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How to Configure an Additional IP as an Alias in Ubuntu

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Iceman
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An additional IP is an address that is not bound to a specific device and therefore can be moved. In this article, we will cover how to configure an additional IP as an alias within Ubuntu.

Prerequisites

Topics

Configuring an Additional IP Address within Ubuntu 16.04

To begin, use the ip command to list the network interface name(s):

$ ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether 00:00:5e:00:53:3a brd ff:ff:ff:ff:ff:ff
    inet <SERVER'S_IP>/24 brd <SERVER'S_BROADCAST> scope global eth0
       valid_lft forever preferred_lft forever
    inet6 <SERVER'S_IPv6>::/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::a6bf:1ff:fe1a:57c2/64 scope link 
       valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group default qlen 1000
    link/ether 00:00:5e:00:53:3b brd ff:ff:ff:ff:ff:ff
Note: Except for the Game server, all OVHcloud servers come with at least two NICs (Network Interface Cards). The first one, eth0 in our example, is used for public (i.e., Internet) traffic. The second one, eth1 in our example, is used for private networking. For more information on this, please check out our Getting Started with vRack article.

From the output, note the public interface name, which for us is eth0.

Next, open the /etc/network/interfaces file using a text editor of your choice:

$ sudo vi /etc/network/interfaces

Then, assign your additional IP to a subinterface, also known as a virtual interface, by adding the following to the file:

auto eth0:1
iface eth0:1 inet static
        address <ADDITIONAL_IP>
        netmask 255.255.255.255
Note: Please be sure to update the interface name (e.g., eth0) with your actual interface name followed by an unused subinterface (e.g., :1) and an additional IP address that is assigned to your server.

Finally, use the systemctl command to restart the networking service:

$ sudo systemctl restart networking
Warning: If you receive the following error:

Job for networking.service failed because the control process exited with error code. See "systemctl status networking.service" and "journalctl -xe" for details.

Comment (#) out the following lines under the iface eth0 inet6 static section of the /etc/network/interfaces file:

#post-up sleep 5; /sbin/ip -family inet6 route add <IPv6_GATEWAY> dev <INT>
#post-up sleep 5; /sbin/ip -family inet6 route add default via <IPv6_GATEWAY>

Then, flush the interface and restart the networking service with the following command:
$ sudo ip addr flush dev <INT> && systemctl restart networking

Finally, from your local machine, use the ping command to test that the system can communicate via the additional IP address:

$ ping <ADDITIONAL_IP>

If you do not receive a response, review the configuration for any inaccuracies and/or restart the operating system.

Configuring an Additional IP Address within Ubuntu 18.04

Canonical, the developer of Ubuntu, implemented the use of Netplan for easy-to-use network configuration as of version 17.x. However, as of this writing, a known issue with IPv6 configuration exists between Netplan and systemd.network (the system service that manages networks). Although we are working with an IPv4 address, each Dedicated server is configured with an IPv6 address, which means network configurations are stored with systemd.network directly. As a result, we will be adding to its configuration file, bypassing Netplan entirely.

To begin, open the /etc/systemd/network/50-default.network file using a text editor of your choice:

$ sudo vi /etc/systemd/network/50-default.network

Then, add the following to the file, under the [Network] section:

address=<ADDITIONAL_IP>/32

Finally, use the systemctl command to restart the networking service, which applies the configuration:

$ sudo systemctl restart systemd-networkd

Lastly, from your local machine, use the ping command to test that the system can communicate via the additional IP address:

$ ping <ADDITIONAL_IP>

If you do not receive a response, review the configuration for any inaccuracies and/or restart the operating system.

Configuring an IP Address within Ubuntu 20.04

To begin, log into your Ubuntu instance via SSH. Find and open your network configuration file in a text editor of your choice. On a Dedicated Server, it will be the following file:

$ sudo vi /etc/netplan/50-cloud-init.yaml

Add the line in red to the file, replacing the IP address in the example with your additional IP address. In our example, our additional IP address is 203.0.113.1.

# This file is generated from information provided by the datasource. Changes
# to it will not persist across an instance reboot. To disable cloud-init's
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    version: 2
    ethernets:
        eno1:
            dhcp4: true
            addresses: [203.0.113.1/32]
            match:
                macaddress: d0:50:99:d4:f3:63
            set-name: eno1

Next apply the configuration using the following command:

$ sudo netplan apply

If you have any syntax errors in your file, follow the instructions from the output of this command. If you have no syntax errors, you will see no output. Make sure the alias is properly configured by exiting your server and pinging the additional IP address.

Conclusion

Having read this article, you should be familiar with the steps required to configure an interface with an additional IP address within Ubuntu.

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