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Re: Ping and LAN Problem

Subject: Re: Ping and LAN Problem
From: Marius Gedminas
Date: Mon, 13 Aug 2007 18:43:49 +0300
On Mon, Aug 13, 2007 at 08:59:42PM +0530, das ààà wrote:
> I have two Ethernet cards, named by OS as eth0 and eth2. My DSL router
> modem is connected on eth2. It is working fine (that is how I am sending
> this mail :)). But, my laptop (on Ubuntu 7.04 too) connected to this
> machine on eth0 of the desktop. The eth0 is fixed IP: and
> the laptop is Earlier, I have always connected like this,
> with ipmasq on in the Desktop. But, for quite some time, I did not use
> the laptop. Now, when I am going to connect, the Desktop is giving a
> queer message:
> <<<
> [email protected]:~$ ping
> PING ( 56(84) bytes of data.
> ping: sendmsg: Operation not permitted
> ping: sendmsg: Operation not permitted
> ping: sendmsg: Operation not permitted
> ping: sendmsg: Operation not permitted

You get this message when your firewall blocks an outgoing packet.

> From the laptop too, the ping is not working. It is getting stuck
> without any output till I press 'Ctrl-c', though the self-ping with
> 'ping' from the laptop is working fine. 
> Why this is happening? Earlier I have done all my works through this
> LAN. And I know so little of this networking things. 

The symptoms are consistent with the theory that you have a firewall on
your desktop that doesn't allow it to talk to your laptop (and talking
includes replying to the laptop's pings).

ipmasq sets up the iptables firewall for you automatically.  You could
try rerunning it to see whether that fixes things.

> I am quoting the output of the 'ifconfig -a' on the desktop:
> <<<
> [email protected]:~$ ifconfig -a
> eth0      Link encap:Ethernet  HWaddr 00:A1:B0:09:0C:8F  
>           inet addr:  Bcast:  Mask:

The broadcast address is wrong, it should be  Strange,
usually it is determined automatically and correctly.  Have you
overriden it by hand?  How does your /etc/network/interfaces look like?

An incorrect broadcast address doesn't affect regular connections; you''
need to fix your firewall in any case.

>           inet6 addr: fe80::2a1:b0ff:fe09:c8f/64 Scope:Link
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
> eth2      Link encap:Ethernet  HWaddr 00:E0:7D:89:F6:2F  
>           inet addr:  Bcast: Mask:
>           inet6 addr: fe80::2e0:7dff:fe89:f62f/64 Scope:Link
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

Marius Gedminas
A Law of Computer Programming:
        Make it possible for programmers to write in English
        and you will find that programmers cannot write in English.
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