Table 14-1 Icmp Commands That Trigger Alerts; Table 14-2 Legal Netbios Commands; Table 14-3 Legal Smtp Commands; Icmp Vulnerability - ZyXEL Communications ZyWALL 10 User Manual

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:
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.
AUTH DATA
QUIT RCPT
Introducing the ZyWALL Firewall

Table 14-1 ICMP Commands That Trigger Alerts

5 REDIRECT
13 TIMESTAMP_REQUEST
14 TIMESTAMP_REPLY
17 ADDRESS_MASK_REQUEST
18 ADDRESS_MASK_REPLY

Table 14-2 Legal NetBIOS Commands

MESSAGE
REQUEST:
POSITIVE:
NEGATIVE:
RETARGET:
KEEPALIVE

Table 14-3 Legal SMTP Commands

EHLO ETRN
RSET SAML
ZyWALL 10 Internet Security Gateway
EXPN HELO
SEND SOML
HELP MAIL
TURN VRFY
NOOP
14-3