Packets dropped by kernel without firewall rules

[Ambrogio]

Hi all,
I'm analizing some behavior of my FC2 installation.

This is what appens:
ping an address I obtain response from the host.

When I run nmap on this host I obtain the result:
Starting nmap 3.50 ( http://www.insecure.org/nmap/ ) at 2004-07-08 16:20
CEST
Note: Host seems down. If it is really up, but blocking our ping probes,
try -P0
Nmap run completed -- 1 IP address (0 hosts up) scanned in 12.126
seconds

This is very strange because I'm able to ping the host and is directly
connected.

If I open another shell and run the tcpdump command the nmap works well

Starting nmap 3.50 ( http://www.insecure.org/nmap/ ) at 2004-07-08 16:23
CEST
Interesting ports on 168.159.176.115:
(The 1654 ports scanned but not shown below are in state: closed)
PORT     STATE SERVICE
66/tcp   open  sql*net
135/tcp  open  msrpc
139/tcp  open  netbios-ssn
445/tcp  open  microsoft-ds
8081/tcp open  blackice-icecap

Nmap run completed -- 1 IP address (1 host up) scanned in 3.021 seconds

At the end of the tcpdump sometime I obtain a lot of packets dropped by
kernel, but I don't have iptables rules activated.
Why my kernel drop some packets?
And whi nmap don't work well?

Someone know the answer?
Thanks in advance to all
Bye
	Ambrogio