Adding a resolv.conf configuration module (LP: #1100434)

Managing resolv.conf can be quite handy when running in an environment
where you would like to control DNS resolution, despite being provided
DNS server information by DHCP. This module will allow one to define the
 structure of their resolv.conf and write it PER_ONCE.

Right now this makes the most sense on RedHat, and therefore, has defined 'distros' as such.

Note that when a config drive is used, and dhcp is not used that this module
may not be applicable since in the RedHat based distros the config drive typically
contains a ubuntu style network configuration which contains nameservers that can
be used in /etc/resolv.conf, but for other types of datasources it is useful to
have a module which can be specifically configured to write out /etc/resolv.conf

cloudinit/config/cc_resolv_conf.py

@@ -0,0 +1,107 @@
+# vi: ts=4 expandtab
+#
+#    Copyright (C) 2013 Craig Tracey
+#
+#    Author: Craig Tracey <craigtracey@gmail.com>
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU General Public License version 3, as
+#    published by the Free Software Foundation.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU General Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License
+#    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#    Note:
+#    This module is intended to manage resolv.conf in environments where
+#    early configuration of resolv.conf is necessary for further
+#    bootstrapping and/or where configuration management such as puppet or
+#    chef own dns configuration. As Debian/Ubuntu will, by default, utilize
+#    resovlconf, and similarly RedHat will use sysconfig, this module is
+#    likely to be of little use unless those are configured correctly.
+#
+#    For RedHat with sysconfig, be sure to set PEERDNS=no for all DHCP
+#    enabled NICs.  And, in Ubuntu/Debian it is recommended that DNS
+#    be configured via the standard /etc/network/interfaces configuration
+#    file.
+#
+#
+#    Usage Example:
+#
+#    #cloud-config
+#    manage_resolv_conf: true
+#
+#    resolv_conf:
+#      nameservers: ['8.8.4.4', '8.8.8.8']
+#      searchdomains:
+#        - foo.example.com
+#        - bar.example.com
+#      domain: example.com
+#      options:
+#        rotate: true
+#        timeout: 1
+#
+
+
+from cloudinit.settings import PER_ONCE
+from cloudinit import templater
+from cloudinit import util
+
+frequency = PER_ONCE
+
+distros = ['fedora', 'rhel']
+
+
+def generate_resolv_conf(cloud, log, params):
+    template_fn = cloud.get_template_filename('resolv.conf')
+    if not template_fn:
+        log.warn("No template found, not rendering /etc/resolv.conf")
+        return
+
+    flags = []
+    false_flags = []
+    if 'options' in params:
+        for key, val in params['options'].iteritems():
+            if type(val) == bool:
+                if val:
+                    flags.append(key)
+                else:
+                    false_flags.append(key)
+
+    for flag in flags + false_flags:
+        del params['options'][flag]
+
+    params['flags'] = flags
+    log.debug("Writing resolv.conf from template %s" % template_fn)
+    templater.render_to_file(template_fn, '/etc/resolv.conf', params)
+
+
+def handle(name, cfg, _cloud, log, _args):
+    """
+    Handler for resolv.conf
+
+    @param name: The module name "resolv-conf" from cloud.cfg
+    @param cfg: A nested dict containing the entire cloud config contents.
+    @param cloud: The L{CloudInit} object in use.
+    @param log: Pre-initialized Python logger object to use for logging.
+    @param args: Any module arguments from cloud.cfg
+    """
+    if "manage_resolv_conf" not in cfg:
+        log.debug(("Skipping module named %s,"
+                   " no 'manage_resolv_conf' key in configuration"), name)
+        return
+
+    if not util.get_cfg_option_bool(cfg, "manage_resolv_conf", False):
+        log.debug(("Skipping module named %s,"
+                   " 'manage_resolv_conf' present but set to False"), name)
+        return
+
+    if not "resolv_conf" in cfg:
+        log.warn("manage_resolv_conf True but no parameters provided!")
+
+    generate_resolv_conf(_cloud, log, cfg["resolv_conf"])
+    return

doc/examples/cloud-config-resolv-conf.txt

@@ -0,0 +1,20 @@
+#cloud-config
+#
+# This is an example file to automatically configure resolv.conf when the
+# instance boots for the first time.
+#
+# Ensure that your yaml is valid and pass this as user-data when starting
+# the instance. Also be sure that your cloud.cfg file includes this
+# configuration module in the appropirate section.
+#
+manage-resolv-conf: true
+
+resolv_conf:
+  nameservers: ['8.8.4.4', '8.8.8.8']
+  searchdomains:
+    - foo.example.com
+    - bar.example.com
+  domain: example.com
+  options:
+    rotate: true
+    timeout: 1

templates/resolv.conf.tmpl

@@ -0,0 +1,39 @@
+#
+# Your system has been configured with 'manage-resolv-conf' set to true.
+# As a result, cloud-init has written this file with configuration data
+# that it has been provided. Cloud-init, by default, will write this file
+# a single time (PER_ONCE).
+#
+
+#if $varExists('nameservers')
+#for $server in $nameservers
+nameserver $server
+#end for
+#end if
+#if $varExists('searchdomains')
+search #slurp
+#for $search in $searchdomains
+$search #slurp
+#end for
+
+#end if
+#if $varExists('domain')
+domain $domain
+#end if
+#if $varExists('sortlist')
+sortlist #slurp
+#for $sort in $sortlist
+$sort #slurp
+#end for
+
+#end if
+#if $varExists('options') or $varExists('flags')
+options #slurp
+#for $flag in $flags
+$flag #slurp
+#end for
+#for $key, $value in $options.items()
+$key:$value #slurp
+#end for
+
+#end if