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Thread: network issues

  1. #1
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    network issues

    I've put together a small lab of salvaged computers running Lubuntu at my University. Everything worked and I went on vacation. When I got back I noticed none of the Lubuntu computers could connect to the internet, but were on the local network. My personal Mac and the rest of the Windows computers were unaffected. I'm not so sure how to begin tracking down the problem. Could someone suggestion some commands I should run that might get to the bottom of this?
    Last edited by Kanefa; October 27th, 2013 at 05:53 PM.

  2. #2
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  3. #3
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    Re: network issues

    I was able to run a couple tests from the article. I ran them both on my Mac Laptop and my Lubuntu box. Both are sitting on my desk and I am using the same ethernet cable. The Mac is on the network and I can connect to the internet. The Lubuntu box says it is connected, but is unable to connect to the internet.

    1. Ping the router - Unsure if I used the correct router number. For the Mac I got it from Network Preferences -> Ethernet -> Router. For the Lubuntu box I got it from the Network icon (LXPanel) -> Information -> Default Route.
    Mac - 9 packets transmitted, 9 packets received, 0.0% packet loss
    Lubuntu - 10 packets transmitted, 10 packets received, 0% packet loss, time 8996ms

    2. Ping commonly known internet IP (ping 8.8.8.8)
    Mac - 30 packets transmitted, 6 packets received, 80.0% packet loss
    Lubuntu - 248 packets transmitted, 0 received, 100% packet loss, time 248976ms

    3. see if DNS is working (ping google.com)
    Mac - 21 packets transmitted, 6 packets received, 71.4% packet loss
    Lubuntu - ping: unknown host google.com


    That's the results today. Yesterday the Lubuntu box was working perfectly fine which is an exception to the norm. However, when the Lubuntu box doesn't work the Mac and Windows computers still do. The University's network along with the ISP are unreliable (I am a volunteer in a developing country), but what confuses me the most is why the Mac and Windows computers typically work when the Lubuntu doesn't. I have no idea what that points to. Any thoughts?

  4. #4
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    Re: network issues

    Thread moved to Networking & Wireless

    Hopefully you'll have more luck here. Quite an odd problem you have. I would take this as a given then forget about it, though: The Mac and Win boxes work regardless of what the Lubuntu box is doing, so they are not part of this equation. They only go to prove that internet is working all the time so as that's a fact, forget them. The problem is with the Lubuntu machine only so focus there.

    Considering this, you appear to be dealing with the internet dropping in and out on the Lubuntu machine, works sometimes and not others, so time to search for what might be causing that. Does the connection die when you are actually using it? If so, immediately open a terminal and check the result of:

    dmesg

    You also give no clue as to how the Lubuntu machine is connecting to the internet. As these are computer graveyard bits and pieces this could be a hardware issue (if not the card then perhaps the PCI slot, you could try another). What card does it use? Please provide the output of:

    Code:
    sudo lshw -C network
    As the Mac and Win machines are probably using totally different cards, even more reason to pay them no attention (apart from proving you have an active internet cable coming out of whatever AP you are using).

    So, give complete details of how you are connecting the Linux machine (only) to the net, and the hardware you have used to build it.
    Last edited by Bucky Ball; October 23rd, 2013 at 10:22 AM.

  5. #5
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    Re: network issues

    The connection does not come and go that often. I'm starting to log the days it does work, so I have some idea of what the frequency is. That being said the Lubuntu computers may not have connectivity for a week and then all of a sudden work for two days straight. It's not a daily issue, so running "dmesg" at the right time would be lucky.

    We do have a few Dell Dimension 2400 which are all identical, so I ran the "sudo lshw -C network" on one of these. However, let me give you a little more background information (I'm not sure if it will be relevant). The Dell's were all donated running a version of Windows. We used them as is for a while and they never exhibited this problem. In fact, we still have one I haven't switch from Windows to Lubuntu and it continues to work. Lastly, when I put the lab together I used Clonezilla. Every computer is an exact clone of the original, but as a post install step I make the references to hostname unique in both etc/hostname and etc/hosts.

    *-network
    description: Ethernet interface
    product: BCM4401 100Base-T
    vendor: Broadcom Corporation
    physical id: 9
    bus info: pci@0000:01:09.0
    logical name: eth0
    version: 01
    serial: 00:0d:56:51:c7:3f
    size: 10Mbit/s
    capacity: 100Mbit/s
    width: 32 bits
    clock: 33MHz
    capabilities: pm bus_master cap_list rom ethernet physical mii 10bt 10bt-fd 100bt 100bt-fd autonegotiation
    configuration: autonegotiation=on broadcast=yes driver=b44 driverversion=2.0 dublex=half latency=64 link=no multicast=yes port=twisted pair speed=10Mbit/s
    resources: irq:17 memory:fe9ee000-fe9effff memory:fea00000-fea03fff

  6. #6
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    Apr 2012
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    Re: network issues

    Just a couple of observations

    Unless there's a more complicated network topology that you haven't shared with us, the router IP should be the same whether you get it from the Windows boxes or the Lubuntu ones - if it's not then that would be the first thing to look in to. You might find that doing an 'ifconfig' on the *buntus and comparing to an 'ipconfig' on the Windows boxes to be helpful

    The fact that you apparently get a near 9s ping time off the router from the *buntu box suggests either it's *not* the correct IP or the routing is screwed up OR driver/hardware issues

    In any case, an 'ifconfig' from the *buntu box would be a useful place to start, preferably with the corresponding 'ipconfig' from Windows by way of comparison

  7. #7
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    Re: network issues

    Quote Originally Posted by steeldriver View Post
    driver/hardware issues

    In any case, an 'ifconfig' from the *buntu box would be a useful place to start, preferably with the corresponding 'ipconfig' from Windows by way of comparison
    Based on the data and history, I'd expect the issue is with the physical cable and/or ports either on the router or the machines. Can you swap those around between the different machines? That would eliminate much .... or confirm it is just the Ubuntu machines.

    Hope this helps. The ideas from others above are smart too, BTW.
    Thanks for doing those tests from the article. The results really did help narrow down the thousands of possibilities.

    If you haven't already found the root cause, run and post the results for a few commands on OSX, Win, and Linux machines:
    ifconfig - ipconfig /all
    route - route print
    dig google.com - nslookup google.com

    Linux on the left, windows on the right.

  8. #8
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    Jul 2013
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    Re: network issues

    I'm going to jump ahead for a minute. I was looking into steeldriver's question about the topology. I'm really not sure what the topology is (it may be fairly complex). I wasn't sure if I got the routers IP correctly either (that's why I specified how I got them).

    What I did was borrow a router from a friend. This should at least simplify getting the IP address for the router. I plugged the ethernet cable directly into the router and then my Mac and Lubuntu computers. I open Firefox up and enter the router's IP (which I get from the "Connection Information" in Lubuntu). On the Mac I end up at the Cable/DSL Gateway Router Setup Utility page. On the Lubuntu computer I get "The connection has timed out."

    That must mean something. The Lubuntu computer can see the router's IP address and the rest, but than cannot access it. Also, considering this was the first test in the "Linux Troubleshooting 101" article I figure it must mean something is going wrong quickly.
    Last edited by Kanefa; October 23rd, 2013 at 07:11 PM.

  9. #9
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    Kubuntu 14.04 Trusty Tahr

    Re: network issues

    Please post the results of "route -n" in [code][/code] tags. My routing table looks like this:

    Code:
    Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
    0.0.0.0         192.168.100.1   0.0.0.0         UG    0      0        0 wlan0
    192.168.100.0   0.0.0.0         255.255.255.0   U     9      0        0 wlan0
    Your results should list "eth0" rather than "wlan0". My computer's IP is 192.168.100.110, and the gateway is 192.168.100.1. I see other computers in the 192.168.100.0/24 network via broadcasts; all other traffic goes to the gateway.

    My network's topology is rather similar to yours because my gateway router is behind my Internet-facing router. I can see the Internet from all my machines because both routers do "masquerading" and impersonate the machines behind them. The router at 192.168.100.1 masquerades the traffic from 192.168.100.0/24 as if it is coming from its upstream-facing IP of 192.168.1.2. The Internet facing router has local interface 192.168.1.1 that talks to 192.168.1.2 and masquerades its traffic again using the public Internet address on its "WAN" side.

    Try this experiment. Connect with the Mac and identify its IP address and the gateway's address; let's say these are 192.168.1.10 and 192.168.1.1 respectively. Now disconnect the Mac from the network. On the lubuntu box, use these commands

    Code:
    sudo ifconfig eth0 down
    sudo ifconfig eth0 192.168.1.10 netmask 255.255.255.0 
    sudo ip route add default via 192.168.1.1
    ping 192.168.1.1
    ping 8.8.8.8
    How are we doing? Can you connect to the router's configuration page now?

    Oh, just one other thing, did you reset the router to its factory defaults after borrowing it? If not, I would do that first. There usually is a small switch behind a hole in the back of the router that you can depress with a paper clip. Turn off the router, then turn it back while holding down the switch. Hold it down for 30 seconds or so, then turn off the router and turn it back on again.
    Last edited by SeijiSensei; October 23rd, 2013 at 08:46 PM.
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  10. #10
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    Re: network issues

    Please post the results of "route -n" in tags.
    SeijiSensei, it looks like the code did not make it in.

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