RE: Iptables DROP packets but Nmap show the ports opened !!

That’s a correct behavior of iptables. If you want another error behavior use: “ …. –j REJECT –reject-with icmp-host-unreachable” instead.

From man:

REJECT

This is used to send back an error packet in response to the matched packet: otherwise it is equivalent to DROP so

it is a terminating TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD and OUTPUT

chains, and user-defined chains which are only called from those chains. The following option controls the nature

of the error packet returned:

--reject-with type

The type given can be

icmp-net-unreachable

icmp-host-unreachable

icmp-port-unreachable

icmp-proto-unreachable

icmp-net-prohibited

icmp-host-prohibited or

icmp-admin-prohibited (*)

which return the appropriate ICMP error message (port-unreachable is the default). The option tcp-reset

can be used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back. This

is mainly useful for blocking ident (113/tcp) probes which frequently occur when sending mail to broken

mail hosts (which won't accept your mail otherwise).

(*) Using icmp-admin-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT

Saludos. Pablo.

De: Robin-Vinet Mathieu [mailto:robin@elda.org]
Enviado el: Jueves, 06 de Abril de 2006 11:07 a.m.
Para: debian-firewall@lists.debian.org
Asunto: Iptables DROP packets but Nmap show the ports opened !!

Hi,

I've got a question, about how DROPPED packets are shown to TCP scanners such as Nmap.

I've done an IPtables script wich does what i want it to do, but even if unautorised packets are dropped and logged, when i nmap my server, almost all tcp ports are shown as opened.
Of course, some of those ports are (eg. TCP 80), but others are not (eg. TCP 445), i think it is clearly unsafe, cause hackers knows that there is a server behind those closed ports.
In my mind, a good firewall would show the firewalled TCP ports as "stealth" or "filtered" or in the last "closed", but i'd prefer "stealth".

Is it normal ? If not, do you know how can i solve that ?

Thanks a lot.

Regards,

--
Robin-Vinet Mathieu