Re: NMAP shows local machine as down

[luto () stanford edu]

Quoting micro dev <microdev1 () yahoo com>:

> Thanks kevin.
> Where can I find that doc. Please give me the pointer to that doc.

[snip]

>  
> I tried to snoop the packets thru Ethereal. And what I deduce that I
> always see this stuff.
> Source - 10.25.125.203
> Destination - 11.25.125.203 
> Protocol - ICMP
> Info - Echo (ping) reply
>  


I don't remember, but since I wrote it, here goes ;)

winpcap (up to v2.3 at least -- i haven't played with v3) can neither 
monitor nor inject packets into the loopback interface.  For some 
reason I couldn't get winpcap to scan localhost by using a different 
interface either -- I assume that windows does not see injected packets 
as recieved, even though ethereal (which uses winpcap) does, so these 
packets are effectively dropped -- so there is no way (other than 
connect()) to scan localhost.  SOCK_RAW will not bind localhost on 
win2k or xp.

Ugly diagram of what might be happening (assuming IP 10.0.0.1)

IP stack                winpcap layer                  NIC
10.0.0.1->10.0.0.2 -----------------------------------> (10.0.0.2 here)
                    10.0.0.1->10.0.0.1 ------------> (no listener here)
   ^                                      ^
  windows misses it                    Ethereal catches
  because there is no                  the packet here
  packet here

The strange behavior you see is due to some logic I added that, IIRC, 
falsifies the routing information for the local computer when it is in 
a range of other IPs.  I think I did this because the alternative would 
be to try to route the packets to the local machine over the localhost 
interface, which confused something.  Fixing this oddity would be 
pointless, because it is still impossible to scan the local machine.

Hope this helps.


Andy


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