General Questions

I want  monitor the traffic from ever IP,so I run in exec.php

ipfw pipe 100  config mask dst-ip 0xffffffff
ipfw add 30000 pipe 100 ip from any to any out via vr1
ipfw pipe show 100

Then I can see the every IP's traffic .but ,when I clear it

ipfw pipe zero 100

The system tell me "ipfw pipe zero 100
ipfw: rule 100: setsockopt(IP_FW_ZERO): Invalid argument"

Why and How can I zero cleaning?My version is 1.235
Thank you very much!