I have a laptop that practically never moves from my desktop so it connects over wired.
I had to take it away and now when restoring all connections it absolutely refuses to connect over wired, it only detects the wireless connection.
I tried everything: resetting router to default, clearing device list, rebooting router and computer and still no luck.
It works the same with wicd or plasma-nm, the wired connection is connected but it shown no traffic.
I tried setting up a new wired connection, in the Connection editor, because there was nothing there, and cloned MAC address to the MAC address of the network card, it does show but there is no way to activate it.

I don't think that it has to do with OS, and I am not a fan of closed source like MAC.
I have wicd on a smaller 32-bit netbook, it works flawlessy, upon connecting the same wire that the problem computer hooks into, it automatically switches to wired connection and disconnects wlan0, upon disconnecting ethernet it switches back to wireless seamlessly.
So there must be something in the problem computer, the ethernet cable and socket work, the OS correctly detects the wired connection when it is plugged or unplugged, but there is no connection to the internet.
I uninstalled plasma-nm and network manager and still have same issue with wicd installed, cannot connect to wired network.
Tried cli as root: ip link set eth0 up and ip link set dev eth0 up, no luck.
Reset router to defaults, cleared device list, disconnected all the network, restarted router and reconnected devices one by one, still same old problem.
Utterly bewildered.

A notebook PC that has been accessing the internet may suddenly lost the connection. This problem may be accompanied by a browser error like, "Cannot find Server." Possible causes may be:

The cable connection is loose or damaged
The router or modem fails
The device driver is missing or corrupted
The Internet Service Provider (ISP) service or network fails

Please post the output of "ifconfig" (may need to be run as root) with both the ethernet cable connected and disconnected.

Thanks for all posts, now posting output.
Wire disconnected.

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root@papimalo:/home/ckosloff# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:0f:b0:c5:32:d7 
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

eth0:avahi Link encap:Ethernet  HWaddr 00:0f:b0:c5:32:d7 
          inet addr:169.254.21.42  Bcast:169.254.255.255  Mask:255.255.0.0
          UP BROADCAST MULTICAST  MTU:1500  Metric:1


Wire connected.

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root@papimalo:/home/ckosloff# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:0f:b0:c5:32:d7 
          inet6 addr: fe80::20f:b0ff:fec5:32d7/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

eth0:avahi Link encap:Ethernet  HWaddr 00:0f:b0:c5:32:d7 
          inet addr:169.254.21.42  Bcast:169.254.255.255  Mask:255.255.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

Now a more extensive test: disconnected wireless, stopped samba, wire connected, using ifupdown utility.

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root@papimalo:/home/ckosloff# /etc/init.d/samba stop
[ ok ] Stopping Samba AD DC daemon: samba.
[ ok ] Stopping SMB/CIFS daemon: smbd.
[ ok ] Stopping NetBIOS name server: nmbd.

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root@papimalo:/home/ckosloff# ifdown eth0
Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/eth0/00:0f:b0:c5:32:d7
Sending on   LPF/eth0/00:0f:b0:c5:32:d7
Sending on   Socket/fallback
DHCPRELEASE on eth0 to 192.168.1.254 port 67
Reloading /etc/samba/smb.conf: smbd.

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root@papimalo:/home/ckosloff# ifup eth0
Internet Systems Consortium DHCP Client 4.2.4
Copyright 2004-2012 Internet Systems Consortium.
All rights reserved.
For info, please visit https://www.isc.org/software/dhcp/

Listening on LPF/eth0/00:0f:b0:c5:32:d7
Sending on   LPF/eth0/00:0f:b0:c5:32:d7
Sending on   Socket/fallback
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 4
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 6
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 10
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 11
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 12
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 10
No DHCPOFFERS received.
No working leases in persistent database - sleeping.

Cable and router should be disregarded as causes because a netbook with same configuration connects seamlessly to wired and goes back to wireless when unplugged on same cable.
I think that this issue it not related to KDE but to a broken ethernet transceiver, that connection was never reliable, it worked like a false contact, maybe a jolt during transport finally broke it.

It does appear to be detecting the presence of the cable - the state of the connection changes.
It is possible that certain other pairs of the cable are degraded though.

Can you check the output of "dmesg" to see if any complaints concerning the adapter are made?
Also, please provide the output of the following, with the cable both connected and disconnected.

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sudo /sbin/ethtool eth0

It might also be worth it to try another cable...

Thank you Mr. Cooksley for your support.
I did try another cable, and I don't think that is the issue because same cable works flawlessly with a netbook.
My cable tester gave up, I already threw it to the garbage, did not get another one yet.
Let me ask you a few questions:
1. Laptop is now connected over wireless, should I disconnect wireless to perform tests?
2. Please let me know what would be syntax for dmesg, just the command as root?
3. I don't use sudo much, just issue commands as root, will check if my user is in the sudoers group.
Thanks again.

The only difference between connected and disconnected is in the last line, yes and no.
Dmesg has a very large output but everything related to ethernet connections looks good.
However...this laptop will not work over a wired connection.

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ckosloff@papimalo:~$ sudo /sbin/ethtool eth0
Settings for eth0:
        Supported ports: [ TP MII ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
        Supported pause frame use: No
        Supports auto-negotiation: Yes
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
        Advertised pause frame use: No
        Advertised auto-negotiation: Yes
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
        Link partner advertised pause frame use: No
        Link partner advertised auto-negotiation: Yes
        Speed: 100Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 32
        Transceiver: internal
        Auto-negotiation: on
        Supports Wake-on: pumbg
        Wake-on: d
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: yes


Hmm, a fault with the connector isn't likely if it managed to auto-negotiate a 100mbit full-duplex link.
Could you use another machine with Wireshark to see if the DHCP packets are being sent?

Thanks for answer Mr. Cooksley.
I don't have a machine ready with WIreshark, can install though, it will take some time.
However, I can give you a quick reply: knemo shows connected, but the Traffic tab shows 0.
Will test later with wireshark installed.

@ Mr. Cooksley,
Installed wireshark on both computers.
Connected wire to problem computer, knemo shows connected with 0 traffic.
Wireshark says that there are no interfaces to capture, refreshing interfaces is of no avail.
Connected working computer and it came up instantly connected through wire, flawless.
I still don't know how to capture or what it is that you would like to test.
Thank you.

What is it with the eht0:avahi? Funny that has an ip but not eth0 - haven't seen that before.

Could you post the contents of your /etc/network/interfaces?

If it looks different to

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auto eth0
allow-hotplug eth0
iface eth0 inet dhcp

please back up your old version and replace it with the above. Restart networking or your computer and things may just work.

toad wrote:Could you post the contents of your /etc/network/interfaces?

Thanks for answer.
Here is my old interfaces file

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# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
allow-hotplug eth0
iface eth0 inet dhcp

This exact same file works fine with no- problem computer

In problem computer I added line auto eth0 just above line 9, saved, restarted computer, kernel read the changes, plugged in wire.
Upon GUI loading, I attempted connection via wire, no luck, now connected over wireless

note: thread was set to solved so I reverted it back to unsolved

My next guess would be that your access point is fixed on giving you a certain ip that has already been given to another device or is otherwise blocked.

Try assigning a manual ip in your /etc/network/interfaces. Something like

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auto eth0
iface eth0 inet static
       address 169.254.21.98
       netmask 255.255.255.0
       network 169.254.21.98
       broadcast 169.254.21.255
       gateway 169.254.21.1

should do the trick (hopefully). Again, restart networking or the computer.