate the attack via attack mitigation reports & mitigation flow logs. The list of anomalies is pre-set in any policy you create. Azure DDoS Protection standard provides detailed attack insights and visualization with DDoS Attack Analytics.
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