Stateful Inspection; Table 9-4 Legal Smtp Commands - Nortel Contivity 251 User Manual

9-8 Firewalls
Table 9-3 Legal NetBIOS Commands
POSITIVE:
NEGATIVE:
RETARGET:
KEEPALIVE:
All SMTP commands are illegal except for those displayed in the following tables.

Table 9-4 Legal SMTP Commands

AUTH DATA EHLO
QUIT RCPT RSET
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.
4. 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 Contivity 251 blocks all
IP Spoofing attempts.

9.5 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
317516-A Rev 00
ETRN EXPN HELO
SAML SEND SOML
HELP MAIL NOOP
TURN VRFY