[Bug 1538812] Re: Relies on DNS to resolve own hostname

[Florian Haas]

Just as a reference point. It appears that what this charm helper
*really* wants to do is get the host's "default" IP address. Recognizing
that a reverse name lookup is a rather poor way of finding that out,
Ansible does an ip route get to two well-known IP addresses (v4 and v6)
to determine the outgoing interface for the default route, and then
returns that interface's address as ansible_default_ipv4.address, and
ansible_default_ipv6.address.

See
https://github.com/ansible/ansible/blob/devel/lib/ansible/module_utils/facts.py#L1918
for details.

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https://bugs.launchpad.net/bugs/1538812

Title:
  Relies on DNS to resolve own hostname

Status in Charm Helpers:
  Triaged
Status in ceph package in Juju Charms Collection:
  New
Status in cinder package in Juju Charms Collection:
  New
Status in neutron-gateway package in Juju Charms Collection:
  New
Status in nova-compute package in Juju Charms Collection:
  New

Bug description:
  In charm/hooks/utils.py, the get_host_ip method seems to rely on DNS
  to resolve host names:

  @cached
  def get_host_ip(hostname=None):
      if config('prefer-ipv6'):
          return get_ipv6_addr()[0]

      hostname = hostname or unit_get('private-address')
      try:
          # Test to see if already an IPv4 address
          socket.inet_aton(hostname)
          return hostname
      except socket.error:
          # This may throw an NXDOMAIN exception; in which case
          # things are badly broken so just let it kill the hook
          answers = dns.resolver.query(hostname, 'A')
          if answers:
              return answers[0].address

  Firstly, the dns.resolver.query call strikes me as incredibly silly.
  What if the other node in not resolvable via DNS, but its name is in
  /etc/hosts? What if the other node *is* in DNS, but is a CNAME? And,
  bottom line, why not simply use socket.gethostbyname() here?

  Secondly, this currently (as of today) breaks a Ceph deployment. It
  definitely didn't do so a month ago, so whatever it was that changed
  in the interim, this is a regression. I don't know if this code path
  wasn't there earlier, or whether it was just never hit. But it
  definitely used to work, and no longer does.

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