If I had to block port 144 for udp on eth1 in an active way, for packets with source port 8080, but block it actively so not silently ignore, but to send active signal to the sender of this packet that this port is blocked, how would I do it under linux?


Your iptables rule should REJECT with an ICMP port unreachable, e.g.:

-j REJECT --reject-with icmp-port-unreachable

or for IPV6:

-j REJECT --reject-with icmp6-port-unreachable

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