Changing the networking settings of a CentOS or Ubuntu server is an easy task, no matter if you know to work with a linux distribution or not.
So let’s get started.

CentOS

The first method I’ll use is the manual one, using an editor like nano.
The file you are going to edit is: /etc/sysconfig/network-scripts/ifcfg-eth0

nano /etc/sysconfig/network-scripts/ifcfg-eth0

And here is how you should edit this file:

DEVICE=eth0
BOOTPROTO=static
BROADCAST=192.168.1.255
HWADDR=00:0E:0C:C1:E2:00
IPADDR=192.168.1.2
NETMASK=255.255.255.0
NETWORK=192.168.1.0
ONBOOT=yes

Now let me explain a bit some of this options.
DEVICE – the device you are configuring
BOOTPROTO=static – the ip configuration is set to static, no DHCP
IPADDR – the ip address assigned to this interface
BROADCAST, NETWORK and NETMASK – you can calculate this options using an online ipcalc
ONBOOT=yes – all this options will apply at system startup
HWADDR – this is the mac address of the interface

Now let’s edit /etc/sysconfig/network

nano /etc/sysconfig/network

Edit this file in order to look like this:

NETWORKING_IPV6=no
NETWORKING=yes
GATEWAY=192.168.1.1
HOSTNAME=ServerHostName

I guess there’s no need to discuss this options.
Don’t forget to replace 192.168.1.x with your gateway, do this for the previous file also.

If you need to change the dns ip addresses also:

nano /etc/resolv.conf

The file looks like this:

nameserver 192.168.1.1
nameserver 192.168.2.1

Don’t forget that all 192.168.x.x ip’s are used only as an example, replace them with your own.
As a final step you should restart the server or simply restart networking like so:

/etc/init.d/network restart

Now after you learned this the hard way, you can try the easy graphical tool, run:

system-config-network

Follow the steps and you are done.

Ubuntu

Ubuntu is a bit different then CentOS, that applies for configuration file location and format.
Let’s start by editing /etc/network/interfaces

nano /etc/network/interfaces

And here is an example of this file configured with an 192.168.x.x ip address:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
 address 192.168.1.2
 netmask 255.255.255.0
 network 192.168.1.0
 broadcast 192.168.1.254
 gateway 192.168.1.1
 dns-nameservers 192.168.1.1

The description I gave for CentOS applies here also, with a few exceptions:
address – this is the ip address
dns-nameservers – this is the dns that usually is configured into /etc/resolv.conf
And in the end you need to reboot the server or restart networking like this:

/etc/init.d/networking restart

Claudiu Popescu

VIsit Claudiu Popescu's website