Networking not working - Cannot connect to the Internet

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Whenever troubleshooting networking problems, the output of ifconfig would be required:

$ ifconfig  -a
eth0      Link encap:Ethernet  HWaddr 00:24:1d:d0:d7:cb 
          inet addr:192.168.1.101  Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::224:1dff:fed0:d7cb/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:182370750 errors:0 dropped:0 overruns:0 frame:0
          TX packets:190535711 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:130894066157 (130.8 GB)  TX bytes:131294765683 (131.2 GB)
          Interrupt:42 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:816196 errors:0 dropped:0 overruns:0 frame:0
          TX packets:816196 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:160307139 (160.3 MB)  TX bytes:160307139 (160.3 MB)

You should see at least the 'lo' network interface, and another one ('eth0' or other).

Hardware issues

If no eth0 nor other interfaces show in ifconfig above, check for the presence of the hardware:

# lspci | grep Ethernet
02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 02)

# lshw -C network
  *-network              
       description: Ethernet interface
       product: RTL8111/8168B PCI Express Gigabit Ethernet controller
       vendor: Realtek Semiconductor Co., Ltd.
       physical id: 0
       bus info: pci@0000:02:00.0
       logical name: eth0
       version: 02
       serial: 00:24:1d:d0:d7:cb
       size: 100Mbit/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 ip=192.168.1.101 latency=0 link=yes multicast=yes port=MII speed=100Mbit/s
       resources: irq:42 ioport:de00(size=256) memory:fdfff000-fdffffff memory:fdfe0000-fdfeffff memory:fdf00000-fdf0ffff

The above output is normal and may be useful to fix your system.
However, if either command return nothing, then there is a hardware problem:

# lspci | grep Ethernet
# lshw -C network

If so, here is a list of items to consider, and if all fails, then it's definitely a hardware issue and you should replace the ethernet port, whether on board or not, with a new discrete PCI ethernet card.

- Rule out a problem with the operating system. Try the above with a live rescue CD. Boot the computer with the live CD and use it to perform diagnostics. You may want to unplug the hard drives so that the original OS is never booted until a proper diagnostic has been established. Try all the steps below, always rebooting with the live CD to check the results.
- Reset all bios settings to their default.
- Clear the CMOS cache (RTC RAM) using the onboard jumpers (check your mother board manual).
- unplug the computer and the router and leave them unplugged for a few minutes before trying again.
- make sure the router and the ethernet cable are working with another computer. Pay special attention to the lights both on the router and in the ethernet port on the back panel of your computer. With a working computer/connection, do you get green lights? With the broken computer/connection, are some lights orange or missing?
- Try cleaning the connectors in the ethernet port with some q-tip and alcohol.

If all fails, the ethernet port appears to be dead. You should consider buying a discrete ethernet card to replace it:
http://linux.overshoot.tv/wiki/ethernet_port_onboard_or_ethernet_card

Issues related to this page:

ProjectSummaryStatusPriorityCategoryLast updatedAssigned to
Linux softwareMy eth0 has gone and I don't have internet and …activenormalbug report39 weeks 6 days