Packet flow ingress and egress: FortiGates without network processor offloading

Packet flow ingress and egress: FortiGates without network processor offloading

This section describes the steps a packet goes through as it enters, passes through and exits from a FortiGate unit. This scenario shows all of the steps a packet goes through if a FortiGate does not contain network processors (such as the NP6).



All packets accepted by a FortiGate pass through a network interface and are processed by the TCP/IP stack. Then if DoS policies or Access Control List (ACL) policies have been configured the packet must pass through these as well as automatic IP integrity header checking.

DoS scans are handled very early in the life of the packet to determine whether the traffic is valid or is part of a DoS attack. The DoS module inspects all traffic flows but only tracks packets that can be used for DoS attacks (for example, TCP SYN packets), to ensure they are within the permitted parameters. Suspected DoS attacks are blocked, other packets are allowed.

IP integrity header checking reads the packet headers to verify if the packet is a valid TCP, UDP, ICMP, SCTP or GRE packet. The only verification that is done at this step to ensure that the protocol header is the correct length. If it is, the packet is allowed to carry on to the next step. If not, the packet is dropped.

Incoming IPsec packets that match configured IPsec tunnels on the FortiGate are decrypted after header checking is done.

If the packet is an IPsec packet, the IPsec engine attempts to decrypt it. If the IPsec engine can apply the correct encryption keys and decrypt the packet, the unencrypted packet is sent to the next step. Non-IPsec traffic and IPsec traffic that cannot be decrypted passes on to the next step without being affected. IPSec VPN decryption is offloaded to and accelerated by CP8 or CP9 processors.


Admission Control

Admission control checks to make sure the packet is not from a source or headed to a destination on the quarantine list. If configured admission control then imposes FortiHeartBeat protection that requires a device to have FortiClient installed before allowing packets from it. Admission control can also impose captive portal authentication on ingress traffic.



Once a packet makes it through all of the ingress steps, the FortiOS kernel performs the following checks to determine what happens to the packet next.


Destination NAT

Destination NAT checks the NAT table and determines if the destination IP address for incoming traffic must be changed using DNAT. DNAT is typically applied to traffic from the Internet that is going to be directed to a server on a network behind the FortiGate. DNAT means the actual address of the internal network is hidden from the Internet. This step determines whether a route to the destination address actually exists. DNAT must take place before routing so that the FortiGate unit can route packets to the correct destination.



Routing uses the routing table to determine the interface to be used by the packet as it leaves the FortiGate unit. Routing also distinguishes between local traffic and forwarded traffic. Firewall policies are matched with packets depending on the source and destination interface used by the packet. The source interface is known when the packet is received and the destination interface is determined by routing.


Stateful inspection/Policy lookup/Session management

Stateful inspection looks at the first packet of a session and looks in the policy table to make a security decision about the entire session. Stateful inspection looks at packet TCP SYN and FIN flags to identity the start and end of a session, the source/destination IP, source/destination port and protocol. Other checks are also performed on the packet payload and sequence numbers to verify it as a valid session and that the data is not corrupted or poorly formed.

When the first packet is a session is matched in the policy table, stateful inspection adds information about the session to its session table. So when subsequent packets are received for the same session, stateful inspection can determine how to handle them by looking them up in the session table (which is more efficient than looking them up in the policy table).

Stateful inspection makes the decision to drop or allow a session and apply security features to it based on what is found in the first packet of the session. Then all subsequent packets in the same session are processed in the same way.

When the final packet in the session is processed, the session is removed from the session table. Stateful inspection also has a session idle timeout that removes sessions from the session table that have been idle for the length of the timeout.

See the Stateful Firewall Wikipedia article ( for an excellent description of stateful inspection.


Session helpers

Some protocols include information in the packet body (or payload) that must be analyzed to successfully process sessions for this protocol. For example, the SIP VoIP protocol uses TCP control packets with a standard destination port to set up SIP calls. To successfully process SIP VoIP calls, FortiOS must be able to extract information from the body of the SIP packet and use this information to allow the voice-carrying packets through the firewall.

FortiOS uses session helpers to analyze the data in the packet bodies of some protocols and adjust the firewall to allow those protocols to send packets through the firewall. FortiOS includes the following session helpers:


l  PPTP  


l  H323  


l  RAS  


l  TNS  


l  TFTP  


l  RTSP  


l  FTP  




User authentication

User authentication added to security policies is handled by the stateful inspection, which is why Firewall authentication is based on IP address. Authentication takes place after policy lookup selects a policy that includes authentication.


Device identification

Device identification is applied if required by the matching policy.



Local SSL VPN traffic is treated like special management traffic as determined by the SSL VPN destination port. Packets are decrypted and are routed to an SSL VPN interface. Policy lookup is then used to control how packets are forwarded to their destination outside the FortiGate. SSL encryption and decryption is offloaded to and accelerated by CP8 or CP9 processors.


Local management traffic

Local management traffic terminates at a FortiGate interface. This can be any FortiGate interface including dedicated management interfaces. In multiple VDOM mode local management traffic terminates at the management interface. In Transparent mode, local management traffic terminates at the management IP address.

Local management traffic includes administrative access, some routing protocol communication, central management from FortiManager, communication with the FortiGuard network and so on. Management traffic is allowed or blocked according to the Local In Policy list which lists all management protocols and their access control settings. You configure local management access indirectly by configuring administrative access and so on.

Management traffic is processed by applications such as the web server which displays the FortiOS web-based manager, the SSH server for the CLI or the FortiGuard server to handle local FortiGuard database updates or FortiGuard Web Filtering URL lookups.

Local management traffic is not involved in subsequent stateful inspection steps.

SSL VPN traffic terminates at a FortiGate interface similar to local management traffic. However, SSL VPN traffic uses a different destination port number than administrative HTTPS traffic and can thus be detected and handled differently.

This entry was posted in FortiOS 5.4 Handbook and tagged on by .

About Mike

Michael Pruett, CISSP has a wide range of cyber-security and network engineering expertise. The plethora of vendors that resell hardware but have zero engineering knowledge resulting in the wrong hardware or configuration being deployed is a major pet peeve of Michael's. This site was started in an effort to spread information while providing the option of quality consulting services at a much lower price than Fortinet Professional Services. Owns PacketLlama.Com (Fortinet Hardware Sales) and Office Of The CISO, LLC (Cybersecurity consulting firm).

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