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126Chapter 14 - Filters and Rules
Cyclades-PR4000
Exterior Router
The exterior router is the network’s first defense against attacks. For this reason, it is reasonable to prohibit all
packets except for those explicitly allowed. This is done by choosing the
Default Scope
to be
Deny
. Thus, ALL
desired traffic must be expressly allowed by the rules in the rule list.
Let
e-mail in
W
o
r
l
d
o
f
P
o
s
s
i
b
l
e
P
a
c
k
e
t
s
Let
e-mail out
DENY
DENY
DENY
Let Telnet
Connections Out
FIGURE 12.3 DENY AS DEFAULT SCOPE
In Figure 12.3, a conceptual equivalent of the interface is shown. All packets except those which fall into the holes
in the ball will be denied entry in to or out of the network.