Security 15-5
How filter sets work
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
INSPECTOR
FROM:
APPROVET : D
FROM:
TO:
FROM:
TO:
A filter inspects data packets like a customs inspector scrutinizing packages.
Filter priority
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.