NMAP probing of network ports

Garrett Cooper youshi10 at u.washington.edu
Thu Sep 15 11:56:48 PDT 2005


On Sep 15, 2005, at 11:43 AM, Boris Karloff wrote:

> Hello:
>
> How do I cause freeBSD 5.4 to not respond to an nmap
> inquiry? I have already tried creating a line in rc.firewall
> that says:
>
> ${fwcmd} deny all from any to any
> ${fwcmd} drop all from any to any
>
> I know these are active, since 1) I see them on the screen
> at startup, and 2) pinging from any computer to any computer
> results in a timeout.
>
> (both of these should drop all TCP packets; but apparently,
> they cause a RESET message to be sent.)
>
> I've also tried adding the following to sysctl.conf:
>
> net.inet.tcp.blackhole=2
> net.inet.udp.blackhole=1
>
> Again, these don't seem to prevent my freeBSD from sending a
> packet (probably a RESET or UNREACHABLE-HOST ack).
>
> Once the person sending the nmap to this machine has the IP,
> its a simple step for them to ip-flood this machine; or
> worse.
>
> How do I make freeBSD not acknowledge the fingerprint from
> nmap?
>
> Thanks in advance.
>
> Harold.

     One thing to note is that if you have a listening server, nmap  
will always get a response regardless of whether or not you want it  
to because that's how servers function (unless you block the traffic  
completely which is silly because then no one could connect to your  
machine from anywhere). As for ICMP traffic, you should block ICMP if  
you don't want to send ping replies, etc.
-Garrett


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