How to retrieve instance hostname from ucd

Q&A
1

I’ve read whatever documentation and --help output I can get from ucd and ucd-data-fetch

I know that ucd generates /var/lib/cloud/aws-user-data when the systemd oneshot runs.

What I want to understand is whether ucd also fetches the instance hostname from metadata → the same output that you’d get from curl --silent http://169.254.169.254/latest/meta-data/hostname on an EC2.

The Ask

When ucd runs at boot, does it save and/or output that hostname somewhere?

The reason I ask is because putting curl --silent http://169.254.169.254/latest/meta-data/hostname > /opt/hostname.txt into a systemd oneshot appears to take about 5 seconds to run, and ucd is significantly snappier.

See below:

root@ip-10-0-10-170.us-west-2.compute.internal~ # systemd-analyze  blame
5.314s set-hostname-metadata.service
1.248s ucd.service

Reason

Even those 5 seconds make a difference in our use-case.

Any help or info would be much appreciated!

2

The current implementation only picks up SSH keys from the user metadata APIs. It should grab the hostname if it’s defined in the standalone cloud-config, but not from e.g. .../meta-data/hostname. Enabling setting a hostname based on the user metadata API would be a good feature request here: Issues · clearlinux/micro-config-drive · GitHub

3

Thank you for the quick and helpful reply. :slight_smile:

As suggested, I’ve created the feature request.