Figure 9-4 Smurf Attack; Table 9-2 Icmp Commands That Trigger Alerts; Table 9-3 Legal Netbios Commands - ZyXEL Communications ZyWALL 5 User Manual

(pings). Since the destination IP address of each packet is the broadcast address of the network, the
router will broadcast the ICMP echo request packet to all hosts on the network. If there are
numerous hosts, this will create a large amount of ICMP echo request and response traffic. If a
hacker chooses to spoof the source IP address of the ICMP echo request packet, the resulting
ICMP traffic will not only clog up the "intermediary" network, but will also congest the network
of the spoofed source IP address, known as the "victim" network. This flood of broadcast traffic
consumes all available bandwidth, making communications impossible.
ICMP Vulnerability
ICMP is an error-reporting protocol that works in concert with IP. The following ICMP types trigger
an alert:
Illegal Commands (NetBIOS and SMTP)
The only legal NetBIOS commands are the following - all others are illegal.
All SMTP commands are illegal except for those displayed in the following tables.
Firewalls

Figure 9-4 Smurf Attack

Table 9-2 ICMP Commands That Trigger Alerts

5 REDIRECT
13 TIMESTAMP_REQUEST
14 TIMESTAMP_REPLY
17 ADDRESS_MASK_REQUEST
18 ADDRESS_MASK_REPLY

Table 9-3 Legal NetBIOS Commands

MESSAGE:
REQUEST:
POSITIVE:
NEGATIVE:
RETARGET:
KEEPALIVE:
ZyWALL 5 Internet Security Appliance
9-5