amazon-ebs builder: Attach an existing elastic network interface?

[Rob Capellini]

Is it possible to attach an existing elastic network interface (ENI) to an instance when using any of the amazon builders?  I need eth0 to have the specific MAC address of the ENI.  I have a pre-provisioned ENI available and need to attach it to the instance at device index 0.  I have tried to script this using shell scripts, but unfortunately it seems you can't replace eth0 after the instance has been stood up.  Is there a way to do this that I am missing?

[Rickard von Essen]

Every instance in a VPC has a default network interface, called the primary network interface (eth0). You cannot detach a primary network interface from an instance. 

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html#eni-basics

 Seems like it's not possible.

Why do you need it as eth0?

On Mon, Jul 1, 2019 at 5:44 AM Rob Capellini <rob.ca...@gmail.com> wrote:

Is it possible to attach an existing elastic network interface (ENI) to an instance when using any of the amazon builders?  I need eth0 to have the specific MAC address of the ENI.  I have a pre-provisioned ENI available and need to attach it to the instance at device index 0.  I have tried to script this using shell scripts, but unfortunately it seems you can't replace eth0 after the instance has been stood up.  Is there a way to do this that I am missing?

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[Rob Capellini]

 Seems like it's not possible.

No, it's not possible after the instance has already been created. 

Why do you need it as eth0?

A requirement of some software that we're using (quite draconian, but a requirement nonetheless).

I've modified the packer source and got it to work by adding my own 'network_interface' option, but 'there be dragons' since you can no longer define a subnet and security group and there could be other complications.  I would try to submit a PR, but I think that this is such an edge case that it's most likely not worth the maintenance burden for the few people who would need it.  I think my best option is to pursue a custom builder plugin.

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[Rickard von Essen]

Yes this is probably very much an edge case but it should be fairly simple to add network_interface as a config option and disallow setting security_groups and security_group_filters if it is set. 

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