[TriLUG] Any help diagnosing an intermittent network issue?

[Steve Litt]

On Sun, 26 Oct 2014 10:42:24 -0400
Rodney Radford <ncgadgetry at gmail.com> wrote:

> Wow lots of ideas, and lots of questions - let me try to address some
> of the questions/suggestions given:
> 
> Here is the ifconfig eth0 output when everything is working correctly:
> 
>    eth0      Link encap:Ethernet  HWaddr 2c:27:d7:06:56:54
>           inet addr:10.42.0.1  Bcast:10.42.0.255  Mask:255.255.255.0
>           inet6 addr: fe80::2e27:d7ff:fe06:5654/64 Scope:Link
>           UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>           RX packets:8754 errors:0 dropped:0 overruns:0 frame:0
>           TX packets:145646 errors:0 dropped:0 overruns:0 carrier:0
>           collisions:0 txqueuelen:1000
>           RX bytes:8881902 (8.8 MB)  TX bytes:23985517 (23.9 MB)
> 
> And here it is when it is dead:
> 
>     eth0      Link encap:Ethernet  HWaddr 2c:27:d7:06:56:54
>           inet6 addr: fe80::2e27:d7ff:fe06:5654/64 Scope:Link
>           UP BROADCAST MULTICAST  MTU:1500  Metric:1
>           RX packets:8752 errors:0 dropped:0 overruns:0 frame:0
>           TX packets:145577 errors:0 dropped:0 overruns:0 carrier:0
>           collisions:0 txqueuelen:1000
>           RX bytes:8881218 (8.8 MB)  TX bytes:23974762 (23.9 MB)
> 
> When I connect the network cable between the laptop and the RPi,
> ifconfig starts toggling between those two outputs on a 1-2 second
> period. When the connection is stable, this does not happen.

Based on this symptom description and the tests you've done already to
rule out the acable and the RPi, I'd suggest the following two tests:

1) Boot System Rescue CD on the laptop and see if the RPi/Laptop
   connection remains stable for long periods. This rules in/out all
   software on your laptop.

2) Back in your native laptop operating system, just as a diagnostic
   test, uninstall network-manager, properly hard
   code /etc/network/interfaces, and temporarily make your interfaces
   file immutable with the chattr i command, reboot, and try again. See
   if the instability goes away. If it does, start exploiting the
   differences until you get to the root cause, fix that, and undo your
   diagnostics.

3) If #1 and #2 make no difference, try a known good different physical
   laptop.

SteveT

Steve Litt                *  http://www.troubleshooters.com/
Troubleshooting Training  *  Human Performance