Re: LAN Recognition Problem
On Wed, Oct 16, 2013 at 08:58:36PM +0100, Karl E. Jorgensen wrote:
> Hi
>
> On Wed, Oct 16, 2013 at 01:53:52PM -0400, Thomas H. George wrote:
> > I have two computers connected to the LAN, one my desktop running Wheezy
> > and the other a RaspberryPi running Wheezy-Raspbian.
> >
> > The RaspberryPi can successfully ping all the devices on the LAN, the
> > desktop, the printer, the gateway. It immediately found the desktop's
> > hostname and sucessfully pings it by hostname.
>
> Sounds like your desktop advertises itself with e.g. avahi, and the
> RaspberryPi picks up on this.
>
> > The desktop cannot successfully ping the RaspberryPi but can
> > successfully ping the printer and the gateway.
>
> A couple of questions:
>
> (1) Are you ping'ing by IP address or by name? To separate out DNS
> problems from connectivity, it is useful to stick with IP addresses
> first. If ping by IP address works, then name resolution is a likely
> culprit.
>
> Either way, it is useful to use ping's "-n" option to avoid confusion:
> the reverse DNS lookup that ping does can impose delays which (to the
> untrained eye) can look like lack of ping responses
>
> (2) Is there a firewall on the RaspberryPi ? It may block incoming
> pings... Even when a device does not respond to ping, its presence
> can still be detected by looking at the arp cache, as it *must*
> respond to arp requests[1]. The lack of an arp response is a good
> indication of lack of physical connectivity:
>
> root@RaspberryPi# ping -c 5 -n ${ip-of-desktop}
> ... wait for ping to give up
> root@RaspberryPi# arp -an
>
> If the target IP address appears in the arp cache, then it *is* on the
> network. If it still does not respond to ping, then ping responses may
> be disabled or firewalled.
>
> (3) Have you checked for IP address collisions? If two or more
> devices have the same address weird things happen. It is worth
> cross-checking that the devices see each other with the expected MAC
> address by checking their respective arp caches. (But don't bother
> with this until all else fails - it cumbersome).
>
> > I realize this may be a problem with Wheezy-Raspbian and have posted the
> > problem to the RaspberryPi troubleshooting forum. As yet the only
> > response was to question whether the difficulty might be a firewall on
> > the desktop. I have not installed a firewall on the desktop.
>
> [1] Yes. I know. There are weird corner cases where arp responses
> undergo severe filtering in the name of security. I do not expect that
> to be the case here.
>
> Hope this helps
> --
>
Thank you for your comments. While they did not solve my problem they
made me face up to it and find a correction. In addition I learned a
little bit about arp which I had never used before.
The problem was as follows: I live in an apartment in the far corner of
the house and have strung an ethernet cable from the wireless router
which connects to the internet to my apartment. For historical reasons
the wireless route that connects to the internet has the gateway address
192.168.2.1 (When this system was set up years ago Verizon used
192.168.1.1 for their modem and we had a firewall computer behind their
modem which relayed 192.168.2.1 to 192.168.1.1)
>
In my apartment I have a five port ethernet switch.
One port for the cable to the wireless router
One port for my desktop
One port for my printer
One port to continue the cable to a pc in the room below
One port free
Then I acquired a tablet, the original HP tablet. The wireless signal
from the other end of the house didn't reach me. So I added another
wireless router but you can't do that. The new wireless router has
three ethernet ports so I connected one of these ethernet ports to the
open port on the five port switch and let this router have the address
192.168.2.1! Works beautifully. The tablet upgraded to Android
courtsey of cyanogen connects to wirelessly to the new router and via
the ethernet cables and switch to the old router and so to the internet.
Now I have a RaspberryPi. I innocently ran an ethernet cable from the
Pi to an open port on the new switch. Result: Using gateway 192.168.1.1
Pi can talk to everyone on the LAN and can reach the internet via the
old wireless router but no one on the LAN can reach the Pi.
Solution: Pull the ethernet cable to the pc downstairs from the five
position switch and connect Pi in its place. Now using gateway
192.168.2.1 Pi can talk to everyone on the LAN and everyone on the LAN
can talk to Pi.
Of course I put the ethernet cable to the computer downstairs to an open
port on the wireless router. I have no idea whether that will work but
who cares. The only thing down there is a cheap Windoze laptop
connected to the TV in case someone wants to watch Netflix. No one has
bothered in months.
Tom
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