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There is something wrong with my Ubuntu 14.04 Laptop's eth0. It works fine after booting up. However, if I don't use it for a short period of time (few minutes), then it will not work. When this happens, connecting to Internet and pinging 192.168.1.1 (my ADSL router's IP address) will fail. To solve this issue, I need to disconnect and reconnect Ethernet1 (i.e., eth0) in Ubuntu desktop environment.

That eth0 connects to a port of that ADSL router which another Windows PC also connects to (the other port of it). The network of that Windows PC always works fine.

Could anybody comment on this strange issue?

Oliver Chen
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2 Answers2

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Lacking data, I must guess that you are getting your IP Address via DHCP from the ADSL router. Check for a short "lease time". If this is the case, fix it on the ADSL router. Use the command: 

zgrep dhclient /var/log/syslog*

to investigate.

waltinator
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  • Yes, Ubuntu's eth0 gets IP address via DHCP from the ADSL router. I'll check and feedback later. Thanks for your comment! – Oliver Chen Dec 01 '15 at 23:19
  • bound to 192.168.1.103 -- renewal in 37972 seconds. – Oliver Chen Dec 03 '15 at 01:54
  • In those /var/log/syslog* files there are many lines containing string like: ... bound to 192.168.1.103 -- renewal in 37972 seconds. And, after checking the DHCP server setting of my ADSL router, the leased time is set to 24 hours. According to these information, this doesn't look like an "lease time" issue, right? – Oliver Chen Dec 03 '15 at 02:06
  • @user3558781 Right. Look in `/var/log/*` for events that happened between the last net UP and the first net down. – waltinator Dec 03 '15 at 02:28
  • After trying to search the /var/log/* files using few key words like "UP" or "net", I found I've no idea about how to identify events that happened between the last net UP and the first net down. I'm wondering if you could provide more information how to do this? – Oliver Chen Dec 03 '15 at 06:19
  • @user3558781: The first three fields of a `/var/log/syslog` entry contain the time and date of the entry. Record the time when your net works, go away, come back when net is down. Look for time/date stamps between the recorded time and now. – waltinator Dec 03 '15 at 15:46
  • OK. I understand. Let me check the content of that file. Thanks again for your information and kind help! – Oliver Chen Dec 04 '15 at 01:21
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After posting this problem I encounter the same symptom on that Windows PC. Fortunately, after debugging this weird problem for weeks, now the root cause seems to be clear: there is something wrong with the firmware in that DSL modem. By upgrading that FW to the latest version, everything has gone smoothly for two days. Anyway, many thanks to waltinator's kind help!

Oliver Chen
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