Setting Up Network Interfaces in Linux

Here are the things that will cause the failure of your network:

• Device conflicts (a modem can conflict with the Ethernet card)
• Interface is not compatible with the hardware
• Wrong drivers were installed
• Bad cables
• Interface does not work (bad card)
• Wrong configuration settings (incorrect routing)
• Incorrect gateway prevents LAN from accessing the Internet
• The same IP address for two interfaces
• Firewall (iptables) prevents connection
• Incorrect cable (crossover instead of straight through)
• No IP forwarding or NAT prevents LAN from accessing the Internet
• Hub/switch auto negotiation fails

Here are the problems grouped according to the general nature of their reason:

No data goes from CPU to interface:

• Interface is not working (need to buy a new Ethernet card)
• Hardware not compatible (need a new Ethernet card, or even a new computer)
• Wrong driver installed (download a newer/proper driver)
• Device conflicts (need to remove/replace) conflicting device
• Iptables (firewall) blocking (edit iptables config, open port/protocol)
• Kudzu conflicts (disable kudzu with ntsysv or chkconfig)

Data gets to the interface, but not to the hub:

• Two interfaces with the same IP address (change one IP address to a different subnet)
• Bad cables (need to buy a new cat5 cable)
• Hub/switch autonegotiation fails (disable autonegotiating)

Data gets to the hub and apparently to the second computer, but not from second computer’s interface to its CPU:

• Check the first group (No data goes from CPU to interface)

LAN computer does not get through the firewall to the Internet:

• No IP forwarding or NAT (enable IPv4 forwarding, enable masquerading in iptables, enable NAT)
• Incorrect gateway (it’s the IP address of the gateway computer on the same subnet as the computer using the gateway)
• Iptables blocking (add a rule to your iptables config to allow needed connection type)

Have you ever thought about the reason why in the past the amount of Linux and Unix users was considerably smaller? Absolutely right, it was all about the absence of decent graphical interface that would allow users to configure the system the way they needed. Given operating system – Ubuntu – is extremely easy-to-use even for the beginners, because it has really nice looking and very intuitive graphical interface. However if you spend some time working with the console, you will be able to find out that it has its own important advantages. Though it allows you to do all the things you can do in graphical interface (and even more), the amount of resources it needs is almost negligible. Of course, the way you use the system is totally up to you, but we recommend you at least to try to work with console.