Packet Filtering :
Terms associated:
- Source IP address
- Destination IP address
- Source port #
- Destination port #
- protocol
- interface
Many commercial routers offer a mechanism that augments normal routing and permits a manager to further control packet processing. Casually called a packet filter, the method requires the manager to specify how the router should dispose of each datagram. For example, the manager may choose to filter (i.e.. block) all datagrams that come from a particular source or those used by a particular application, while selecting to route other datagrams to their destination.
The term packet filter arises because the filtering mechanism does not keep a record of interaction or a history of previous datagrams. In its place, the filter considers each datagrams separately. When a datagram 1st arrives, the router passes the datagram through its packet filter before performing any other processing. If the filter discards the datagram, then the router drops it immediately.
For example, In general I won't allow TFTP, open win, RPC, rlogin, rsh message packets to pass through the router whether from inside or outside and router just discards these packets. But It may put some restrictions on telnet, ftp, http, and smtp message packets in order to pass through the router and therefore some processing is to be done before discarding or allowing these packets.
Because TCP/IP does not dictate a standard for packet filters, every router vendor is free to choose the capabilities of their packet filter as well as the interface the manager uses to configure the filter. Some routers allow a manager to configure separate filter actions for each interface, whereas others have a single configuration for all interfaces. Generally, when specifying datagrams that the filter should block, a manager may list any combination of source IP address, destination IP address, source protocol port number, protocol and last one is destination protocol port number. Thus, these filtering rules may become trickier with complex network policies.
Because, Filtering rules are based on port numbers, there is a trouble with RPC applications. 1st, the number of well-known ports is big and growing. Therefore, a manager would need to update such a list repeatedly because a simple error of omission could leave the firewall vulnerable. 2nd, much of the traffic on an internet does not travel to or from a well-known port. In addition to programmers who could choose port numbers for their private client-server applications, services such as Remote Procedure Call (RPC) assigns port dynamically. 3rd, hearing ports of well-known services leaves the firewall vulnerable to tunneling, a technique in which one datagram is temporarily encapsulated in another for transfer across part of an internet.
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