The Firewall TCP SYN Cookie feature implements software to protect the firewall from TCP SYN-flooding attacks, which are a type of DoS attack.
A SYN-flooding attack occurs when a hacker floods a server with a barrage of requests for connection. Because these messages have unreachable return addresses, the connections cannot be established. The resulting volume of unresolved open connections eventually overwhelms the server and can cause it to deny service to valid requests, thereby preventing legitimate users from connecting to a website, accessing e-mail, using FTP service, and so on.
SYN flood attacks are divided into two types:
Host flood—SYN flood packets are sent to a single host aiming to utilize all resources on that host.
Firewall session table flood—SYN flood packets are sent to a range of addresses behind the firewall, with the aim of exhausting the session table resources on the firewall and thereby denying resources to the legitimate traffic going through the firewall.
The Firewall TCP SYN Cookie feature helps prevent SYN-flooding attacks by intercepting and validating TCP connection requests. The firewall intercepts TCP SYN packets that are sent from clients to servers. When the TCP SYN cookie is triggered, it acts on all SYN packets that are destined to the configured VPN Routing and Forwarding (VRF) or zone. The TCP SYN cookie establishes a connection with the client on behalf of the destination server and another connection with the server on behalf of the client and knits together the two half-connections transparently. Thus, connection attempts from unreachable hosts will never reach the server. The TCP SYN cookie intercepts and forwards packets throughout the duration of the connection.
The Firewall TCP SYN Cookie feature provides session table SYN flood protection for the global routing domain and for the VRF domain. Because the firewall saves sessions in a global table, you can configure a limit to the number of TCP half-opened sessions. A TCP half-opened session is a session that has not reached the established state. In a VRF-aware firewall, you can configure a limit to the number of TCP half-opened sessions for each VRF. At both the global level and at the VRF level, when the configured limit is reached, the TCP SYN cookie verifies the source of the half-opened sessions before creating more sessions.