Add an option to use short hostname in RHEL

[hamza]

cloud init with RHEL always use the FQDN over the hostname even if the hostname is sent like below
#cloud-config
---
hostname: shortname

the problem with always using the FQDN as hostname is the linux constrain on hostname length

getconf HOST_NAME_MAX
64

the FQDN is used for DNS and the DNS servers has no limit on that but using as hostname raise an error when cloud init try to set the hostname and fail cloud init

an option should be provided for RHEL like below

#cloud-config
---
enable_shortname: true

to change the default behavior from FQDN to the
1. hostname if send
2. short hostname from the FQDN

and the /etc/resolv.conf
search <domain>

so if the hostname is test.domain.com

the hostname will be *test*
and the
/etc/resolv.conf
search domain.com

[hamza (alqtaishat)]

and that is already the behavior for ubuntu, opensuse, ... but rhel
https://github.com/canonical/cloud-init/blob/36ddf1ebed3f264fa86ef4f657dce29244c2e068/cloudinit/distros/__init__.py#L259

[hamza (alqtaishat)]

or we can check if the FQDN > 64 and set the hostname
since it will fail the cloud init any way

[Chad Smith (chad.smith)]

Thanks for the bug Hamze and for making cloud-init better.

I feel like this is a reasonable and concise request for a bug fix. Hamza, do you have the ability to run `cloud-init collect-logs` on an affected instance to show the Traceback that occurs with fqdn on RedHat is > 64 or the invalid behavior on such as system when trying to resolve hostanmes?

Note that `cloud-inig collect-`logs collects /var/log/cloud-init.log or userdata, if you specify custom passwords or something in userdata you may want to redact that from the logs before attaching them to this bug.

[Chad Smith (chad.smith)]

What I imagine is we could add smarts to rhel.py if len(fqdn) > HOST_NAME_MAX then use 'hostname' if present in userdata. Then we wouldn't have to change default behavior on anything except broken systems.

[James Falcon (falcojr)]

This bug is believed to be fixed in cloud-init in version 21.2. If this is still a problem for you, please make a comment and set the state back to New

Thank you.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3860