its a fresh 12.04 install 64bits. wifi works fine, wired stays off with cable connected and network-manager shows as if cable is disconnected. Turning off networking lights up my network-cards leds, turning networking on shuts off the leds and no communication is possible.

I already tried, turning off the network-manager (sudo service network-manager stop) and setting up my eth0 manually, as soon as I switch off the network-manager my leds light up, but after setting up manually eth0 (sudo ifconfig eth0 10.2.10.114 netmask 255.255.0.0 up) the leds turn off again. I am still dual booting with 10.04 where I have no issues at all, leaving the cable connected all time to my notebook and a switch.

Here is some hardware info: lshw:

*-network
            description: Ethernet interface
            product: RTL8111/8168B PCI Express Gigabit Ethernet controller
            vendor: Realtek Semiconductor Co., Ltd.
            physical id: 0
            bus info: pci@0000:03:00.0
            logical name: eth0
            version: 03
            serial: c8:0a:a9:d7:05:97
            size: 10Mbit/s
            capacity: 1Gbit/s
            width: 64 bits
            clock: 33MHz
            capabilities: pm msi pciexpress msix vpd bus_master cap_list rom ethernet physical tp mii 10bt 10bt-fd 100bt 100bt-fd 1000bt 1000bt-fd autonegotiation
            configuration: autonegotiation=on broadcast=yes driver=r8169 driverversion=2.3LK-NAPI duplex=half firmware=rtl_nic/rtl8168d-2.fw latency=0 link=no multicast=yes port=MII speed=10Mbit/s
            resources: irq:42 ioport:2000(size=256) memory:f0004000-f0004fff memory:f0000000-f0003fff memory:f0010000-f001ffff

lspci:

    02:00.0 Network controller: Atheros Communications Inc. AR9285 Wireless Network Adapter (PCI-Express) (rev 01)
03:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 03)

ifconfig eth0:

eth0      Link encap:Ethernet  HWaddr c8:0a:a9:d7:05:97  
      inet addr:10.2.10.114  Bcast:10.2.255.255  Mask:255.255.0.0
      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)
      Interrupt:42 Base address:0xc000 

cat /etc/network/interfaces: (already tried here with and w/o eth0)

auto lo eth0
iface lo inet loopback

cat /etc/NetworkManager/NetworkManager.conf

 [main]
plugins=ifupdown,keyfile
dns=dnsmasq

[ifupdown]
managed=false

Any help is welcome ;) Laket