[Bug 1375252] Re: Hostname change is not preserved across reboot on Azure Ubuntu VMs

[Dan Watkins]

This turned out to be something of an involved change, which I'm still
working on.

Essentially, the cloud-init Azure data source will _always_ set the
hostname to that provided by the fabric, even before the configuration
modules that would normally do it (i.e. set_hostname and
update_hostname). It does this so that the virtual IP address will be
assigned correctly (by renewing the DHCP lease), but does this on every
boot.

It has to set the hostname and bounce DHCP on every boot, because we
can't know if this is the first boot of an instance until we have got
our instance ID from SharedConfig.xml. We can't get SharedConfig.xml
without network. We can't have network (on first boot) until we have
performed the bounce[0]. Very Ouroborian.

My change sounds simple: after we have performed the DHCP bounce and
walinuxagent has fetched SharedConfig.xml, we will set the hostname back
to what it was before, to allow the regular cloud-init modules to
perform hostname changing.

Unfortunately, this has lead to fairly major restructuring of the code
in question, so this won't make it in to vivid's cloud-init (as we are
releasing in just over a week). But we are making good progress!

[0] We also can't try to get SharedConfig.xml and then do the bounce if
we fail, because this will add about a minute on to first boot times.

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Title:
  Hostname change is not preserved across reboot on Azure Ubuntu VMs

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