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Warning

Stopping and restarting the instance after adding a second network interface will release any automatically assigned public IP address.

Additionally, adding a second network interface to a non-Amazon Linux EC2 instance causes traffic flow issues. This occurs when there is only one routing table containing one default gateway and traffic that arrives at the second network interface leaves the instance using the first network interface or vice-versa. 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 only one interface has an Elastic IP address, restart your network and test reachability via the Elastic IP address.
  • If both interfaces only have private IP addresses or both have public IP addresses,
    • configure an additional routing table, and then
    • set up rules in the custom routing table policy database so that traffic for each interface uses its own routing table.

Refer to the AWS documentation and to section Example of a More Complex Network Configurationfor more information. 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 instance after adding a network interface with an Elastic IP address to your instance. Do not restart 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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