ZyXEL Communications ZYWALL10 - Manual (Page 172)

ZyWALL 10 Internet Security Gateway
Introducing the ZyWALL Firewall 14-3
ICMP Echo
A brute-force attack, such as a "Smurf" attack, targets a feature in the IP specification known as directed or
subnet broadcasting, to quickly flood the target network with useless data. A Smurf hacker floods a router
with Internet Control Message Protocol (ICMP) echo request packets (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:
Table 14-1 ICMP Commands That Trigger Alerts
5 REDIRECT
13 TIMESTAMP_REQUEST
14 TIMESTAMP_REPLY
17 ADDRESS_MASK_REQUEST
18 ADDRESS_MASK_REPLY
Illegal Commands (NetBIOS and SMTP)
The only legal NetBIOS commands are the following - all others are illegal.
Table 14-2 Legal NetBIOS Commands
MESSAGE:
REQUEST:
POSITIVE:
NEGATIVE:
RETARGET:
KEEPALIVE:
All SMTP commands are illegal except for those displayed in the following tables.
Table 14-3 Legal SMTP Commands
AUTH DATA EHLO ETRN EXPN HELO HELP MAIL NOOP
QUIT RCPT RSET SAML SEND SOML TURN VRFY BDAT