How to debug a network problem in Linux

[Super Gyandev]

I am not able to connect the Ethernet in my system. I am using Linux and not able to find the exact problem. Initially I thought there might be hardware issue with the onboard ethernet port. So I tried adding separate NIC card which also not worked. So I removed that port. I have installed the OS properly. I don't think there is some issue in OS. Can somebody help me on this topic.?

[Big Fish]

I know how to debug the network problem. The first thing is to check whether you have properly configured the network card. Also check the settings of it. You can change the network parameters with a command "ifconfig". When you run this command without any parameters you will get a list of network interfaces. Now you will have to load the network drivers. After loading drivers you will have to configure the interface. To check that the drivers are loaded properly the command "dmesg | grep -3 -i eth". dmesg returns information that your system discovered at boot time and the grep allows us to focus on the ethernet. Now you should be able to ping your system. After your system gets pinged try to ping your gateway. If it is not connecting to gateway check that you had plugged the cable properly. And if you can ping the gateway then later you will have to ping past gateway. If everything is done you can connect to the network.

[Shen]

Many times your ethernet device works properly but it doesn't have an IP address. To establish the IP address manually type

dhclient eth0

If this code results into positive then try some browser (Firefox, etc..) on internet to connect. You can also connect (say for Yahoo) to the URL by using code

ping any site

If this is also working then I would suggest you to disable the network service and enable the network manager by using code

su
chkconfig network off
chkconfig NetworkManager on
reboot

If everything goes good NetworkManager will start network when it needs it.

[Jardina]

Even I was having the similar problem. I was succeeded in solving that problem. Here are steps that helped me a lot. When I removed and reloaded the network kernel module there was some message from udev mentioning that the network device was renamed to eth0 to eth1. When I googled this issue I came to conclusion that I had conflicting entries in the networking device naming rules which is located in /etc/udev/rules.d/z25_persistent-net.rules. After removing conflicts my problem was fixed. Hope this will also help you.