I recently dual-booted my system with Windows 7 and 12.04. I can't access Internet through wired connection, in spite of LAN cable in the socket. It says network cable unplugged, but I can access Internet from Windows 7.
During the instillation phase for Ubuntu, when it tries to check the network connection, it was giving the same message that network cable is unplugged.
Previously, I was using 10.04 along with Windows XP and it was working fine.
Here is some information you may find useful.
ifconfig -a
eth0 Link encap:Ethernet HWaddr 00:21:70:e7:cc:93
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:44 Base address:0xa000
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:16436 Metric:1
RX packets:1328 errors:0 dropped:0 overruns:0 frame:0
TX packets:1328 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:105328 (105.3 KB) TX bytes:105328 (105.3 KB)
wlan0 Link encap:Ethernet HWaddr 00:24:2b:dc:04:ed
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)
sudo lshw -class network
*-network
description: Network controller
product: BCM4312 802.11b/g LP-PHY
vendor: Broadcom Corporation
physical id: 0
bus info: pci@0000:06:00.0
version: 01
width: 64 bits
clock: 33MHz
capabilities: pm msi pciexpress bus_master cap_list
configuration: driver=b43-pci-bridge latency=0
resources: irq:19 memory:f4000000-f4003fff
*-network DISABLED
description: Ethernet interface
product: RTL8111/8168B PCI Express Gigabit Ethernet controller
vendor: Realtek Semiconductor Co., Ltd.
physical id: 0
bus info: pci@0000:07:00.0
logical name: eth0
version: 02
serial: 00:21:70:e7:cc:93
size: 1Gbit/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=full firmware=N/A latency=0 link=no multicast=yes port=MII speed=1Gbit/s
resources: irq:44 ioport:5000(size=256) memory:f8610000-f8610fff memory:f8600000-
f860ffff memory:f8620000-f862ffff
*-network DISABLED
description: Wireless interface
physical id: 2
logical name: wlan0
serial: 00:24:2b:dc:04:ed
capabilities: ethernet physical wireless
configuration: broadcast=yes driver=b43 driverversion=3.2.0-23-generic-pae firmware=N/A
link=no multicast=yes wireless=IEEE 802.11bg
sudo ethtool eth0
Settings for eth0:
Supported ports: [ TP MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Advertised pause frame use: Symmetric Receive-only
Advertised auto-negotiation: Yes
Link partner advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Link partner advertised pause frame use: Symmetric Receive-only
Link partner advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: MII
PHYAD: 0
Transceiver: internal
Auto-negotiation: on
Supports Wake-on: pumbg
Wake-on: pumbg
Current message level: 0x00000033 (51)
Link detected: no
http://askubuntu.com/questions/177083/wifi-and-ethernet-connectivity-problems-after-upgrading-to-12-04
– Prasad Mar 29 '13 at 12:01