Re: Weird Nmap Behavior

[yaroslav <ambivalenced () gmail com>]

Hi arvind.

from little research with tcpdump and nmap 4.85 it seems like
standart nmap behaviour, I think, taking into account that -PN switch
was specified. this is the cause that nmap threat all hosts alive
_except_ LAN. When you're scanning host in the LAN, firstly ARP
request issued to obtain ethernet-address of host in question. and if
it fails - host marked as down. In case of WAN you can not get any ARP
responces, thereby you (well...nmap) has nothing more than assume host
alive.
to deal with this, read nmap man page, section "HOST DISCOVERY". there
are some advices to deal with different situations, some may be
applicable to you.

On Mon, Oct 5, 2009 at 9:38 PM, arvind doraiswamy
<arvind.doraiswamy () gmail com> wrote:
> Hey Pplz,
> I wanted to check if any of you guys have come across this behavior.
> We routinely scan large networks using Nmap - so we thought we'd use
> it to also try and discover what IP's were live.
>
> Now note that this discussion covers hosts on the Internet and not on
> the LAN. So while testing out Nmap 4.76/5.00 we scanned one of our own
> IP ranges to check if it detected what was up and what was down.
>
> Now note that we know for a fact that out of the 16 IP's we scanned
> not all were live. So we did expect atleast some to be down. But
> strangely Nmap said that all 16 IP's were "up". Sure all ports were
> filtered - but the IP's were up. We're running SYN scans with a -PN
> switch as well and am quite sure it wasn't our firewall doing this -
> because we weren't doing any blocking as such( 3 IP's were live -
> ping).
>
> Now I'm a little confused - Firstly ofcourse an IP can be live while
> having say 65535 ports filtered coz its behind a firewall. Which then
> brings me to the next 2 questions:
> --- If every port is filtered and ping is blocked(Internet) how does
> Nmap decide that a host is up?
> --- How would you explain behavior like the above where I know for a
> fact an IP hasn't been assigned to a server/device/anything?
>
> Lastly if I want to test known "down" IP's are there any such IP's?
> Not misspelt domain names as of now - just test "down" IP addresses.
>
> Finally if this behavior for Nmap is how it is and can't be
> changed(due to whatever stack dependencies etc , just shooting in the
> air here) isn't this giving in accurate results? What is a workaround?
>
> Thnx
> Arvind
>
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