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Re: Nmap says Host down when actually host is up.
From: kx <kxmail () gmail com>
Date: Fri, 26 Oct 2007 08:46:55 +0200

I can't say for ICMP, but I have definitely written a generic UDP
server on a Solaris box before that had multiple IP addresses, that
was listening on all IPs, and when the server would reply to a UDP
packet, the kernel behavior would be to reply from the IP addresses on
the Solaris box that was closest to the source, not necessarily from
the IP address it received the packet on.

Now, in this case it made sense, I would send a packet from a subnet
connected to the Solaris box, but I would send it to the IP address
not on the subnet. The response would come back from the IP address on
my subnet.

Example

Solaris has IP 10.10.1.5 and 10.10.100.5
I am IP 10.10.1.6

10.10.1.6 -- UDP --> 10.10.100.5
10.10.1.6 <-- UDP -- 10.10.1.5

Not as clear as is what is going on below, and as Kris stated, it
shouldn't happen with ICMP, but just throwing it out for
consideration.

Cheers,
  kx



On 10/26/07, Dario Ciccarone (dciccaro) <dciccaro () cisco com> wrote:
Hm. 10.10.209.18 *could be* the network address for subnet
10.10.10.209.108/30 - hosts being .109 and .110, broadcast .111 - still
wouldn't explain why .2 is replying. Funny.

Got the whole packet capture for this? The ICMP echo request should
include the whole content of the payload section of the ICMP echo
request. Can you add some payload and see what you get back ? see if it
also changes the data ?

I would theorized .2 has the wrong network mask for the subnet, the
router for .108/30 is translating the ping to a subnet-level broadcast
and .2 is replying - but using .30 implies a P2P link, not a broadcast
medium w/ multiple hosts on it . . .


Dario


-----Original Message-----
From: nmap-dev-bounces () insecure org
[mailto:nmap-dev-bounces () insecure org] On Behalf Of Fyodor
Sent: Thursday, October 25, 2007 5:23 PM
To: Swapnali
Cc: nmap-dev () insecure org
Subject: Re: Nmap says Host down when actually host is up.

On Thu, Oct 25, 2007 at 09:08:05AM -0500, Swapnali wrote:
Following is verbose output.

Nmap says Host 10.10.209.108 seems to be a subnet broadcast address
(returned 1 extra pings)

D:\>nmap -sP -vv --packet-trace 10.10.209.108

Starting Nmap 4.20 ( http://insecure.org ) at 2007-10-23
08:40 Central
Daylight
Time
SENT ( 0.2340s) ICMP 10.205.42.40 > 10.10.209.108 Echo request
(type=8/code=0) ttl=56 id=10663 iplen=28
RCVD (0.2660s) ICMP 10.204.100.2 > 10.205.42.40 Echo reply
(type=0/code=0)
ttl=249 id=10663 iplen=28

Are you sure this host is really up?  If so, it is strange that it is
replying to the ping packet from a different IP than the one the ping
was sent to.  I normally only see that with subnet-directed broadcast
addresses, so Nmap does not treat the machine as being up unless it
receives the response from the same address it sent to.  It is also
interesting that this target host apparently didn't reply to the port
80 request.  Again, are you sure it is actually up?  What OS is it
running?

Does anyone know if the RFC even allows a machine receiving an ICMP
echo request to respond from a different IP address?  I doubt that is
allowed, but I'm not certain.

Cheers,
-F

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