[TriLUG] Any help diagnosing an intermittent network issue?

[R Radford]

I have my laptop (running Ubuntu 14.04) corrected via wifi to my home
router - that connect is up and stable.

I then connect the wired network of the laptop to my Raspberry Pi.  I have
configured both the RPi and my laptop as static on a different subnet.

On most days, all is well - I can ssh to my RPi, and I have ICS set up
between the RPi through my laptop to my wifi, so I can grab updates on my
RPi and ssh to it.

However some days the connection between my laptop and the RPi is very
unstable.  I plug in the Ethernet cable between the two, watch the
interface (eth0) come up and can connect, but then it drops a few seconds
later.  Running a watch command on "ifconfig eth0" shows that it toggles
between up and down every few seconds.

There are no errors listed on the ifconfig, and nothing is in dmesg
indicating an error.

Ifconfig output when everything is fine:

rradford at asimov:~$ ifconfig
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:8607 errors:0 dropped:0 overruns:0 frame:0
          TX packets:132232 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:8871528 (8.8 MB)  TX bytes:21755527 (21.7 MB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:847951 errors:0 dropped:0 overruns:0 frame:0
          TX packets:847951 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:134296144 (134.2 MB)  TX bytes:134296144 (134.2 MB)

wlan0     Link encap:Ethernet  HWaddr 68:a3:c4:d5:57:7a
          inet addr:192.168.0.82  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::6aa3:c4ff:fed5:577a/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:344266 errors:0 dropped:0 overruns:0 frame:0
          TX packets:290361 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:260820609 (260.8 MB)  TX bytes:61832612 (61.8 MB)

Static interface on the RPi is configured via network-manager GUI.  The
eth0 interface is not in /etc/network/interfaces.

Watching the eth0 on the RPi does not show any issues.

I don't understand why the interface bounces up and down every few seconds
(and usually finally gives up) on some days, but with no changes works fine
another day. I know this will be very difficult to debug remotely, so what
I am looking for is help on where I should look to help diagnose this
problem.

Btw, this was working reliably with no problems before the 14.04 upgrade a
few months ago, if that matters.

Any assistance or ideas are greatly appreciated.

Thanx,
Rodney

PS: I hope this doesn't get posted twice by accident (first post was from
an unregistered address so I am sending it again).