set_hostname: When present in metadata, set it before network bringup.

When instance meta-data provides hostname information, run
cc_set_hostname in the init-local or init-net stage before network
comes up.

Prevent an initial DHCP request which leaks the stock cloud-image default
hostname before the meta-data provided hostname was processed.
A leaked cloud-image hostname adversely affects Dynamic DNS which
would reallocate 'ubuntu' hostname in DNS to every instance brought up by
cloud-init. These instances would only update DNS to the cloud-init
configured hostname upon DHCP lease renewal.

This branch extends the get_hostname methods in datasource, cloud and
util to limit results to metadata_only to avoid extra cost of querying
the distro for hostname information if metadata does not provide that
information.

LP: #1746455

1 files changed, 17 insertions, 4 deletions

diff --git a/cloudinit/sources/__init__.py b/cloudinit/sources/__init__.py
index a05ca2f6..df0b374a 100644
--- a/cloudinit/sources/__init__.py
+++ b/cloudinit/sources/__init__.py
@@ -276,21 +276,34 @@ class DataSource(object):
             return "iid-datasource"
         return str(self.metadata['instance-id'])
 
-    def get_hostname(self, fqdn=False, resolve_ip=False):
+    def get_hostname(self, fqdn=False, resolve_ip=False, metadata_only=False):
+        """Get hostname or fqdn from the datasource. Look it up if desired.
+
+        @param fqdn: Boolean, set True to return hostname with domain.
+        @param resolve_ip: Boolean, set True to attempt to resolve an ipv4
+            address provided in local-hostname meta-data.
+        @param metadata_only: Boolean, set True to avoid looking up hostname
+            if meta-data doesn't have local-hostname present.
+
+        @return: hostname or qualified hostname. Optionally return None when
+            metadata_only is True and local-hostname data is not available.
+        """
         defdomain = "localdomain"
         defhost = "localhost"
         domain = defdomain
 
         if not self.metadata or 'local-hostname' not in self.metadata:
+            if metadata_only:
+                return None
             # this is somewhat questionable really.
             # the cloud datasource was asked for a hostname
             # and didn't have one. raising error might be more appropriate
             # but instead, basically look up the existing hostname
             toks = []
             hostname = util.get_hostname()
-            fqdn = util.get_fqdn_from_hosts(hostname)
-            if fqdn and fqdn.find(".") > 0:
-                toks = str(fqdn).split(".")
+            hosts_fqdn = util.get_fqdn_from_hosts(hostname)
+            if hosts_fqdn and hosts_fqdn.find(".") > 0:
+                toks = str(hosts_fqdn).split(".")
             elif hostname and hostname.find(".") > 0:
                 toks = str(hostname).split(".")
             elif hostname: