Re: how nmap can know my firewalled servers ?

["Arturas Zalenekas" <security () zalenekas net>]

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Just a thought .... :)
How does TCP protocol work !? How should it replay, e.g. to a closed port
!? With ICMP packet !? No, it has to answer with an RST.
So, your service, e.g. telnet, is running on external interface and on
localhost (loopback). Set the iptables INPUT chain so, that on the
external interface packets will be rejected with RST packets and packets
on the loopback interface will be allowed.
But I don't really understand the purpose of your test, b/c there are a
lot of different techniques to identify, if a service will be protected.
It doesn't matter if it a real FW or a router with ACL etc.


- --
Kind regards,
Arturas Zalenekas
Network Security Engineer and Analyst


On Thu, April 13, 2006 04:59, Alexey Eremenko wrote:
> > I am assuming you are using a DROP rule on your firewall.  NMAP knows
> > that if it does not receive a response for a TCP connection then it is
> > firewalled.  Dropping traffic at a firewall violates RFC and makes it
> > much easier to know when there is a firewall between the scanner and the
> > end host.  I recommend using REJECT
> >
> > -A INPUT -j REJECT --reject-with icmp-host-unreachable
> >
> > That will conform to RFC (I'm pretty sure) and will make it harder to
> > detect a firewall with NMAP.
>
> True, I am using DROP state on my iptables, but even when I changed
> the state of my firewall to "REJECT --reject-with
> icmp-host-unreachable" - nmap _still_ knows that my services are
> running (telnet) and are filtered !
>
> linux:/ # iptables -A INPUT -p tcp --dport 23 -j REJECT --reject-with
> icmp-host-unreachable
> linux:/ # nmap  localhost
>
> Starting nmap 3.81 ( http://www.insecure.org/nmap/ ) at 2006-04-13 12:59
> GMT+2
> Interesting ports on localhost (127.0.0.1):
> (The 1648 ports scanned but not shown below are in state: closed)
> PORT      STATE    SERVICE
> 22/tcp    open     ssh
> 23/tcp    filtered telnet
> ...
>
> linux:/ # iptables -L
> Chain INPUT (policy ACCEPT)
> target     prot opt source               destination
> REJECT     tcp  --  anywhere             anywhere            tcp
> dpt:telnet reject-with icmp-host-unreachable
>
> Chain FORWARD (policy ACCEPT)
> target     prot opt source               destination
>
> Chain OUTPUT (policy ACCEPT)
> target     prot opt source               destination
>
>
> So how do I fool nmap that my service is really down ?




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