Re: nmap.bind to set NSE socket source address

[David Fifield <david () bamsoftware com>]

On Sun, Oct 11, 2009 at 06:48:49PM -0500, Ron wrote:
> On 10/11/2009 06:11 PM, David Fifield wrote:
> > Ron, I added the function nmap.bind in r15813. In the DHCP script you
> > will use it like this:
> >
> >      try(socket:bind(nil, 68))
> >
> > Let me know if it provides what you need.
>
> The changes work as advertised, thanks for making them!
>
> I am, however, still having a problem, and it's because of something I  
> hadn't counted on in the protocol. When I wrote the dhcp-discover  
> script, I was using DHCPINFORM, which comes directly back to my ip  
> address. However, I just realized that DHCPDISCOVER doesn't come back to  
> me; rather, it comes back to 255.255.255.255.
>
> Take a look at the attached .pcap file -- I did DHCPINFORM, which worked  
> fine with the changes, then I did DHCPDISCOVER, which didn't.
>
> Do you know if there's any way to receive broadcast packets without a pcap?

The DHCP server in my router sends back a unicast DHCP offer in response
to DHCPDISCOVER, so it must not be every server that does that.

I think what you would normally do is make a socket bound to
255.255.255.255:67 and try to receive on it. In Python this works:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM, socket.IPPROTO_UDP)
s.bind("255.255.255.255", 67)
print s.recv()

But that currently won't work in NSE, because the socket descriptor
isn't created until a connection is attempted. There's no way to create
a socket without also connecting it. Something about the connection
keeps the receive from working. This is what I tried:

s = nmap.new_socket()
assert(s:bind("255.255.255.255", 67))
assert(s:connect(host.ip, 68, "udp"))
status, data = s:receive()

So we would have to rearchitect part of Nsock to allocate a socket on
nsi_new, which I think is pretty reasonable.

David Fifield

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