Can't see ADSL modem

[Timothy Murphy]

I have a small mystery on my home network.
The machines on the network all run Fedora-16 or CentOS.
All the machines except one, running CentOS-6.2,
can ping the ADSL modem at 192.168.1.254 .

The CentOS-6.2 machine does not get any response to pings
from the modem, but is able to ping sites beyond the modem.

Could some knowledgeable soul suggest a reason for this?

[fedora]

firewall? selinux?

suomi

On 02/02/2012 01:55 AM, Timothy Murphy wrote:
> I have a small mystery on my home network.
> The machines on the network all run Fedora-16 or CentOS.
> All the machines except one, running CentOS-6.2,
> can ping the ADSL modem at 192.168.1.254 .
>
> The CentOS-6.2 machine does not get any response to pings
> from the modem, but is able to ping sites beyond the modem.
>
> Could some knowledgeable soul suggest a reason for this?

[Marko Vojinovic]

On Thursday 02 February 2012 06:59:41 fedora wrote:
> firewall? selinux?

I strongly doubt that SELinux has anything to do with pinging. Firewall, OTOH, 
is a likely culprit.

Best, :-)
Marko

[Marko Vojinovic]

On Thursday 02 February 2012 00:55:31 Timothy Murphy wrote:
> I have a small mystery on my home network.
> The machines on the network all run Fedora-16 or CentOS.
> All the machines except one, running CentOS-6.2,
> can ping the ADSL modem at 192.168.1.254 .
>
> The CentOS-6.2 machine does not get any response to pings
> from the modem, but is able to ping sites beyond the modem.
>
> Could some knowledgeable soul suggest a reason for this?

Are you sure the problem is with the CentOS machine and not the router itself? 
You can try to exchange the ports for that CentOS and one Fedora machine 
(reconnect the cables at the router side), and ping again from both machines. 
If CentOS works and Fedora doesn't, the problem is with the router. I've seen 
routers do these things --- one of the ports behaves differently or has reduced 
functionality or otherwise...

OTOH, if CentOS still doesn't work, then it's most probably the firewall --- 
either the CentOS firewall or the router firewall. Check them both for dropping 
ICMP packets.

I cannot think of any other problem. Can CentOS ping other machines on your 
LAN?

HTH, :-)
Marko

[Timothy Murphy]

Marko Vojinovic wrote:
> > I have a small mystery on my home network.

...
> > The CentOS-6.2 machine does not get any response to pings
> > from the modem, but is able to ping sites beyond the modem.
> >
> > Could some knowledgeable soul suggest a reason for this?
>
> Are you sure the problem is with the CentOS machine and not the router
> itself?

...
> OTOH, if CentOS still doesn't work, then it's most probably the firewall
> --- either the CentOS firewall or the router firewall. Check them both for
> dropping ICMP packets.
>
> I cannot think of any other problem. Can CentOS ping other machines on
> your LAN?

Thanks for all the responses.

I have to confess that my error was probably at kindergarten level ...
The modem is at 192.168.1.254 ,
while my LAN is 192.168.2.0 , which the computer accesses through eth1 .

I found (from "route") that this computer was trying to send 
packets for the modem through eth0, to which nothing was connected.
After I ran
    [tim@grover ~]$ sudo ifconfig eth0 down
I was able to access the modem.

I looked at /etc/sysconfig/network-scripts/ifcfg-eth0
and I saw that "ONBOOT=yes" was set.
(I had copied this from my working server.)

I sort of thought that this would not be activated
unless something was connected to the ethernet socket ...