Basics of In-Vehicle Ethernet Switches: What is ACL (Access Control List)?

An ACL (Access Control List) is a list required to enforce control of network traffic. This list allows you to journal packets that pass through or reject packets. It also contributes to improving security and optimizing the network by controlling network communications. For example, blocking Access from specific IP addresses can prevent unauthorized Access and attacks.

Standard ACLs can restrict communications from specific source addresses, making it possible to block Access from specific hosts or networks. The control method is to process rules that allow or deny communications in order, and control communications based on the first rule that matches. Therefore, incorrect or inappropriate configurations can lead to blocking legitimate communications or allowing illegitimate communications. It is important to set the order and priority of rules appropriately.

Specifically, it is controlled as follows. Look at the packet from PC1 in the diagram below. This IP address is specified as denied on the first line, but allowed on the third line. However, since the conditions are checked in order from the first line as an ACL rule, if a denial is found on the first line, the conditions that follow are not applied. As a result, the setting on the third line is not reflected, and packets from PC1 are denied.

It is important to note that ACLs have an implicit rule called "deny any." "deny any" is automatically added to the last line of the created ACL. This function also sets the rule to deny all packets. Packets that you want to allow must be set in the ACL.