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filters to selectively admit or refuse TCP/IP connections from certain remote networks and specific hosts. You
will also use filters to screen particular types of connections. This is commonly called firewalling your network.
Before creating filtersets, you should read the next few sections to learn more about how these powerful secu-
rity tools work.
Parts of a filter
A filter consists of criteria based on packet attributes. A typical filter can match a packet on any one of the fol-
lowing attributes:
The source IP address (where the packet was sent from)
The destination IP address (where the packet is going)
The type of higher-layer Internet protocol the packet is carrying, such as TCP or UDP
Other filter attributes
There are three other attributes to each filter:
The filter’s order (i.e., priority) in the filterset
Whether the filter is currently active
Whether the filter is set to forward packets or to block (discard) packets
Design guidelines
Careful thought must go into designing a new filterset. You should consider the following guidelines:
Be sure the filterset’s overall purpose is clear from the beginning. A vague purpose can lead to a faulty set,
and that can actually make your network less secure.
Be sure each individual filter’s purpose is clear.
Determine how filter priority will affect the set’s actions. Test the set (on paper) by determining how the fil-
ters would respond to a number of different hypothetical packets.
Consider the combined effect of the filters. If every filter in a set fails to match on a particular packet, the
packet is:
• Forwarded if all the filters are configured to discard (not forward)
• Discarded if all the filters are configured to forward
• Discarded if the set contains a combination of forward and discard filters
An approach to using filters
The ultimate goal of network security is to prevent unauthorized access to the network without compromising
authorized access. Using filtersets is part of reaching that goal.
Each filterset you design will be based on one of the following approaches:
That which is not expressly prohibited is permitted.
That which is not expressly permitted is prohibited.
It is strongly recommended that you take the latter, and safer, approach to all of your filterset designs.
Summary of Contents for NVG595
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