Netopia 2200 manual 157, filtering rule, Parts of a filter

Firewall Tutorial

A filtering rule

The criteria are based on information contained in the packets. A filter is simply a rule that prescribes certain actions based on certain conditions. For example, the following rule qualifies as a filter:

“Block all Telnet attempts that originate from the remote host 199.211.211.17.”

This rule applies to Telnet packets that come from a host with the IP address 199.211.211.17. If a match occurs, the packet is blocked.

Here is what this rule looks like when implemented as a filter in Netopia Firmware Version 7.6:

To understand this particular fil- ter, look at the parts of a filter.

Parts of a filter

A filter consists of criteria based on packet attributes. A typical fil- ter can match a packet on any one of the following attributes:

•The source IP address and subnet mask (where the packet was sent from)

•The destination IP address and subnet mask (where the packet is going)

•The TOS bit setting of the packet. Certain types of IP packets, such as voice or mul- timedia packets, are sensi-

tive to delays introduced by the network. A delay-sensitive packet is identified by a special low-latency setting called the TOS bit. It is important for such packets to be received rapidly or the quality of service degrades.

•The type of higher-layer Internet protocol the packet is carrying, such as TCP or UDP

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