Sunday, October 23, 2005

RHCE: Networking and Network Configuration

One of the greatest things about Linux is the ability to easily network systems. Linux excels when using wired or wireless networking. There are several basic requirements that must be met for your Linux machine to communicate with other machines or devices on a network.

The first requirement is that your machine must have a means of communicating with other machines through hardware. This requirement is typically met through a network interface card (NIC) at the host level and a switch or router at the local area network (LAN) level. These devices are connected through ethernet cables or another suitable medium (which may also include wireless devices). This requirement is also referred to as the Physical Layer in the OSI Reference Model.

The next requirement is that each network interface must be configured properly. Each interface must be configured with an IP address, netmask, and gateway. These simple parameters allow the interface to communicate with other interfaces on the network. Each interface may be configured through the GUI tools which are provided by Red Hat, by editing configuration files with a text editor, or they can be manually configured from the command line. I will review the command line and text file configurations only. This requirement covers layers 2 and 3 of the OSI Reference Model.

Another requirement that must be met if communication with the network outside of the immediate network is desired is proper configuration of the /etc/resolv.conf file with the IP address of a valid DNS server:

The /etc/resolv.conf file, when configured properly, will allow the machine to obtain IP addresses for hosts which are known only by host name. It is critical that you do not allow any spaces before the nameserver directive in this file or it will not function. This file is configured automatically when using DHCP and manually when using a static IP.

There is a configuration file for each interface in the following directory:
These files serve as parameter files for the network startup script which is:
When the network service is started, either manually or on system boot, the network startup script reads the configuration file to determine whether or not the interface should be configured to start and how to configure the interface. If the interface is configured to obtain an IP address via DHCP then it will broadcast for a DHCP server and set the IP address, netmask, default gateway, /etc/resolv.conf file, and possibly the hostname for the machine, depending on how the system is configured. If the interface is configured to use a static IP address then most of the above settings will be configured through the configuration file in:
The hostname and /etc/resolv.conf file must be configured manually. An example of this file properly configured with a static IP address is as follows:

[root@primary network-scripts]# cat ifcfg-eth0
Most of the file is self-explanatory and fulfills the requirements listed above with regard to the interface configuration. If the interface is configured to use DHCP, the file will resemble the following:

[root@primary ~]# cat /etc/sysconfig/network-scripts/ifcfg-eth0
This simple file will cause the interface to obtain an IP address, netmask, and gateway on startup and configure the /etc/resolv.conf file with the nameserver used by the DHCP server. This is a very common configuration and makes network management very robust.

The last part of this article illustrate how to configure an interface manually on the command line. I will merely list the steps required to configure the same interface that is listed in the configuration file above with the same parameters:

ifconfig eth0
route add default gw
ifconfig eth0 up
You can now verify your settings with:

ifconfig devicename
netstat -r
To manually obtain an IP address via DHCP, use the dhclient utility. If no interface is specified then the utility will attempt to obtain a DHCP address for each non-multicast address on the system. An example of using this utility is as follows:

dhclient eth0
This command will configure the eth0 interface with an IP address, netmask, default gateway, and setup /etc/resolv.conf with the proper nameserver directives.

Through this article I explained what is required to network a Red Hat Enterprise Linux system. I also explained how to configure an interface so that the machine it is on will be able to communicate with other machines and devices on a network. The configuration can be done manually on the command line, through editting text files, and with the GUI tools provided by Red Hat with the operating system. The ability to configure Linux to interact with other systems is an essential skill, as a computer without the ability to network is useless.

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