Motorola Enterprise Series Routers 用户手册

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 cer tain ser vices and deny anything else. If the other rule is used, you would have to figure 
out ever ything that you want to disallow, now and in the future. 

Firewall design is a test of logic, and filter rule ordering is critical. If a packet is for warded through a series of 
filter rules and then the packet matches a rule, the appropriate action is taken. The packet will not for ward 
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.

and a packet goes through these rules destined for FTP, the packet would for ward through the first rule (WWW), 
go through the second rule (FTP), and match this rule; the packet is allowed through.

If you had this filter set for example....

Allow WWW access;

Allow FTP access;

Deny FTP access;

Deny all other packets.

TCP Port

Service

UDP Port

Service

20/21

FTP

161

SNMP

23

Telnet

69

TFTP

25

SMTP

387

AURP

80

WWW

144

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