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Ubuntu is unable to connect to internet, both live CD and installed Ubuntu can't connect to network. The system sees the Ethernet interface from the look of ifconfig output:

  Link encap:Ethernet  HWaddr 00:1d:04:4b:c4:bd  
  inet6 addr: fe80::23d:9ff:fe4b:c4bd/64 Scope:Link        
  RX packets:302 errors:0 dropped:0 overruns:0 frame:0   
  TX packets:343 errors:0 dropped:0 overruns:0 carrier:0      
  collisions:0 txqueuelen:1000        
  RX bytes:20208 (20.2 KB)  TX bytes:74524 (74.5 KB)         

  Link enc ap:Local Loopback      
  inet addr:  Mask:        
  inet6 addr: ::1/128 Scope:Host         
  UP LOOPBACK RUNNING  MTU:16436  Metric:1         
  RX packets:756 errors:0 dropped:0 overruns:0 frame:0        
  TX packets:756 errors:0 dropped:0 overruns:0 carrier:0         
  collisions:0 txqueuelen:0        
  RX bytes:87621 (87.6 KB)  TX bytes:87621 (87.6 KB)*

dhclient eth0: 

*RTNETLINK answer: Operation not permitted*
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The problem would appear to be your computer does not have an IP address, and you are trying to invoke the dhclient to get it - but you are not root. You need to be root to attain a DHCP lease (try sudo dhclient eth0)

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sudo dhclient eth0: [1]2591 but no connection – user83171 Mar 12 '13 at 6:45
What happens if you try and bring the link up with a static IP, for example using the commands ifconfig eth0 X.X.X.X; route add default gw X.X.X.Y – davidgo Mar 12 '13 at 6:56
i configured static IP and get ping working, but browser can't find pages claiming DNS lookup failed. – user83171 Mar 12 '13 at 7:17
ok its working, thanks for help – user83171 Mar 12 '13 at 7:23
Does DHCP work for other machines ? It sounds to me like your DHCP server is not handing out DHCP leases. The problem you have with the static IP address can be rectified by editing /etc/resolv.conf and adding a line "nameserver X.X.X.X" - of-course, the point of setting it up statically was to eliminate potential problems - we now know that your ethernet interface is working and connected to the Internet :> – davidgo Mar 12 '13 at 7:24

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