FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article describes why a SYN-ACK is seen from FortiGate when nmap is initiated toward a non-existing IP address.
Scope
FortiGate.
Solution
When nmap is initiated towards a non existing IP address on both ports 5060 and port 2000, a SYN-ACK is observed on FortiGate.
This an expected behavior when the ALG configuration is set to a proxy-based mode.
In a proxy-based policy, the TCP connection is proxied by the FortiGate. A TCP 3-way handshake can be established with the client, even though the server didn't complete the handshake, and vice versa. This behavior is anticipated, given that there is a corresponding Firewall policy in place, and the ALG is set to proxy-based.
If the VoIP algorithm is changed to kernel-helper-based, SYNC-ACK will not be observed on FortiGate
config system settings
set default-voip-alg-mode proxy-based * | kernel-helper-based
