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Setting up Additional IP address to an EC2 Instance.

Suppose you want to add an additional IP to your Ubuntu Web Server, Here the steps are as follows:

1) Make sure your EC2 instance type will support more than 2 Network Interfaces. If not then you have to upgrade your Instance Type.
2) Add a New Network Interface. Make sure you give the same AZ, Subnet and Security Group as of the web server.
3) Now Add an Elastic IP and associate this elastic ip to the Web server.
4) Now attach the new Network Interface to the Webserver.

Now the final part is , to create 2 route tables with default routes for each interface and use rules on inbound traffic to assign which table to use for the response traffic.

5) Here is a sample interface file that performs this:

root@webserver:~# cat /etc/network/interfaces.d/eth0.cfg
auto eth0
iface eth0 inet dhcp
post-up ip route add default via 10.0.1.1 dev eth0 tab 1
post-up ip rule add from 10.0.1.81 tab 1
pre-down ip rule del from 10.0.1.81 tab 1
pre-down ip route del default via 10.0.1.1 dev eth0 tab 1


root@webserver:~# cat /etc/network/interfaces.d/51-eth1.cfg
auto eth1
iface eth1 inet static
address 10.0.1.136
netmask 255.255.255.0
gateway 10.0.1.1
post-up ip route add default via 10.0.1.1 dev eth1 tab 2
post up ip rule add from 10.0.1.136 tab 2
pre-down ip rule del from 10.0.1.136 tab 2
pre-down ip route del default via 10.0.1.1 dev eth1 tab 2


NB: The gateway of eth1 should be the first ip of the private subnet 10.0.1.0/24, which is 10.0.1.1

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