Netopia Router PN Series Manuel d'utilisateur télécharger pdf (Page 151)

Security 7-9

How individual ﬁlters work

As described above, a ﬁlter applies criteria to an IP packet and then

takes one of three actions:

A ﬁlter’s actions

■ Passes the packet to the local or remote network

■ Blocks (discards) the packet

■ Ignores the packet

A ﬁlter passes or blocks a packet only if it ﬁnds a match after

applying its criteria. When no match occurs, the ﬁlter ignores the

packet.

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:

A ﬁltering rule

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 on the

Netopia Router:

+-#--Source IP Addr--Dest IP Addr-----Proto-Src.Port-D.Port--On?-Fwd

+--------------------------------------------------------------------+

1 199.211.211.17 0.0.0.0 TCP 23 Yes No

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

1 2 ... 146 147 148 149 150 151 152 153 154 155 156 ... 300 301

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Netopia Router PN Series Manuel d'utilisateur Page 151