networking, Strange network problems - pings to host are fine, pings from host fail

comp.os.linux.networking - Networking and communications under Linux


networking >> Strange network problems - pings to host are fine, pings from host fail

by David Brown » Tue, 02 Dec 2008 01:56:48 GMT

Sorry for the long post - I've tried to outline the symptoms of my
problem, and what I've tried to fix it.


I'm having some networking problems with a PC running an old version of
Red Hat (kernel 2.2). The PC came with a machine we have bought
second-hand - as far as I know, everything was working before the
machine was moved to our company.

To keep things simple, we have the Red Hat machine connected to a
Windows XP machine that came with it, with only a simple switch in
between. Each machine is set up with a fixed IP address on the same
network.

From the XP machine, I can ping the Red Hat machine reliably and
quickly. From the Red Hat machine, pings to the XP machine /generally/
fail - typically there are about 80%-90% failures. Those pings that
don't fail, are fast (reply time about 1 ms). On the XP machine, it's
easy to see the packet counters showing packets in and replies out. On
the Red Hat machine, ifconfig shows similar packet rx and tx counts, and
zero error counts.

arping to the XP machine from the Red Hat machine is reliable and fast.

When we try a browser on the XP machine and address the web server on
the Red Hat host, there is generally a long pause (perhaps minutes),
then suddenly the page appears.

We have tried using another Linux box in place of the original XP
machine, with the same results from the Red Hat system.

We have tried replacing the cables and switch, with no effect - given
that arpings are working perfectly it's hard to see how it could be a
hardware problem.

I'm not very familiar with Red Hat or a 2.2 kernel (my experience is
mostly with Debian and related distros, and with 2.4 and 2.6 kernels).
But "ipchains -L" shows no firewalling (everything accepted), "ifconfig"
and "route -n" have the expected setup, and I could not stop anything
unexpected with "sysctrl".

One of my colleagues will try a different network card this evening.


Any ideas or tips would be much appreciated. We've tried pretty much
every sensible idea we can think of, so I'm ready to listen to any crazy
or unlikely tricks.

mvh.,

David

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networking >> Strange network problems - pings to host are fine, pings from host fail

by Balwinder S Dheeman » Tue, 02 Dec 2008 02:44:45 GMT





I think, you need to check DNS settings/entries on both of your XP as
well as RH machines; better automate all these and, or other such things
with the help of a DHCP server.

Moreover, try syncing your C:\WINDOWS\system32\drivers\etc\hosts file on
XP machine with the one /etc/hosts on you RH machine.

Or, better run a DNS server having zones for local/private network/segments.

Hope that helps,
--
Dr Balwinder S "bsd" Dheeman Registered Linux User: #229709
Anu'z Linux@HOME (Unix Shoppe) Machines: #168573, 170593, 259192
Chandigarh, UT, 160062, India Gentoo, Fedora, Debian/FreeBSD/XP
Home: http://cto.homelinux.net/ ~bsd/ Visit: http://counter.li.org/


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networking >> Strange network problems - pings to host are fine, pings from host fail

by Robert Harris » Tue, 02 Dec 2008 02:52:36 GMT



You may well have a hardware problem in your network. Try connecting
your Red Hat machine directly to your XP machine with a crossover cable
and try pinging again.

Robert

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Strange network problems - pings to host are fine, pings from host fail

by Tauno Voipio » Tue, 02 Dec 2008 03:17:29 GMT




Have you tried ping -n ?

--

Tauno Voipio
tauno voipio (at) iki fi


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Strange network problems - pings to host are fine, pings from host fail

by David Brown » Tue, 02 Dec 2008 04:38:38 GMT

alwinder S Dheeman wrote:

It's not a DNS problem - I'm pinging by IP address. I'm trying to keep
things as simple as possible until I know what's going wrong.

We have a DNS server on the router to which these machines will be
attached, once we've got this problem figured out. And yes, I'll be
using the hosts files on both machines to make sure the naming is right.

Thanks for the ideas anyway.

mvh.,

David

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Strange network problems - pings to host are fine, pings from host fail

by David Brown » Tue, 02 Dec 2008 04:43:39 GMT

obert Harris wrote:

We thought of that, but I couldn't find a crossover cable at the office
(we haven't needed crossover cables for a decade or so). We have tried
connecting using only a modern switch, and also using an old 10 MBit
hub, as well as connecting them on the LAN side of a small router (a
LinkSys WRT54GL running openwrt). There were absolutely no differences,
so I don't think the problem can be there.

If there is a hardware problem, it must lie with the network card on the
Red Hat machine, as everything else has been tested (different cables,
different target machines, etc.). It's conceivable that the card has
come lose while transporting the machine to our offices, I suppose.
I'll know a little more in the morning (after my colleague has tried a
different network card).

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Strange network problems - pings to host are fine, pings from host fail

by David Brown » Tue, 02 Dec 2008 05:15:42 GMT





I'm already pinging by ip address, so "ping -n" should not make a
difference. Or is it the case that with this setup (either Red Hat
specific, or kernel 2.2, or older versions of ping and the resolver
libraries), that ping without the "-n" will try to do a reverse DNS
lookup to find a name for the ip address I'm pinging? I must admit that
I haven't worried too much about DNS so far, since I am sticking to IP
addresses first. The DNS server specified on the Red Hat machine does
not exist in our network (it is a local address on the original user's
network), so perhaps ping is hanging when trying to resolve a name for
the address. A similar thing may be happening with the web server
accessed by the client machine. This is not a problem I've seen before
(I've seen other DNS issues, and also set up nodes and networks with no
DNS at all), but it may just be the problem.

I'll try out some fixes first thing in the morning when I'm back at the
office (firstly by putting the ip addresses I'm pinging in the hosts file).

Thanks for the hint!

mvh.,

David

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Strange network problems - pings to host are fine, pings from host fail

by Allen McIntosh » Tue, 02 Dec 2008 09:08:30 GMT





By default, ping tries reverse DNS on the source IP address of the
packet it gets back.

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Strange network problems - pings to host are fine, pings from host fail

by David Brown » Tue, 02 Dec 2008 16:06:06 GMT






That was the problem. A couple of lines in the hosts file (and correct
DNS setup once the system was connected to the rest of the network), and
everything worked perfectly.

Many thanks to those who replied on this thread - I've learned something
new, and fixed our problem.

mvh.,

David



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> 
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> 192.168.1.0     0.0.0.0         255.255.255.0   U     0      0        0
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> 172.16.1.0      0.0.0.0	  255.255.255.0   UG    0      0        0 eth1
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with no gateway on either entry.)  The 172.16.1.0 entry looks funny too. 
  What is the address of eth1 supposed to be?


I think you need to sit down with a piece of paper and draw your 
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