Fail fast optimization to is_resolvable.

[joe-usa-amzn]

Proposed Commit Message

On top of this previous optimization to is_resolvable: https://github.com/canonical/cloud-init/pull/2040
an additonal optimization is to fail early when the hostname is not resolvable at all.  
Original author Perez Pestana, Alejandro <aalepp@amazon.com>

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

Is there any reason the author can't submit this code themselves?

When do you expect this to happen?

Please provide logs where where you observed poor performance, as well as logs from a system with this change applied.

[joe-usa-amzn]

Hi @holmanb, I obtained permission from the original author to contribute this as I am the designated cloud-init SME for our team.
This is not a performance optimization as much as an optimization against badname checking from being called. The issue was our DNS service route 53 saw a bunch of calls to "does-not-exist.example.com.", "example.invalid.", "cloud_init_expected_not_found" and after these changes and the one mentioned here: #2040 we were able to reduce the number of DNS calls to these junk addresses. As far as logging you wouldn't find anything different in cloud-init.log unless you trigger this redirect attack. I can try to ask around for the route 53 logs if you need, but it's been almost year since we added this change in Amazon Linux and it has been stable ever since.

[joe-usa-amzn]

Would you like to see anything else? @holmanb

[holmanb]

That is insufficient, sorry. Please have the author contribute it.

[joe-usa-amzn]

I will have him do so, thanks for taking a look.

[holmanb]

I will have him do so, thanks for taking a look.

Thank you.