Re: LAN Recognition Problem
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
--
Karl E. Jorgensen
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