Contents

Configuring TCP attack protection· 1

Overview·· 1

Enabling the SYN Cookie feature· 1

Displaying TCP attack protection· 1

Configuring TCP attack protection

Overview

Attackers can attack the device during the process of TCP connection establishment. To prevent such attacks, the device provides the SYN Cookie feature.

Enabling the SYN Cookie feature

As a general rule, the establishment of a TCP connection involves the following three handshakes:

1.     The request originator sends a SYN message to the target server.

2.     After receiving the SYN message, the target server establishes a TCP connection in SYN_RECEIVED state, returns a SYN ACK message to the originator, and waits for a response.

3.     After receiving the SYN ACK message, the originator returns an ACK message, establishing the TCP connection.

Attackers may mount SYN Flood attacks during TCP connection establishment. They send a large number of SYN messages to the server to establish TCP connections, but they never make any response to SYN ACK messages. As a result, a large number of incomplete TCP connections are established, resulting in heavy resource consumption and making the server unable to handle services correctly.

The SYN Cookie feature can prevent SYN Flood attacks. After receiving a TCP connection request, the server directly returns a SYN ACK message, instead of establishing an incomplete TCP connection. The server can establish a connection only after receiving an ACK message from the client. The server then enters ESTABLISHED state. In this way, incomplete TCP connections can be avoided to protect the server against SYN Flood attacks.

To enable the SYN Cookie feature:

If you enable MD5 authentication for TCP connections, the SYN Cookie configuration is ineffective. Then, if you disable MD5 authentication for TCP connections, the SYN Cookie configuration automatically becomes effective.

With the SYN Cookie feature enabled, only the maximum segment size (MSS) is negotiated during TCP connection establishment, instead of the window's zoom factor and timestamp.

Displaying TCP attack protection