Configuring IP aliasing

Last updated 16th September 2021

Objective

IP aliasing is a special network configuration for your OVHcloud dedicated servers, which allows you to associate multiple IP addresses with a single network interface.

This guide explains how to add failover IP addresses to your network configuration.

Requirements

• a dedicated server in your OVHcloud account
• a failover IP address or a failover IP block (RIPE)
• administrative access (root) via SSH or GUI to your server
• basic networking and administration knowledge

Instructions

The following sections contain the configurations for the most commonly used distributions/operating systems.

Debian 10

Step 1: Create a backup

First, make a copy of the config file, so that you can revert at any time:

cp /etc/network/interfaces.d/50-cloud-init /etc/network/interfaces.d/50-cloud-init.bak

Step 2: Edit the config file

You can now modify the config file:

editor /etc/network/interfaces.d/50-cloud-init

You then need to add a secondary interface:

auto eth0:0
iface eth0:0 inet static
address FAILOVER_IP
netmask 255.255.255.255

To ensure that the secondary interface is enabled or disabled whenever the eth0 interface is enabled or disabled, you need to add the following line to the eth0 configuration:

post-up /sbin/ifconfig eth0:0 FAILOVER_IP netmask 255.255.255.255 broadcast FAILOVER_IP
pre-down /sbin/ifconfig eth0:0 down

If you have two failover IPs to configure, the /etc/network/interfaces.d/50-cloud-init file should look like this:

auto eth0
iface eth0 inet dhcp

auto eth0:0
iface eth0:0 inet static
address FAILOVER_IP1
netmask 255.255.255.255

auto eth0:1
iface eth0:1 inet static
address FAILOVER_IP2
netmask 255.255.255.255

Or like this:

auto eth0
iface eth0 inet dhcp

# IPFO 1
post-up /sbin/ifconfig eth0:0 FAILOVER_IP1 netmask 255.255.255.255 broadcast FAILOVER_IP1
pre-down /sbin/ifconfig eth0:0 down

# IPFO 2
post-up /sbin/ifconfig eth0:1 FAILOVER_IP2 netmask 255.255.255.255 broadcast FAILOVER_IP2
pre-down /sbin/ifconfig eth0:1 down

Step 3: Restart the interface

You now need to restart your interface:

/etc/init.d/networking restart

Debian 6/7/8 and derivatives

Step 1: Create a backup

First, make a copy of the config file, so that you can revert at any time:

cp /etc/network/interfaces /etc/network/interfaces.bak

Step 2: Edit the config file

You can now modify the config file:

editor /etc/network/interfaces

You then need to add a secondary interface:

auto eth0:0
iface eth0:0 inet static
address FAILOVER_IP
netmask 255.255.255.255

To ensure that the secondary interface is enabled or disabled whenever the eth0 interface is enabled or disabled, you need to add the following line to the eth0 configuration:

post-up /sbin/ifconfig eth0:0 FAILOVER_IP netmask 255.255.255.255 broadcast FAILOVER_IP
pre-down /sbin/ifconfig eth0:0 down

If you have two failover IPs to configure, the /etc/network/interfaces file should look like this:

auto eth0
iface eth0 inet static
address SERVER_IP
netmask 255.255.255.0
broadcast xxx.xxx.xxx.255
gateway xxx.xxx.xxx.254

auto eth0:0
iface eth0:0 inet static
address FAILOVER_IP1
netmask 255.255.255.255

auto eth0:1
iface eth0:1 inet static
address FAILOVER_IP2
netmask 255.255.255.255

Or like this:

auto eth0
iface eth0 inet static
address SERVER_IP
netmask 255.255.255.0
broadcast xxx.xxx.xxx.255
gateway xxx.xxx.xxx.254

# IPFO 1
post-up /sbin/ifconfig eth0:0 FAILOVER_IP1 netmask 255.255.255.255 broadcast FAILOVER_IP1
pre-down /sbin/ifconfig eth0:0 down

# IPFO 2
post-up /sbin/ifconfig eth0:1 FAILOVER_IP2 netmask 255.255.255.255 broadcast FAILOVER_IP2
pre-down /sbin/ifconfig eth0:1 down

Step 3: Restart the interface

You now need to restart your interface:

/etc/init.d/networking restart

Debian 9+, Ubuntu 17.04, Fedora 26+ and Arch Linux

On these distributions, the naming of interfaces as eth0, eth1 (and so on) is abolished. We will therefore use systemd-network more generally.

Step 1: Create a backup

First, make a copy of the config file, so that you can revert at any time:

cp /etc/systemd/network/50-default.network /etc/systemd/network/50-default.network.bak

Step 2: Edit the config file

You can now add your failover IP to the config file, as follows:

editor /etc/systemd/network/50-default.network

[Address]
Address=FAILOVER_IP/32
Label=failover1 # optional

The label is optional. It’s just for distinguishing between your various failover IPs.

Step 3: Restart the interface

You now need to restart your interface:

systemctl restart systemd-networkd

Ubuntu 17.10 and following

Each failover IP address will need its own line in the configuration file. The configuration file is called "50-cloud-init.yaml" and is located in /etc/netplan.

Step 1: Determine the interface

ifconfig

Note the interface name and its MAC address

Step 2: Create the configuration file

Connect to your server via SSH and run the following command:

editor /etc/netplan/50-cloud-init.yaml

Next, edit the file with the content below, replacing INTERFACE_NAME MAC_ADDRESS and FAILOVER_IP:

network:
    version: 2
    ethernets:
        INTERFACE_NAME:
            dhcp4: true
            match:
                macaddress: MAC_ADDRESS
            set-name: INTERFACE_NAME
            addresses:
            - FAILOVER_IP/32

Save and close the file. You can test the configuration with the following command:

netplan try

Step 3: Apply the change

Next, run the following commands to apply the configuration:

netplan apply

CentOS and Fedora (25 and earlier)

Step 1: Create the config file

First, make a copy of the source file so that you can use it as a template:

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

Step 2: Edit the config file

You can now modify the eth0:0 file in order to replace the IP:

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

First, replace the name of the device, then replace the existing IP with the failover IP you have received:

DEVICE="eth0:0"
ONBOOT="yes"
BOOTPROTO="none" # For CentOS use "static"
IPADDR="FAILOVER_IP"
NETMASK="255.255.255.255"
BROADCAST="FAILOVER_IP"

Step 3: Start the alias interface

You now need to start your alias interface:

ifup eth0:0

Gentoo

Step 1: Create a backup

First, make a copy of the config file, so that you can revert at any time:

cp /etc/conf.d/net /etc/conf.d/net.bak

Step 2: Edit the config file

Now you have to edit the file to add the failover IP. In Gentoo, an alias is added directly in the eth0 interface. You do not need to create an eth0:0 interface like in other distributions.

All you need to do is add a line break after the netmask 255.255.255.0 and add your failover IP (SERVER_IP must be replaced by your server’s primary IP).

editor /etc/conf.d/net

You therefore need to add the following:

config_eth0=( "SERVER_IP netmask 255.255.255.0" "FAILOVER_IP netmask 255.255.255.255 brd FAILOVER_IP" )

The /etc/conf.d/net file must contain the following:

#This blank configuration will automatically use DHCP for any net.
# scripts in /etc/init.d. To create a more complete configuration,
# please review /etc/conf.d/net.example and save your configuration
# in /etc/conf.d/net (this file :]!).
config_eth0=( "SERVER_IP netmask 255.255.255.0"
"FAILOVER_IP netmask 255.255.255.255 brd FAILOVER_IP" )
routes_eth0=( "default gw SERVER_IP.254" )

In order to ping your failover IP, simply restart the network interface.

Step 3: Restart the interface

You now need to restart your interface:

/etc/init.d/net.eth0 restart

openSUSE

Step 1: Create a backup

First, make a copy of the config file, so that you can revert at any time:

cp /etc/sysconfig/network/ifcfg-ens32 /etc/sysconfig/network/ifcfg-ens32.bak

Step 2: Edit the config file

Open the file:

editor /etc/sysconfig/network/ifcfg-ens32

Then add the following:

IPADDR_1=FAILOVER_IP
NETMASK_1=255.255.255.255
LABEL_1=ens32:0

Finally, reboot your server to apply the changes.

cPanel (on CentOS 6)

Step 1: Create a backup

First, make a copy of the config file, so that you can revert at any time:

cp /etc/ips /etc/ips.bak

Step 2: Edit the config file

You then need to edit the /etc/ips file:

editor /etc/ips

Then add the failover IP to the file:

FAILOVER_IP:255.255.255.255:FAILOVER_IP

Next, add the IP in `/etc/ipaddrpool``:

FAILOVER_IP

Step 3: Restart the interface

You now need to restart your interface:

/etc/init.d/ipaliases restart

Windows Servers

Windows servers are often DHCP-enabled in the network configuration. If you have already set up a failover IP or switched your configuration to a fixed IP, go directly to the next step.

Otherwise, you need to first switch from a network-level DHCP configuration to a fixed IP configuration.

Open the command prompt cmd or powershell, then type the following command:

ipconfig /all

This will return a result similar to the following example:

Identify and write down your IPv4, subnet mask, default gateway and the name of the network interface controller (network adapter).

In our example, the server IP is 94.23.229.151.

You can perform the next steps via either a command-line interface or the graphical user interface.

Via a command-line interface (recommended)

In the commands below, you need to replace:

In the command prompt:

1. Switch to a fixed IP

netsh interface ipv4 set address name="NETWORK_ADAPTER" static IP_ADDRESS SUBNET_MASK GATEWAY

1. Set the DNS server

netsh interface ipv4 set dns name="NETWORK_ADAPTER" static 213.186.33.99

1. Add a failover IP

netsh interface ipv4 add address "NETWORK_ADAPTER" IP_ADDRESS_FAILOVER 255.255.255.255

Your failover IP is now functional.

Via the graphical user interface

1. Go to Start> Control Panel>Network and Internet> Network and Sharing Centre> Change Adapter Settings(in the left-hand menu).
2. Right-click on Local Area Connection.
3. Click on Properties.
4. Select Internet Protocol Version 4 (TCP/IPv4), then click on Properties.
5. Click on Use the following IP address and type in your server’s primary IP, subnet mask and default gateway information obtained by using the ipconfig command above. In the "Preferred DNS Server" box, type 213.186.33.99.

Then click on Advanced (still in the TCP/IP Settings).

In the IP Address section, click Add:

Type in your failover IP and the subnet mask 255.255.255.255.

Click on Add.

Your failover IP is now functional.

FreeBSD

Step 1: Determine the interface

Determine the name of your primary network interface. You can use the ifconfig command for this operation:

ifconfig

This will return the following:

ifconfig
>>> nfe0: flags=8843 metric 0 mtu 1500
>>> options=10b
>>> ether 00:24:8c:d7:ba:11
>>> inet 94.23.196.18 netmask 0xffffff00 broadcast 94.23.196.255
>>> inet 87.98.129.74 netmask 0xffffffff broadcast 87.98.129.74
>>> media: Ethernet autoselect (100baseTX )
>>> status: active
>>> lo0: flags=8049 metric 0 mtu 16384
>>> options=3
>>> inet6 fe80::1%lo0 prefixlen 64 scopeid 0x2
>>> inet6 ::1 prefixlen 128
>>> inet 127.0.0.1 netmask 0xff000000 v comsdvt#

In our example, the name of the interface is therefore nfe0.

Step 2: Create a backup

Next, make a copy of the config file, so that you can revert at any time:

cp /etc/rc.conf /etc/rc.conf.bak

Step 3: Edit the config file

Edit the /etc/rc.conf file:

editor /etc/rc.conf

Then add this line at the end of the file: ifconfig_INTERFACE_alias0="inet FAILOVER_IP netmask 255.255.255.255 broadcast FAILOVER_IP".

Replace INTERFACE and FAILOVER_IP with the name of your interface (identified in the first step) and your failover IP, respectively. Here is an example:

ifconfig_nfe0_alias0="inet 87.98.129.74 netmask 255.255.255.255 broadcast 87.98.129.74"

Step 4: Restart the interface

You now need to restart your interface:

/etc/rc.d/netif restart && /etc/rc.d/routing restart

Solaris

Step 1: Determine the interface

Determine the name of your primary network interface. You can use the ifconfig command for this operation:

ifconfig -a

This will return the following:

ifconfig -a
lo0:     flags=2001000849 mtu 8232 index 1 
         inet 127.0.0.1 netmask ff000000 
e1000g0: flags=1000843 mtu 1500 index 2 
         inet 94.23.41.167 netmask ffffff00 broadcast 94.23.41.255 
         ether 0:1c:c0:f2:be:42

In our example, the name of the interface is therefore e1000g0.

Step 2: Create the config file

editor /etc/hostname.e1000g0:1

In this file, enter the following: FAILOVER_IP/32 up, where FAILOVER_IP is your failover IP. For example:

188.165.171.40/32 up

Step 3: Restart the interface

You now need to restart your interface:

svcadm restart svc:/network/physical:default

Troubleshooting

First, restart your server from the command line or its GUI. If you are still unable to establish a connection from the public network to your alias IP and suspect a network problem, you need to reboot the server in rescue mode. Then you can set up the failover IP address directly on the server.

Once you are connected to your server via SSH, enter the following command:

ifconfig eth0:0 FAILOVER_IP netmask 255.255.255.255 broadcast FAILOVER_IP up

To test the connection, simply ping your failover IP from the outside. If it responds in rescue mode, that probably means that there is a configuration error. If, however, the IP is still not working, please inform our support teams by creating a support request in your OVHcloud Control Panel for further investigations.

Go further

Activating and using rescue mode

Join our community of users on https://community.ovh.com/en/.

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