Руководство Пользователя для Netopia 4741 2a4na

As described above, a filter applies criteria to an IP packet and then takes one of three actions:

■

For wards the packet to the local or remote network

■

Blocks (discards) the packet

■

Ignores the packet

A filter for wards 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 cer tain 
actions based on cer tain 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 4741:

To understand this par ticular filter, look at the par ts of a filter.

A filter consists of criteria based on packet attributes. A typical 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 carr ying, such as TCP or UDP

A filter can also match a packet’s por t number attributes, but only if the filter’s protocol type is set to TCP or 
UDP, since only those protocols use por t numbers. The filter can be configured to match the following:

■

The source por t number (the por t on the sending host that originated the packet)

■

The destination por t number (the por t on the receiving host that the packet is destined for)

By matching on a por t number, a filter can be applied to selected TCP or UDP ser vices, such as Telnet, FTP, and 
World Wide Web. The following tables show a few common ser vices and their associated por t numbers:

  +-#--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  |
  +--------------------------------------------------------------------+