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12-24 User’s Reference Guide
Firewall Logic
Firewall design is a test of logic, and filter rule ordering is critical. If a packet is passed through a series of filter
rules and then the packet matches a rule, the appropriate action is taken. The packet will not pass through the
remainder of the filter rules.
For example, if you had the following filter set...
Allow WWW access;
Allow FTP access;
Allow SMTP access;
Deny all other packets.
and a packet goes through these rules destined for FTP, the packet would pass through the first rule (WWW), go
through the second rule (FTP), matches this rule and 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 pass 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.
Binary Representation
It is easiest when doing filtering to convert the IP address and mask in question to binary. This will allow you to
perform the logical AND to determine if a packet matches a filter rule.
Logical ANDing
When a packet is compared (in most cases) a logical AND is performed. First the IP addresses and subnet
masks are converted to binary and then ANDed together. The rules for logical ANDing are as follows:
0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1
For example:
Filter rule:
Deny
IP: 163.176.1.15 BINARY: 10100011.10110000.00000001.00001111
Mask: 255.255.255.255 BINARY: 11111111.11111111.11111111.11111111