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11-2 Creating Custom Rules
11.2 Rule Logic Overview
Study these points carefully before configuring rules.
11.2.1
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 is the direction connection: from the LAN to the Internet, or from the
Internet to the LAN?
4. What IP services will be affected?
5. What computers on the LAN 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.
11.2.2
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?
317516-A Rev 00
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