Firewall Design Rules; Firewall Logic - Netopia 2200 series Software User's Manual

Example TCP/UDP Ports

Firewall design rules

There are two basic rules to firewall design:
• "What is not explicitly allowed is denied."
and
• "What is not explicitly denied is allowed."
The first rule is far more secure, and is the best approach to firewall design. It is far easier
(and more secure) to allow in or out only certain services and deny anything else. If the
other rule is used, you would have to figure out everything that you want to disallow, now
and in the future.

Firewall Logic

Firewall design is a test of logic, and filter rule ordering is critical. If a packet is forwarded
through a series of filter rules and then the packet matches a rule, the appropriate action
is taken. The packet will not forward through the remainder of the filter rules.
For example, if you had the following filter set...
Allow WWW access;
Allow FTP access;
Allow SMTP access;
Deny all other packets.
148
TCP Port Service
20/21 FTP
23 Telnet
25 SMTP
80 WWW
144 News
UDP Port Service
161 SNMP
69 TFTP