13-6 Administration Guide
How individual filters work
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.
A filtering rule
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 4752:
To understand this par ticular filter, look at the par ts of a filter.
Parts 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
Port numbers
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 |
+--------------------------------------------------------------------+
Содержание 4752
Страница 1: ...Netopia 4752 SDSL Integrated Access Device Administration Guide ...
Страница 12: ...Administration Guide ...
Страница 18: ...2 4 Administration Guide ...
Страница 30: ...4 8 Administration Guide ...
Страница 34: ...5 4 Administration Guide ...
Страница 40: ...6 6 Administration Guide ...
Страница 58: ...Administration Guide ...
Страница 82: ...9 24 Administration Guide ...
Страница 110: ...10 28 Administration Guide ...
Страница 172: ...12 30 Administration Guide ...
Страница 206: ...13 34 Administration Guide ...
Страница 236: ...Administration Guide ...
Страница 242: ...A 6 Administration Guide ...
Страница 258: ...C 14 Administration Guide ...
Страница 264: ...E 4 Administration Guide ...
Страница 272: ...F 8 Administration Guide Speed Dialing by dialing a feature code Three Way Calling Custom Ringing Distinctive Ringing ...
Страница 284: ...2 Administration Guide ...