Chapter 14 Firewalls

14.4.2.1 ICMP Vulnerability

ICMP is an error-reporting protocol that works in concert with IP. The following ICMP types trigger an alert:

Table 75 ICMP Commands That Trigger Alerts

5REDIRECT

13TIMESTAMP_REQUEST

14TIMESTAMP_REPLY

17ADDRESS_MASK_REQUEST

18ADDRESS_MASK_REPLY

14.4.2.2 Illegal Commands (NetBIOS and SMTP)

The only legal NetBIOS commands are the following - all others are illegal.

Table 76 Legal NetBIOS Commands

MESSAGE:

REQUEST:

POSITIVE:

VE:

RETARGET:

KEEPALIVE:

All SMTP commands are illegal except for those displayed in the following tables.

Table 77 Legal SMTP Commands

AUTH

DATA

EHLO

ETRN

EXPN

HELO

HELP

MAIL

NOOP

 

 

 

 

 

 

 

 

 

QUIT

RCPT

RSET

SAML

SEND

SOML

TURN

VRFY

 

 

 

 

 

 

 

 

 

 

14.4.2.3 Traceroute

Traceroute is a utility used to determine the path a packet takes between two endpoints. Sometimes when a packet filter firewall is configured incorrectly an attacker can traceroute the firewall gaining knowledge of the network topology inside the firewall.

Often, many DoS attacks also employ a technique known as "IP Spoofing" as part of their attack. IP Spoofing may be used to break into systems, to hide the hacker's identity, or to magnify the effect of the DoS attack. IP Spoofing is a technique used to gain unauthorized access to computers by tricking a router or firewall into thinking that the communications are coming from within the trusted network. To engage in IP spoofing, a hacker must modify the packet headers so that it appears that the packets originate from a trusted host and should be allowed through the router or firewall. The ZyXEL Device blocks all IP Spoofing attempts.

14.5 Stateful Inspection

With stateful inspection, fields of the packets are compared to packets that are already known to be trusted. For example, if you access some outside service, the proxy server remembers things about your original request, like the port number and source and destination addresses. This “remembering” is called saving the state. When the outside system responds to your request, the firewall compares the received packets with the saved state to determine if they

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P-2602H(W)(L)-DxA User’s Guide