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.
