Configuring TCP and ICMP attack protection· 1

TCP and ICMP attack protection overview·· 1

Enabling the SYN Cookie feature· 1

Enabling protection against Naptha attacks 2

Disabling forwarding of ICMP fragments 2

Displaying and maintaining TCP and ICMP attack protection· 3

TCP and ICMP attack protection overview

An attacker can attack the device during the process of TCP connection establishment or by sending a large number of ICMP fragments. To prevent such attacks, the switch provides the following features:

·           SYN Cookie

·           Protection against Naptha attacks

·           Disabling of ICMP fragments forwarding

This document describes the attacks these features can prevent, working mechanisms of these features, and configuration procedures.

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 the 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. Thus, the TCP connection is established.

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 amount of incomplete TCP connections are established, resulting in heavy resource consumption and making the server unable to handle services normally.

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. Only after receiving an ACK message from the client can the server establish a connection, and then enter the ESTABLISHED state. In this way, large amounts of incomplete TCP connections could be avoided to protect the server against SYN Flood attacks.

To enable the SYN Cookie feature:

Enabling protection against Naptha attacks

Naptha attacks are similar to the SYN Flood attacks. Attackers can perform Naptha attacks by using the six TCP connection states (CLOSING, ESTABLISHED, FIN_WAIT_1, FIN_WAIT_2, LAST_ACK, and SYN_RECEIVED), and SYN Flood attacks by using only the SYN_RECEIVED state.

Naptha attackers control a huge amount of hosts to establish TCP connections with the server, keep these connections in the same state (any of the six), and request for no data so as to exhaust the memory resource of the server. As a result, the server cannot process normal services.

Protection against Naptha attacks reduces the risk of such attacks by accelerating the aging of TCP connections in a state. After the feature is enabled, the device (serving as a TCP server) periodically checks the number of TCP connections in each state. If the device detects that the number of TCP connections in a state exceeds the maximum number, it considers that a Naptha attack occurs and accelerates the aging of TCP connections in this state. The device will stop accelerating the aging of TCP connections when the number of TCP connections in the state is less than 80% of the maximum number (1 at least).

To enable the protection against Naptha attack:

Disabling forwarding of ICMP fragments

To prevent ICMP fragment attacks, you can disable the switch from forwarding ICMP fragments.

To disable the switch from forwarding ICMP fragments:

Displaying and maintaining TCP and ICMP attack protection