Re: Why Linux is blind to this ARP reply ?

From: prg (rdgentry1_at_cablelynx.com)
Date: 02/13/05 

Date: 13 Feb 2005 08:03:01 -0800

andres wrote:
> Hello,
>
> I got this case I can't explain:
>
> Router 2210 (old IBM router) <---cross cable ---->Laptop

Are you sure you need a xover cable here?

> Router: IP: 10.10.0.46/21
> Laptop:
> Windows IP: 10.10.3.112/21
> Linux IP: 10.10.3.99/21
>
> Ping works Ok from Laptop when running windows (windows xp sp2)
> Ping does not work from Laptop when running linux (redhat 9)

Make sure it's not a firewall issue with Linux. Turn off the firewall.

> Now I change the OS on the laptop and use Linux instead, ping does
not
> work any more.
> I placed ethereal in windows and tcpdump in linux.
>
> Ethereal (windows) sees the ARP reply when the router is ping from
> Linux or Windows BUT Tcpdump DOES NOT see the ARP reply

What are you trying to ping? The router? Using IP address or
hostname? Please give command line and output of commands for us to
see. You can use copy-n-paste from an X terminal of your choice.

> Initially the router was placed on the network, but I starting
> simplifying the environment until I got to the cross cable
connection.
> The behaviour is the same when connected to the network and when
using
> the cross cable.
>
> I got the captured packets of both cases (linux and windows) and if
you
> compare the reply of the router in both cases is the same reply,
except
> of course the destination address, which change accordingly.

We can't compare anything -- we can't see anything :(

> NOTE: I have verified issues as subnet mask, broadcast address,
> etc,etc. Everything seems to be in order.

Maybe yes, maybe no. We can't see anything, so we can't verify
anything more than that you think everything is OK. Please post
command line and output of ifconfig -a (and Win's ipconfig /all) as
well as route -n. Router config also needed.

> I used VMWARE because it provided me an easy way to test both OS
> without any hassle, but the results are the same when using native
> Linux boxes instead of vmware.
>
> I suspect something fishy is happening in the handling of the OSI
layer
> 1,but I don't know how to get there.
> Any idea will be greatly appreciated.

It's almost certainly a config error but we can't spot it or really
help without seeing your currrent configs. These must be verified
before looking at any other possible issues.

When using Linux are the pings simply dropped or do you get error
messages?

How are you getting network info at boot? Static? DHCP? Why are the
IPs of Windows and Linux so far apart and different (just curious --
should make no difference)? Are you sure the router is entered as
Linux's default route/GW?

If you entered the network params into RH9 by hand, you likely missed a
key entry in one of the _several_ files. Have you tried setting up the
net params using RH's networking gui? Try $ neat or $
/usr/bin/redhat-config-network from command line.

$ ping 127.0.0.1 < yes?
$ ping 10.10.3.99 < yes?
$ ping 10.10.0.46 < no?

Your route table needs net entries for (at least):
10.10.0.0 netmask 255.255.248.0 < eth0?
127.0.0.0 netmask 255.0.0.0 < lo (loopback)
10.10.0.46 < default route/GW

Does the router see Linux arp requests on the wire? Any way to tell?
Can you at least see its arp cache?

hth,
prg
email above disabled

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