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Bizarre packet loss

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  Bizarre packet loss Gregory Seidman <gsslist+debian@anthropohedron.net> - 2020-10-23 04:10 +0200
    Re: Bizarre packet loss David Christensen <dpchrist@holgerdanske.com> - 2020-10-23 04:50 +0200
    Re: Bizarre packet loss Charles Curley <charlescurley@charlescurley.com> - 2020-10-23 05:20 +0200

#228130 — Bizarre packet loss

First off, here's the network architecture:

ISP (untrusted) router (NAT)
 |
OpenWRT (trusted) router (NAT) ... wireless device
 |
Cisco (unmanaged) switch-----------+
 |                                 |
Netgear (unmanaged) switch        wired device
 |
Debian server

If I ping the Debian server from OpenWRT, the wireless device, or the wired
device I get 25%-30% packet loss *unless* I am pinging an external IP from
the Debian server at the same time, at which point the packet loss goes
below 1%. I have no explanation for this.

But wait, it's even weirder than that. Pinging from one wireless device to
another wireless device *also* shows significant (~20%) packet loss unless
the Debian server is pinging an external IP. Note that pinging an external
IP from a wired or wireless device doesn't seem to have any impact.

I suspect that something bad is happening at the Ethernet layer that I'm
just not seeing, but I don't know where it's coming from or what I'm
looking for. I've used tcpdump from the places where I can (OpenWRT, Debian
server, wired and wireless devices) and looked at it in Wireshark, but nothing
jumps out at me.

There is a pretty strong chance that this isn't coming from the Debian
server at all, but a ping from it has some magical effect on the network.
I've been fighting this for days and I'm just at a loss. I'd very much
appreciate any advice or options to try.

Thanks,
--Gregory

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#228133

On 2020-10-22 19:07, Gregory Seidman wrote:
> First off, here's the network architecture:
> 
> ISP (untrusted) router (NAT)
>   |
> OpenWRT (trusted) router (NAT) ... wireless device
>   |
> Cisco (unmanaged) switch-----------+
>   |                                 |
> Netgear (unmanaged) switch        wired device
>   |
> Debian server
> 
> If I ping the Debian server from OpenWRT, the wireless device, or the wired
> device I get 25%-30% packet loss *unless* I am pinging an external IP from
> the Debian server at the same time, at which point the packet loss goes
> below 1%. I have no explanation for this.
> 
> But wait, it's even weirder than that. Pinging from one wireless device to
> another wireless device *also* shows significant (~20%) packet loss unless
> the Debian server is pinging an external IP. Note that pinging an external
> IP from a wired or wireless device doesn't seem to have any impact.
> 
> I suspect that something bad is happening at the Ethernet layer that I'm
> just not seeing, but I don't know where it's coming from or what I'm
> looking for. I've used tcpdump from the places where I can (OpenWRT, Debian
> server, wired and wireless devices) and looked at it in Wireshark, but nothing
> jumps out at me.
> 
> There is a pretty strong chance that this isn't coming from the Debian
> server at all, but a ping from it has some magical effect on the network.
> I've been fighting this for days and I'm just at a loss. I'd very much
> appreciate any advice or options to try.
> 
> Thanks,
> --Gregory


I would verify that all of the Ethernet cables are good.  Fix or discard 
any that have issues:

https://www.idealind.com/shop/62-200.html

https://www.idealind.com/shop/30-696.html

https://www.idealind.com/shop/85-396.html


David

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#228134

On Thu, 22 Oct 2020 22:07:18 -0400
Gregory Seidman <gsslist+debian@anthropohedron.net> wrote:

> If I ping the Debian server from OpenWRT, the wireless device, or the
> wired device I get 25%-30% packet loss *unless* I am pinging an
> external IP from the Debian server at the same time, at which point
> the packet loss goes below 1%. I have no explanation for this.

Bizarre, indeed.

The first thing I try is to cycle power on all three routers
simultaneously. Leave them off for at least 20 seconds. Sometimes they
just lose their marbles.

Failing that, mtr (package mtr-tiny for the command line version) is an
excellent diagnostic tool for this sort of thing. It will tell you
where a problem lies much better than ping will. E.g.:

mtr -r -i 3 -c 10 debian.org

-- 
Does anybody read signatures any more?

https://charlescurley.com
https://charlescurley.com/blog/

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