Re: LAN Recognition Problem
On Wed, 16 Oct 2013 13:53:52 -0400
"Thomas H. George" <lists@tomgeorge.info> 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.
>
> The desktop cannot successfully ping the RaspberryPi but can
> successfully ping the printer and the gateway.
>
> 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.
>
> Since both computers use the Wheezy operating system I am posting this
> message on the debian-user mailing list in hopes of getting some clues
> as to the source of the problem.
>
Wheezy is the current Debian stable distribution, and while a network
stack problem at that level is not utterly impossible, it is
extraordinarily unlikely.
Asking about a firewall is the correct response, though it is the Pi
which should be the first object of attention, not the desktop. Where
ping is deliberately restricted, it is replying that is prevented, not
sending.
Ping requires routing and hardware to work in *both* directions, which
is sometimes forgotten. If A can get a ping reply from B, then routing
and the physical network path are correct for A *and* B, or at least
adequate for this job. If B cannot get a ping reply from A, then
something is specifically preventing the reply, and that is usually a
firewall. If you had iptables scripts running on both machines you
could log ICMP packets in and out and see quickly what is going on, but
it seems you haven't yet.
If you are certain there is no firewall involved, the first thing to do
is to power cycle everything (not just reboot, and include any network
switch separate from the router) a couple of times. I have to say that
if there is a hardware or initialisation issue here, the Pi is the most
likely suspect.
The next check is whether the TCP/IP configuration of *all* the network
machines is sane and compatible. No duplicated IP addresses, all
netmasks the same. Have a look at the arp tables (sudo arp -a) of both
suspect machines quickly after trying the pings. See if both machines
can see the others' MAC addresses, and that they are consistent.
If you're using the 10. network, the netmask is /8, not anything else.
Yes, I know that's an old one, but Windows 7 still contains Edlin. I've
*seen* very odd networking trouble caused by 10. with other netmasks,
it was a long time ago, but I have no reason to believe it can't still
happen. Some helpful people once hardcoded 10. as a Class A (/8)
network in a few places.
If you're still getting nowhere, it might be time to learn how to drive
Wireshark, after which you will find that the problem is a firewall...
--
Joe
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