Tutorials How to configure floating IP on CentOS

How to configure floating IP on CentOS

Once you get a confirmation from the UpCloud Support that a floating IP has been added, you can find it attached to one of the servers at your UpCloud Control Panel, but using the added IP will require some manual setup. Follow the steps below to find out how to get this done on CentOS servers.

Set up static IP addresses

For this example, the server 1 ( and server 2 ( have a common floating IP, netmask and gateway

Next, you need to configure the servers at the OS level, so start up your systems and log in.

Check your current network settings with the following command.

ip addr

Commonly the second network interface card (NIC) named eth0 has your public IPv4 address assigned to it, see the example below.

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
   link/ether 6e:d7:1b:bf:3a:5f brd ff:ff:ff:ff:ff:ff
   inet brd scope global eth0
      valid_lft 53810sec preferred_lft 53810sec
   inet6 fe80::6cd7:1bff:febf:3a5f/64 scope link
      valid_lft forever preferred_lft forever

In addition to the IP addresses, you will need the default gateway, it can be found with the command underneath.

ip route
default via dev eth0 via dev eth1 dev eth1 proto kernel scope link src dev eth0 proto kernel scope link src

Set the address to static on the NIC with your public IPv4, eth0 in this case.

On CentOS hosts each NIC is controlled by its own configuration file, to edit the eth0 for example, use the command underneath.

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

You’ll need to change the bootproto from dhcp to static, then add the IP address, netmask and gateway to the end.


After making the changes, save the file and exit. You will then need to restart the interface to enable the static configuration.

sudo ifdown eth0 && sudo ifup eth0

If you were connected with SSH, the interface restart should be fast enough to not cause you to disconnect. In case you do loose connection and are unable to reconnect, you can always use the web console at UpCloud Control Panel under your server settings to go through the setup again to make sure everything is entered correctly.

Repeat the steps on your other server to configure it with static IP address as well.

Configure the floating IP

To set up the floating IP, you’ll need to add it as an alias to the network interface with your public static IP configured to.

The easiest way to do this is to duplicate the configuration file of the NIC with your public static IP, this way you don’t need to write the whole file from scratch. For example, with eth0 you can use the following command.

cp /etc/sysconfig/network-scripts/ifcfg-eth0 /etc/sysconfig/network-scripts/ifcfg-eth0:1

Then edit the new alias interface.

sudo vi /etc/sysconfig/network-scripts/ifcfg-eth0:1

Add the same “:1” to the device name, enter a new parameter NM_CONTROLLED=no, replace the IP address with the floating IP and then remove the gateway line.


Save the file and exit after these changes.

Repeat the process to add the alias on any other servers you wish to use the floating IP on.

Enabling traffic to the floating IP

With the new alias interfaces set up, check which server the floating IP is attached to under the Public network list at your UpCloud Control Panel. You should then turn on the alias NIC on that server, in this example using the following command on your Linux host.

sudo ifup eth0:1

Check your network configuration again with the same command as before.

ip addr

The newly added alias shows up under the NIC with your public IP with the floating IP.

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
   link/ether 6e:d7:1b:bf:3a:5f brd ff:ff:ff:ff:ff:ff
   inet brd scope global eth0
   valid_lft forever preferred_lft forever
   inet brd scope global secondary eth0:1
   valid_lft forever preferred_lft forever

Now attempt to connect to your server through the floating IP with SSH or if you have a web server configured open the floating IP on your web browser.

If you get a reply, the floating IP works on that server and you can continue forward. If it didn’t work, make sure you entered the IP address and netmask correctly, that your firewall isn’t blocking your connections, or try another method to connect.

Test the configuration

Your configuration should now be complete, you can test that it works by transferring the floating IP from one server to another.

To do this, turn off the floating IP alias NIC on the server it’s currently attached to, on CentOS servers use the following command.

sudo ifdown eth0:1

Then turn on the alias NIC on your other server.

sudo ifup eth0:1

Enabling the alias on a new host running CentOS will automatically direct any connections through the floating IP to that server.

Test the floating IP again with any method you prefer. When you get a connection you have successfully transferred the floating IP.

Using your new floating IP

You can now transfer the floating IP between your cloud servers by turning off the alias NIC on the current server, and then turning on the alias on the new destination host.

Depending on your intended use case for the floating IP you may wish to continue by setting up automated load balancing, but if you wish to manually transfer the traffic between your servers, it might be useful to create scripts of the required commands.

2 thoughts on “How to configure floating IP on CentOS

    1. Hi Bill, thanks for checking out the guide. Managing a floating IP will require some care to be used appropriately. This guide only covers the examples for enabling and testing the floating IP.

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