ACLs for the USG2000&5000

An access control list (ACL) is a general tool for traffic matching. It can filter and match traffic in terms of MAC addresses, IP addresses, protocols, and time ranges.

ACL Rule and Matching Order
In common cases, any security function can reference multiple ACLs. Therefore, overlaps and conflicts may occur among the traffic defined by these ACLs.

Additionally, to effectively use the ACL, an ACL contains multiple ACL rules, each of which can specify certain traffic, and define the permit or deny action accordingly. As a result, the traffic defined by these rules may overlap and actions for overlapped traffic may conflict with each other.

Therefore, it is necessary to specify the matching orders of ACLs and of multiple rules in an ACL. The matching orders on the USG are as follows:
? ACLs applied to the same function in the same direction are matched according to the configuration time. The earlier the ACL is created; the earlier it is matched. Once the matching succeeds, no subsequent matching is performed.
? ACL rules in the same ACL are matched according to the specified matching type. Two matching types are available:
? Automatic order: indicates automatic matching. It is also called minimal matching or in-depth matching. Actions are performed according to the rule with the minimal matching range. For example, rule 1 allows packets at 192.168.1.0/24 through; rule 2 denies packets at 192.168.1.100. In this case, the final action for packets at 192.168.1.100 is deny. This is because the IP address range specified by rule 2 is smaller and more accurate.
? Configuration order: indicates that ACL rules are matched based on the rule ID. It is the default matching mode. The smaller the rule ID is; the earlier the matching occurs. Once the matching succeeds, no subsequent matching is performed.
Step and Dynamic Insertion of an ACL Rule
After an ACL rule is created, its ID cannot be changed. Therefore, it is difficult for you to manually adjust matching orders of rules in ACLs in configuration order mode. You can only delete existing rules and create new ones. To address this issue, the step function is added.

During the creation of an ACL rule, if no rule ID is specified, the system automatically assigns a rule ID. Rule IDs increase based on the step.

For example, the step is 5. If you create a rule but do not assign a rule ID, the system automatically assigns the minimal ID (which is larger than that of the previous rule and its number takes 5 as the base and increases by 5) to the rule. Suppose that you do not specify the rule ID for rule 1, the system assigns 5 to the rule. When creating rule 2, you assign 12 to it. Then you do not specify the rule ID for rule 3. In this case, the system assigns 15 (larger than 12) to it. Therefore, the IDs of three rules in the ACL are 5, 12, and 15 respectively.

After the step mechanism is used, rule IDs are reserved for rules in an ACL for the further use. In this example, to ensure that rule 4 takes effect between rule 2 and rule 3, you can specify 13 as the ID for the rule during the creation.

Through the dynamic insertion of new rules between two rules, you can control the valid sequences of rules in the ACL.

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