Security 13-7How individual filters workAs 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 packetA filter passes or blocks a packet only if it finds a match after applying its criteria. When no match occurs, thefilter ignores the packet.A filtering ruleThe criteria are based on information contained in the packets. A filter is simply a rule that prescribes certainactions 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 matchoccurs, the packet is blocked.Here is what this rule looks like when implemented as a filter on the Netopia R9100:To understand this particular filter, look at the parts of a filter.Parts of a filterA filter consists of criteria based on packet attributes. A typical filter can match a packet on any one of thefollowing 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 UDPPort numbersA filter can also match a packet’s port number attributes, but only if the filter’s protocol type is set to TCP orUDP, since only those protocols use port numbers. 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 |+--------------------------------------------------------------------+
