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[Brian Rak <brak@xxxxxxxxx>]

Public bug reported:

On CentOS 7 (systemd based), with cloud-init 0.7.5, cc_update_hostname
will *always* overwrite the hostname, even if it shouldn't.

This is pretty easy to reproduce:

1) Add a 'hostname: metadata.example.com' to your user-data
2) Run 'hostnamectl set-hostname test.example.com'
3) Reboot the server
4) Check the hostname, it'll still be 'metadata.example.com'
5) Repeat 2-4, hostname will still be set

Looking at the code, this is because rhel.py doesn't populate the
previous-hostname file correctly:

    def _write_hostname(self, hostname, out_fn):
        if self._dist_uses_systemd():
            util.subp(['hostnamectl', 'set-hostname', str(hostname)])
        else:
            host_cfg = {
                'HOSTNAME': hostname,
            }
            rhel_util.update_sysconfig_file(out_fn, host_cfg)

So, Distros::update_hostname calls _write_hostname to try and save the
previous hostname to /var/lib/cloud/data/previous-hostname .. however,
all this does is run the hostnamectl command, which doesn't actually
update this file.  This means that on the next cloud-init run,
/var/lib/cloud/data/previous-hostname doesn't exist so the hostname gets
blindly updated again.

The fix here seems pretty simple, we just need to update previous-
hostname as well when running hostnamectl.  I attached a patch that does
this, though I don't know the best way to implement this.. my patch
fails if cloud-init's data path is changed

** Affects: cloud-init
     Importance: Undecided
         Status: New

** Patch added: "centos-7-hostname.patch"
   https://bugs.launchpad.net/bugs/1424710/+attachment/4325548/+files/centos-7-hostname.patch

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https://bugs.launchpad.net/bugs/1424710

Title:
  CentOS 7: update-hostname always overwrites hostname

Status in Init scripts for use on cloud images:
  New

Bug description:
  On CentOS 7 (systemd based), with cloud-init 0.7.5, cc_update_hostname
  will *always* overwrite the hostname, even if it shouldn't.

  This is pretty easy to reproduce:

  1) Add a 'hostname: metadata.example.com' to your user-data
  2) Run 'hostnamectl set-hostname test.example.com'
  3) Reboot the server
  4) Check the hostname, it'll still be 'metadata.example.com'
  5) Repeat 2-4, hostname will still be set

  Looking at the code, this is because rhel.py doesn't populate the
  previous-hostname file correctly:

      def _write_hostname(self, hostname, out_fn):
          if self._dist_uses_systemd():
              util.subp(['hostnamectl', 'set-hostname', str(hostname)])
          else:
              host_cfg = {
                  'HOSTNAME': hostname,
              }
              rhel_util.update_sysconfig_file(out_fn, host_cfg)

  So, Distros::update_hostname calls _write_hostname to try and save the
  previous hostname to /var/lib/cloud/data/previous-hostname .. however,
  all this does is run the hostnamectl command, which doesn't actually
  update this file.  This means that on the next cloud-init run,
  /var/lib/cloud/data/previous-hostname doesn't exist so the hostname
  gets blindly updated again.

  The fix here seems pretty simple, we just need to update previous-
  hostname as well when running hostnamectl.  I attached a patch that
  does this, though I don't know the best way to implement this.. my
  patch fails if cloud-init's data path is changed

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