nmap and icmp-replies

Hi list,

while scanning somes hosts I figured out that a scanned packet filter 
sending back icmp destination-unreachable rather slow forces nmap to 
slow down scanning. A packet filter just dropping the scan probes has no 
effect on nmap, so does a host without any packet filter, just sending RST.
My problem is: filtering out the icmp answers with iptables doesn't 
work, as nmap seems to hook into the kernel before iptable does. Is 
there any to tell nmap to ignore icmp replies and just to determine a 
port status via TCP/UDP?

Thanks in advance

Meier

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