Filter
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8-66 User’s Reference Guide
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As described above, a filter applies criteria to an IP packet and then takes one of three actions:
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■ 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.
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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 on the Netopia D-Series:
To understand this particular filter, look at the parts of an IP filter.
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There are two types if filters and filter sets: IP filters and Generic filters. The following discussion applies only to
IP filters and filter sets.
An IP filter consists of criteria based on packet attributes. A typical IP filter can match a packet on any one of
the following 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
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An IP filter can also match a packet’s port number attributes. The filter can be configured to match the
following:
■ The source port number (the port on the sending host that originated the packet)
■ The destination port number (the port on the receiving host that the packet is destined for)
+-#--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 |
+--------------------------------------------------------------------+