Re: port scan--"filtered" ports

On Thu, Jul 15, 2010 at 12:47 AM, Naruto Uzumaki <ageofnaruto@xxxxxxxxx> wrote:
When performing TCP port scanning Nmap marks a port filtered if it
either gets an ICMP port unreachable or no response. Now, this could
be because either a firewall or the scanning host is generating ICMP
ports or silently dropping packets.

By marking them as filtered does it mean that there could be a service
running on these ports but that service is only accessible by internal
users or limited public IPs and blocked for other hosts?


Firewall can be the reason.
While scanning the host with SYN scan and if it's showing the target
port as 'filtered', you can scan the same again with ACK scan
type(using -sA switch in nmap).

In case, there is a firewall, ACK packet is simply supposed to be
dropped, so would show it as 'filtered' and you can be sure that there
*is* a firewall.
And if, it receives RST in response, it would show you 'unfiltered'.


Sagar Belure
Security Analyst
Secfence Technologies

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