A.P. Lawrence

ping and telnet dhcp network routing

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From - Wed Oct  3 07:22:41 2001
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From: Tony Lawrence <tony@aplawrence.com>
Organization: A.P. Lawrence
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Bela Lubkin wrote:
> 

> I would look for a problem in reverse DNS lookups.  Start by trying the
> "-n" flag in your ping attempts.  If that fixes it, the problem is that
> ping is getting hung up trying to translate the IP addresses of received
> replies back into host names.
> 
> If you let the ping run for a long time, DNS will eventually time out
> and you'll see one reply.  I assume you are otherwise interrupting the
> ping attempts.  If you look closely, you should also notice that ping
> says something about having received 1 reply in its final summary, even
> though you never see the reply itself.

I had an interesting one just last night.

A guy calls, says he's adding a SCO box to his home network.  Two
Windows machines that happily talk to each other, but the SCO won't.  I
went through the obvious stuff first, but it all sounded fine.  Correct
supplements, right drivers, good card.  All machines 192,168.2.x. 
Sounds fine.

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I had him ping the SCO from Windows- he said "that's odd- that didn't
work before".  I then had him ping from SCO- no response.  Must be DNS-
had him add the Windows ip to /etc/hosts and try again- but it still
didn't work.

OK, clear out the arp tables and back to the Windows machine to try a
telnet.  Doesn't work and doesn't show up in arp.  Try a ping again-
hmm, doesn't work.  But it did work a few minutes ago.. but the arp
shows the ping got there.

So now I asked  him to fully describe his setup- hubs,  where everything
is plugged, how long are the wires, is it all cat 5, 10/100 what
make/model nics.

All good model cards, all brand new cat 5 cables, a little 8 port 10/100
hub.. and a router.  A router?  Yes, a little internet router-thingy, he
says.  Does that thingy give the Win machines their ip addresses?  Yes,
indeedy, he said.

Ahh.  I asked him to remove the router from the hub and temporarily give
the Windows machines static ip addresses.   OK, mutter, mutter, fumble,
fumble, sound of machines rebooting and Bingo- everything works.

Next question.  What's the DHCP range this router thingy assigns from? 
Dunno, he says.  Set it up a long time ago, don't remember having to
tell it anything, don't know how to get to it.

OK, plug it back in, but this time lets give the SCO box a number way
the heck away from the numbers the Win machines get.  The Win boxes were
getting 192.168.2.8 and 192.168.2.10, so give the SCO box 110.  Son of a
gun, it works.

So- apparently this internet router thingy didn't like the SCO box
trying to use an ip address in its DHCP pool.  Makes sense, but:

Why were 192.168.2.0 packets going to the router anyway?  Should have
just gone through the hub and everybody is happy.  Unless.. oh, darn I
just realized I forgot to check the netmasks!  I'll forgive myself
because I was working over the phone completely free of charge and
really wanted to get back to other things.

I bet that's it.  I bet the router gave them a mask that put them down
in their own little network.  That might explain it.

Anyway, this stuff can be fun.  Especially flying blind on the other end
of the telephone :-)

--
Tony Lawrence
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