[SLL] How to troubleshoot corrupted wget/apt-get/bittorrent file transfers?

[Kurt Buff]

On Mon, Mar 9, 2009 at 19:01, Phil Mocek <pmocek-sll at mocek.org> wrote:
> On Mon, Mar 09, 2009 at 06:26:07PM -0700, Kurt Buff wrote:
>> On Mon, Mar 9, 2009 at 18:20, Phil Mocek <pmocek-sll at mocek.org> wrote:
>> > It looks roughly like this: Cisco 678, with an old Webramp something
>> > running Sonicwall SOHO firmware, both in a nest of power cables,
>> > then CAT5 cable from there running over doors, across floors, and
>> > down a handrail, around the corner, over some doors and through a
>> > couple closets to a desk where there's some no-name switch buried in
>> > a pile of network cables, power cables, and various other gear.  Not
>> > pretty.
>> >
>> > But these are TCP connections.  Isn't any junk on the wire supposed
>> > to be cleaned up at the transport layer?
>>
>> Heh. Not necessarily, though in theory yes. If there's an issue in the
>> network, though, you'll likely see lots of retransmits and other
>> errors.
>
> I think there are multiple issues at play, at least one of which is a
> problem.
>
> I fired up Wireshark, told it to watch eth0, and saw tons and tons
> of errors scrolling by.  I stopped after a few minutes.  "Analyze
> -> expert info composite" reports two errors: about 25,000 TCP bad
> checksums and about 1400 UDP bad checksums.  It also reports
> warnings including unreassembled packets, previous segments lost,
> out of order segments, ACKed lost segments and fast
> retransmissions.  I'm no network engineer, so to me, this all just
> looks like "bad".
>
> So, what now?  Run wireshark on a laptop, moving one step at a time
> along the wired net from the ADSL modem to the switch at the end until I
> see a big increase in trouble, then do some wire maintenance?

Yeah, that's kinda ugly.

I'd trace the cable from the machine to the switch port into which
it's plugged. Is it kinked or twisted? Is something heavy squashing
it? Is it running parallel with and near (less than, say, 3ft.) a long
fluorescent light tube or other source of EMI? Is it heavily coiled
anywhere?

I'd also be quite suspicious of the switch itself. Is that a 5 or 8
port switch, or something larger? Are all the ports full? Try moving
the cable to a different port if not. But, I've had many of those go
bad, and they're relatively cheap to replace.

If you've truly got power and network cables tangled up, and this is a
business, you're probably not within code - definitely separate those
out, so that they're at least physically separated by a few inches,
network in one group, power in the other.

If you can clear those things up, you've made a good start.

Kurt