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Security 13-5
A filter set is a group of filters that work together to check incoming or outgoing data. A filter set can consist of
a combination of input and output filters.
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A filter set acts like a team of customs inspectors. Each filter is an inspector through which incoming and
outgoing packages must pass. The inspectors work as a team, but each inspects every package individually.
Each inspector has a specific task. One inspector’s task may be to examine the destination address of all
outgoing packages. That inspector looks for a certain destination—which could be as specific as a street
address or as broad as an entire country—and checks each package’s destination address to see if it matches
that destination.
A filter inspects data packets like a customs inspector scrutinizing packages.
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Continuing the customs inspectors analogy, imagine the inspectors lined up to examine a package. If the
package matches the first inspector’s criteria, the package is either rejected or passed on to its destination,
depending on the first inspector’s particular orders. In this case, the package is never seen by the remaining
inspectors.
INSPECTOR
FROM:
TO:
FROM:
TO:
FROM:
TO:
APPROVED