What action should be used to inform the source that packets reached the destination, but were not accepted?

The action to inform the source that packets reached the destination but were not accepted is to use "action=reject." When this action is implemented, it sends an ICMP (Internet Control Message Protocol) message back to the source of the traffic. This specific message indicates that the packets were received but not processed due to some criteria not being met, thus informing the sender that their traffic was rejected.

By using "action=reject," network administrators can ensure that the source is aware of the issue and may take appropriate steps to troubleshoot or correct the situation, rather than simply not knowing what happened to their packets.

In contrast, using "action=accept" would allow the packets to pass through to the destination without any notification, while "action=drop" would silently discard the packets without informing the sender of the outcome, leading to possible confusion. The "action=notify" option does not specifically correlate with standard practices of handling rejected packets in networking contexts, thereby making "action=reject" the most suitable choice for this scenario.