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ZyWALL 5/35/70 Series User’s Guide
Chapter 11 Firewall Screens 216
11.3 Rule Logic Overview
Note: Study these points carefully before configuring rules.
11.3.1 Rule Checklist
1 State the intent of the rule. For example, This restricts all IRC access from the LAN to the
Internet. Or, This allows a remote Lotus Notes server to synchronize over the Internet to
an inside Notes server.
2 Is the intent of the rule to forward or block traffic?
3 What direction of traffic does the rule apply to (see Section 10.2 on page 202)?
4 What IP services will be affected?
5 What computers on the LAN or DMZ are to be affected (if any)?
6 What computers on the Internet will be affected? The more specific, the better. For
example, if traffic is being allowed from the Internet to the LAN, it is better to allow only
certain machines on the Internet to access the LAN.
11.3.2 Security Ramifications
Once the logic of the rule has been defined, it is critical to consider the security ramifications
created by the rule:
1 Does this rule stop LAN users from accessing critical resources on the Internet? For
example, if IRC is blocked, are there users that require this service?
2 Is it possible to modify the rule to be more specific? For example, if IRC is blocked for all
users, will a rule that blocks just certain users be more effective?
3 Does a rule that allows Internet users access to resources on the LAN create a security
vulnerability? For example, if FTP ports (TCP 20, 21) are allowed from the Internet to the
LAN, Internet users may be able to connect to computers with running FTP servers.
4 Does this rule conflict with any existing rules?
Once these questions have been answered, adding rules is simply a matter of plugging the
information into the correct fields in the web configurator screens.
11.3.3 Key Fields For Configuring Rules
11.3.3.1 Action
Should the action be to Drop, Reject or Permit?
Note: “Drop” means the firewall silently discards the packet. “Reject” means the
firewall discards packets and sends an ICMP destination-unreachable
message to the sender.