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Configuring TCP and ICMP attack protection
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 device provides the following features:
•
SYN Cookie
•
Protection against Naptha attacks
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 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, 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 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 enter the
ESTABLISHED state. In this way, incomplete TCP connections could be avoided to protect the server
against SYN Flood attacks.
To enable the SYN Cookie feature:
To do...
Command...
Remarks
1.
Enter system view.
system-view
—
2.
Enable the SYN Cookie
feature.
tcp syn-cookie enable
Required
Enabled by default
NOTE:
•
If you enable MD5 authentication for TCP connections, the SYN Cookie configuration is ineffective. If
you disable MD5 authentication for TCP connections, the SYN Cookie configuration automatically
becomes effective. For more information about MD5 authentication, see
Layer 3—IP Routing
Configuration Guide.
•
With the SYN Cookie feature enabled, only the MSS is negotiated during TCP connection
establishment, instead of the window's zoom factor and timestamp.
