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Introduction 39
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Rule
Each chain contains a sequence of rules that control filtering. The rules
address the following issues:
• How the packet should appear in order to match the rule
Some information about the packet is checked according to the rule, for
example, the IP header, the input and output interfaces, the TCP flags and
the protocol.
• What to do when the packet matches the rule
The packet can be accepted, blocked, logged, or jumped to a user-defined
chain.
When a chain is analyzed, the rules of this chain are reviewed one-by-one
until the packet matches one rule. If no rule is found, the default action
for that chain will be taken.
Table 1-15: Levels of IP Filtering (Continued)