Firewall Tutorial
and a packet goes through these rules destined for FTP, the packet would forward through the first rule (WWW), go through the second rule (FTP), and match this rule; the packet is allowed through.
If you had this filter set for example....
Allow WWW access;
Allow FTP access;
Deny FTP access;
Deny all other packets.
and a packet goes through these rules destined for FTP, the packet would forward through the first filter rule (WWW), match the second rule (FTP), and the packet is allowed through. Even though the next rule is to deny all FTP traffic, the FTP packet will never make it to this rule.
Implied rules
With a given set of filter rules, there is an Implied rule that may or may not be shown to the user. The implied rule tells the filter set what to do with a packet that does not match any of the filter rules. An example of implied rules is as follows:
Implied | Meaning |
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Y+Y+Y=N | If all filter rules are YES, the implied rule is NO. |
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N+N+N=Y | If all filter rules are NO, the implied rule is YES. |
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Y+N+Y=N | If a mix of YES and NO filters, the implied rule is NO. |
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