Weird networking problem

Akshay Lamba alamba-KEM+DXFYpnDQT0dZR+AlfA at
Sun Jul 11 10:15:51 UTC 2004

Sounds like a very interesting case marcus. While at this point in time I
don't have any positive thoughts/feedback for you, (give me some time to
chew on it) do keep me updated on how this progresses (alamba (at) bharti
dot com)

BTW, are you doing NAT anywhere on the way from your systems to the sites in
question? Trace the path, even if u're not NATing maybe your ISP is?


-----Original Message-----
From: owner-tlug-lxSQFCZeNF4 at [mailto:owner-tlug-lxSQFCZeNF4 at] On Behalf Of Marcus
Sent: Sunday, July 11, 2004 12:03 PM
To: tlug-lxSQFCZeNF4 at
Subject: [TLUG]: Weird networking problem

Alright, I have a strange problem that I've been up against for the past
two or three days and am running out of ideas so I figured I'd post here
and see if anyone could help.  About 3 days ago my DSL connection seemed
to stop working.  Further analysis showed that the connection was still
there but certain sites/servers were inaccessible.  For instance I was
able to load but was unable to load  Similarly for
many other sites, including an inability to get to my own ISPs website. 
It wasn't simply restricted to http access either as ssh, pings and
traceroutes showed similar behaviour.  Neither was there a browser issue
(tried Mozilla, wget, IE, etc).

Thinking it might be an issue with my firewall I replaced the firewall
with a spare.  This failed to make any difference.  Thinking perhaps it
was a modem issue I also swapped modems again to no avail.  Finally I
called the ISP and, at their urging, I directly connected a Windows XP
laptop to the modem to make their tech support people feel more at
ease.  As expected, this made no difference  They ran line tests and
checked network status.  I was informed that Bell had no issues, they
had no issues and, in fact, by everything they could tell, I had a
picture perfect, low-noise DSL line which was connecting and
authenticating wonderfully.

And the strangest part of the whole thing is how erratic it is in what
hosts it can connect to and what hosts it can't.  For instance, there
are two hosts on the U of T CS network that I have access to.  They are
separated by very little (practically next to each other) and share the
same route out of the U of T networks.  Against all intuition and odds,
one of them is perfectly accessible while the other isn't.  However,
from my machine at home on Rogers both machines are readily accessible

Now, about the only two things that I can think to be a problem is that
either a) something very unusual is going on with my ISPs routers or b)
the IP address has somehow been blacklisted somewhere.  Now, I can't
imagine why b) would be the case and, even if the server was blocked as
a spam relay (which, AFAIK it isn't, and yes I've checked), those
usually block at the mailer level, not the IP level, right?  While I was
able to try a different IP address with my ISP (which worked great) it
doesn't rule out option a) since the address was on a completely
different subnet.

My own feeling is that it is option a) but at the moment I have no way
to show that.  I spent two hours on the phone today convincing the
bloody tech support person that it was *not* a software configuration
issue and he refused to believe that anything could be wrong with their
system that he was unaware of.  Unfortunately, I don't know of any way
to test or determine where the problem is since traceroute has been
inconclusive at best and I'm not a networking expert.

So I guess my question is this: what could I try/test in order to either
rule out or support option a) or b)?  Should I just call back the ISP
and insist that a ticket be filed with their real networking people?  If
that is the case, does anyone have any theory on what might cause such
unusual behaviour so that I can make a better case?  Any other advice?

Thanks to everyone (especially for reading this far!) for any help or
even positive thoughts.  I'm about ready to put my head through the wall
on this one.

Marcus Brubaker <marcus.brubaker-H217xnMUJC0sA/PxXw9srA at>

The Toronto Linux Users Group.      Meetings:
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The Toronto Linux Users Group.      Meetings:
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