Chapter 8 Packet Filters
P-660R-F1 Series User’s Guide
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Packet Filtering
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The router filters packets as they pass through the router’s interface according to the filter rules
you designed.
•
Packet filtering is a powerful tool, yet can be complex to configure and maintain, especially if you
need a chain of rules to filter a service.
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Packet filtering only checks the header portion of an IP packet.
When To Use Filtering
1
To block/allow LAN packets by their MAC addresses.
2
To block/allow special IP packets which are neither TCP nor UDP, nor ICMP packets.
3
To block/allow both inbound (WAN to LAN) and outbound (LAN to WAN) traffic between the specific
inside host/network "A" and outside host/network "B". If the filter blocks the traffic from A to B, it
also blocks the traffic from B to A. Filters cannot distinguish traffic originating from an inside host or
an outside host by IP address.
4
To block/allow IP trace route.
Firewall
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The firewall inspects packet contents as well as their source and destination addresses. Firewalls
of this type employ an inspection module, applicable to all protocols, that understands data in the
packet is intended for other layers, from the network layer (IP headers) up to the application
layer.
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The firewall performs stateful inspection. It takes into account the state of connections it handles
so that, for example, a legitimate incoming packet can be matched with the outbound request for
that packet and allowed in. Conversely, an incoming packet masquerading as a response to a
non-existent outbound request can be blocked.
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The firewall uses session filtering, i.e., smart rules, that enhance the filtering process and control
the network session rather than control individual packets in a session.
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The firewall provides e-mail service to notify you of routine reports and when alerts occur.
When To Use The Firewall
1
To prevent DoS attacks and prevent hackers cracking your network.
2
A range of source and destination IP addresses as well as port numbers can be specified within one
firewall rule making the firewall a better choice when complex rules are required.
3
To selectively block/allow inbound or outbound traffic between inside host/networks and outside
host/networks. Remember that filters cannot distinguish traffic originating from an inside host or an
outside host by IP address.
4
The firewall performs better than filtering if you need to check many rules.
5
Use the firewall if you need routine e-mail reports about your system or need to be alerted when
attacks occur.
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