Re: how nmap can know my firewalled servers ?
- From: "Arturas Zalenekas" <security@xxxxxxxxxxxxx>
- Date: Thu, 13 Apr 2006 13:54:08 -0500 (CDT)
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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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