We had some discussion on this bug today in #cloud-init.
 https://irclogs.ubuntu.com/2017/07/10/%23cloud-init.html

It seems that in order to interact with the metadata service, we need
to configure the instances ip address as a dhcp request would give us.
The service will drop packets if the source address is not "right", and
right does not include ipv4 link local scope.

So there is really no way to talk to the metadata service unless you
do a dhcp.

This little script run in a Ubuntu 16.04 image shows what we were trying to
do and shows a way that it seems to work:
  http://paste.ubuntu.com/25062706/
results of:
  addr: http://paste.ubuntu.com/25062713/
  link-local: http://paste.ubuntu.com/25062728/

So that gives us 2 basic options
a.) use dhclient (or appropriate distro-provided dhcp client)
   This probably can be made to work, but we ultimately would have to work with
   different clients on different distros or image configurations and also have
   to deal with "side affects" of having done dhclient.  Ie, dhclient calls hook
   scripts on up and down.  That may not be a problem, but it definitely could 
be.

b.) use a "built in" dhcp client. 
   http://code.activestate.com/recipes/577649-dhcp-query/

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https://bugs.launchpad.net/bugs/1639030

Title:
  Configure networking based on EC2 metadata source

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