Adding Secondary IP Addresses (CentOS/RHEL)

There are plenty of reasons you would need to add secondary IP addresss (and everyone agrees that SEO is not one of them). Getting a secondary IP address is a simple process if it is done for the right reasons and done correctly. You do NOT need additional NIC cards but you will be creating virtual adapters as the secondary IP will be routing through the primary IP.

Also, this is a great thing to do at home as I've done it to run multiple internal IP addresses on one server to run multiple applications across the same ports (for KISS** sake). Please note that I am doing this is in a virtual testing environment so your settings will definitely be different.

** KISS = Keep It Stupid Simple **

You will need to be the root user and navigate to your /etc/sysconfig/network-scripts

# cd /etc/sysconfig/network-scripts

When getting a list of files in the directory you will see "ifcfg-eth0" (or eth1 if you're doing it for a different adapter)

# ls -l | grep ifcfg-eth
-rw-r--r-- 1 root root   119 Jan 11 19:16 ifcfg-eth0
-rw-r--r-- 1 root root   119 Jan  3 08:45 ifcfg-eth0.bak
-rw-r--r-- 1 root root   119 Feb 24 04:34 ifcfg-eth1
-rw-r--r-- 1 root root   128 Jan 19 18:20 ifcfg-eth1.bak

Now adding the virtual adapters is easy. Basically if the main adapter is called "eth0" you have to call the next (virtual) adapter in a sequential order like so:

  • ifcfg-eth0 (primary adapter, physical)
  • ifcfg-eth0:1 (first virtual adapter to the physical primary adapter)
  • ifcfg-eth0:2 (second virtual adapter to the physical primary adapter)
  • and so on...

That being said, lets go ahead and copy our primary adapter configuration file and name it to be the first virtual adapter for the physical primary:

# cp ifcfg-eth0 ifcfg-eth0:1
# ls -l | grep ifcfg-eth
-rw-r--r-- 1 root root   119 Jan 11 19:16 ifcfg-eth0
-rw-r--r-- 1 root root   119 Feb 24 08:53 ifcfg-eth0:1
-rw-r--r-- 1 root root   119 Jan  3 08:45 ifcfg-eth0.bak
-rw-r--r-- 1 root root   119 Feb 24 04:34 ifcfg-eth1
-rw-r--r-- 1 root root   128 Jan 19 18:20 ifcfg-eth1.bak

Now, we have to configure this virtual adapter to be: a static IP (of course), no hardware address (MAC), configure netmask and of course rename the device.

# vim ifcfg-eth0:1

There is no need to specify a MAC address as it is a virtual adapter and there is also no need to specify a default gateway as it is already routed through the primary adapter. Basically there are only four things that you will need to change:

  • File name for the adapter itself
  • DEVICE=<device name> (should correspond with the file name)
  • IPADDR=<ip address>
  • NETMASK=<netmask>

Afterwards, just restart the networking service:

# service network restart

That's it; lets check ifconfig to make sure the virtual adapter is there and working:

# ifconfig eth0:1
eth0:1    Link encap:Ethernet  HWaddr 08:00:27:ED:05:B7
inet addr:  Bcast:  Mask:

# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=64 time=0.073 ms
64 bytes from icmp_seq=2 ttl=64 time=0.042 ms
64 bytes from icmp_seq=3 ttl=64 time=0.029 ms
64 bytes from icmp_seq=4 ttl=64 time=0.029 ms
--- ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 2999ms
rtt min/avg/max/mdev = 0.029/0.043/0.073/0.018 ms

Per Steven's comment: a second note. If you're not sure if you've done it right and you do not want to restart the entire network server, you can use the following:

# ifup eth0:1
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