How to: Assign multiple IP addresses to one interface in Ubuntu using the Command Line Interface (CLI)?

How to: Assign multiple IP addresses to one interface in Ubuntu using the Command Line Interface (CLI)?

I have been working with Ubuntu more lately and ran into the need to direct traffic going to one server (via IP) to go to a new server but I couldn’t change the client’s configuration. Because part of the services were already migrated over I could not change the IP address to the old server. Because of this I required a way for the new server to answer on the new IP address assigned to it but also listen to the old server’s IP address. To do this you need your server to have more than one IP address which is possible (have done it with Windows Servers for several years) so just needed to find out how to do it on Ubuntu

Solution

1. If you need an additional IP address just for the moment you can add it to any interface on your machine with

    sudo ip address add <ip-address>/<netmask> dev <interface>
   

   for example

    sudo ip address add 172.16.100.17/24 dev eth0
   

   would add 172.16.100.17 using a 24bit netmask to the list of addresses configured for your eth0.

   You can check the result with

   ip address show eth0
   

   and you can delete this address again with

   sudo ip address del 172.16.100.17/24 dev eth0
   

   Of course these changes are lost when you reboot your machine.

2. To make the additional addresses permanent you can edit the file /etc/network/interfaces by adding as many stanzas as needed of the form:

   iface eth0 static
       address 172.16.100.17/24
   

   so if you needed say 2 IP addresses you would do so as follows:

   iface eth0 inet dhcp
   
   iface eth0 inet static
       address 172.16.100.17/24
   
   iface eth0 inet static
       address 172.16.24.15/24
   

   Note we left the dhcp on the first line, so you will end with 3 ip addresses, 2 static ones provided above and one assigned to you by the DHCP server which will be your primary address.

   To activate these settings without a reboot use ifdown/ifup likeso:

   sudo ifdown eth0 && sudo ifup eth0
   

   It is essential to put those two commands into one line if you are remoting into the server because the first one will drop your connection! Given in this way the ssh-session will survive and you’ll be able to reconnect. Exercise caution as if there is an error networking will be dropped but because of the config error it won’t come back up.