[TriLUG] Any help diagnosing an intermittent network issue?

[Steve Litt]

On Sat, 25 Oct 2014 14:27:56 -0400
R Radford <rradford at mindspring.com> wrote:

> 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).

I just got bitten by what turned out to be an intermittent (but I
didn't realize it) because my pfSense firewall's hard drive was going
to hell in a handbasket (smart errors and everything), causing its
DHCP server to sometimes be off.

Anyway, one problem with network problems is there are so many
components, and a lot of times components aren't physically easy to
access, and it can be a PITA.

I'd start with the usual suspects: Ping other points on your LAN, ping
8.8.8.8 (if you have a gateway to the Internet), use ifconfig to see if
you have a (DHCP? address) and the like. Do this stuff both when it's
acting right, and acting wrong, and pay attention to any differences.

Add a second laptop to your wifi, using the same mode (dhcp or static),
and see if both laptops have the same problem at the same time.

Write yourself a shellscript to make a log entry every few seconds with
a timestamp and a "working" or "nonworking" designation, and try to
relate the bad times to other stuff.

Plug your laptop into the wired network for a few days and see if the
problem recurs. Make sure your laptop is still using the same DHCP
server as usual, and for gosh sakes, make sure you don't have dualling
DHCP servers.

If it doesn't happen on wired, that's either because you're on a
different subnet, or because the problem's in the wifi.

I don't envy you, but this intermittent stuff happens to all of us. Ugh!

SteveT

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