Linux Basics - Set A Static IP On CentOS

Version 1.0

This tutorial explains the configuration of a static IP address (IPv4 and IPv6), the hostname and nameservers on CentOS. To be compatible with Desktop and Server Systems, we do the configuration on the shell.

1 Preliminary Note

Suppose you are working in a data center & your boss puts a minimal CentOS server setup & you need to configure it in the running environment. Yes it is little painstaking, but not very tough task. In my case I have a dumb CentOS server which was installed by someone in his networking environment & I want to make it functional in my static IP environment. Suppose I have a vacant IP 192.168.0.100 & I will implement it in my environment. My IP details are as follows:

IPv4

IP address: 192.168.0.100
Subnet: 255.255.255.0
Gateway: 192.168.0.1

IPv-6

IP address: 2001:db8::c0ca:1eaf
Netmask 64
Gateway 2001:db8::1ead:ed:beef

DNS resolving nameservers

Nameserver 1: 8.8.8.8
Nameserver 2: 8.8.4.4

The above values have to be adjusted for your local network enviroment. Just the FNS resolving nameservers can be used on most networks as these IP addresses belong to the public nameservers from Google.

2 Implementation

I will do a configuration file editing with the editor like vi. But you may use any other shell editor like nano or joe instead. The file name for the first network card (eth0) is /etc/sysconfig/network-scripts/ifcfg-eth0

I will first make backup of my original file as /etc/sysconfig/network-scripts/ifcfg-eth0.bak & then proceed for the changes in /etc/sysconfig/network-scripts/ifcfg-eth0

mv /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/ifcfg-eth0.bak

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

I will change the file like this:

#My IP description
# IPv-4

DEVICE="eth0"
NM_CONTROLLED="yes"
ONBOOT=yes
HWADDR=20:89:84:c8:12:8a
TYPE=Ethernet
BOOTPROTO=static
NAME="System eth0"
UUID=5fb06bd0-0bb0-7ffb-45f1-d6edd65f3e03
IPADDR= 2001:db8::c0ca:1eaf
NETMASK=255.255.255.0

Only the Lines IPADDR and NETMASK have to be adjusted. Do not change the other lines.

For IPv6 You just have to add the entries below:

vi /etc/sysconfig/network

[...]
NETWORKING_IPV6=yes

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

[...]
#IPv-6
IPV6INIT=yes
IPV6ADDR=2001:db8::c0ca:1eaf
IPV6_DEFAULTGW=2001:db8::1ead:ed:beef

3 DNS configuration

DNS can be added in the file /etc/resolv.conf

vi /etc/resolv.conf

[...]
nameserver	8.8.8.8
nameserver	8.8.4.4

You can add 2 or more namserver lines. Your system will try the other ones in case that the first nameserver is unreachable.

4 Hostname

In my case the hostname is server1.example.com to set the hostname, I will edit the /etc/hosts file:

vi /etc/hosts

127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
192.168.0.100   server1.example.com     server1

::1         localhost localhost.localdomain localhost6 localhost6.localdomain6

and the resolv.conf file:

vi /etc/resolv.conf

NETWORKING=yes
HOSTNAME=server1.example.com
GATEWAY=192.168.0.1
[...]

and reboot the server.

reboot

Check your hostname with the command below. The new hostname will be applied after reboot only

hostname

5 Advanced networking and virtual network interfaces

I am using CentOS Linux and I would like to create alias for eth0:0 so that I can have multiple IP address. I will implemented by creating a file as follows:

vi /etc/sysconfig/network-scripts/ifcfg-eth0:0

#IP Aliasing

DEVICE="eth0:0"
BOOTPROTO="static"
HWADDR=20:89:84:c8:12:8a
NM_CONTROLLED="no"
ONBOOT="yes"
TYPE="Ethernet"
IPADDR=192.168.0.108
NETMASK=255.255.255.0
GATEWAY=192.168.0.1
DNS1=8.8.8.8
DNS1=8.8.4.4

Here I have done the IP aliasing for the IP 192.168.0.108, it could vary as per your requirement.

6 Services

After any change in the networking files you need to restart the network services as follows:

/etc/init.d/network restart

After the service restart you can check the changes as:

ifconfig

The output will confirm the changes done statically. It will be almost similar like this:

root@server1:~# ifconfig
eth0      Link encap:Ethernet HWaddr 20:89:84:c8:12:8a
          inet addr:192.168.0.100 Bcast:192.168.0.255 Mask:255.255.255.0
          inet6 addr: 2001:db8::c0ca:1eaf/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
          RX packets:200197 errors:0 dropped:67 overruns:0 frame:0
          TX packets:69689 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:64103748 (64.1 MB) TX bytes:14106191 (14.1 MB)

eth0:0    Link encap:Ethernet HWaddr 20:89:84:c8:12:8a
          inet addr:192.168.0.108 Bcast:192.168.0.255 Mask:255.255.255.0
        UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

lo        Link encap:Local Loopback
          inet addr:127.0.0.1 Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING MTU:65536 Metric:1
          RX packets:10365 errors:0 dropped:0 overruns:0 frame:0
          TX packets:10365 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0
          RX bytes:875114 (875.1 KB) TX bytes:875114 (875.1 KB)

Note Above values will differ in your case.