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Warning

Adding a second network interface to a non-Amazon Linux EC2 instance causes traffic flow issues. This occurs because both interfaces are in the same subnet, and there is only one routing table containing one default gateway. Traffic that comes into the second network interface leaves the instance using the first network interface. This is blocked by AWS because a mismatch between MAC address and IP address.

To make the second interface work, perform the following steps:

  • Add the new interface to your running instance.
  • Create a configuration file for the second interface.
  • If one interface has a public Elastic IP address, restart your network and test reachability of the public interface.
  • If both interfaces only have private IP addresses or both have a public Elastic IP address,
    • configure a secondary routing table, and then
    • set up rules in the custom routing table policy database so that traffic for the secondary interface uses the new routing table.

Refer to the AWS documentation and to section Example of a More Complex Network Configuration. Failure to use the proper steps, may make your instance unreachable! 

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An Elastic IP address is a persistent, public IPv4 address to be used for one of your network interfaces or instances. You can associate an Elastic IP address with any instance or network interface in your account.

(info) The advantage of associating the Elastic IP address with the network interface instead of directly with the instance is that you can move the network interface with its attributes easily from one instance to another.

(warning) The initial automatically assigned public IP address will be removed as soon as you restart the network or the instance after adding a network interface with an Elastic IP address to your instance. Do not restart your network or your instance before you are sure you can reach it via the Elastic IP address. The automatically assigned public IP address will also be disabled if you assign an Elastic IP address to the primary Ethernet interface of the instance.

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