Cloud-init Azure

Azure
This datasource finds metadata and user-data from the Azure cloud platform.

Azure Platform
The azure cloud-platform provides initial data to an instance via an attached CD formated in UDF. That CD contains a ‘ovf-env.xml’ file that provides some information. Additional information is obtained via interaction with the “endpoint”.

To find the endpoint, we now leverage the dhcp client’s ability to log its known values on exit. The endpoint server is special DHCP option 245. Depending on your networking stack, this can be done by calling a script in /etc/dhcp/dhclient-exit-hooks or a file in /etc/NetworkManager/dispatcher.d. Both of these call a sub-command ‘dhclient_hook’ of cloud-init itself. This sub-command will write the client information in json format to /run/cloud-init/dhclient.hook/.json.

In order for cloud-init to leverage this method to find the endpoint, the cloud.cfg file must contain:

datasource:
Azure:
set_hostname: False agent_command: __builtin__
If those files are not available, the fallback is to check the leases file for the endpoint server (again option 245).

You can define the path to the lease file with the ‘dhclient_lease_file’ configuration. The default value is /var/lib/dhcp/dhclient.eth0.leases.

dhclient_lease_file: /var/lib/dhcp/dhclient.eth0.leases
walinuxagent
In order to operate correctly, cloud-init needs walinuxagent to provide much of the interaction with azure. In addition to “provisioning” code, walinux does the following on the agent is a long running daemon that handles the following things: – generate a x509 certificate and send that to the endpoint

waagent.conf config
in order to use waagent.conf with cloud-init, the following settings are recommended. Other values can be changed or set to the defaults.

# disabling provisioning turns off all ‘Provisioning.*’ function
Provisioning.Enabled=n
# this is currently not handled by cloud-init, so let walinuxagent do it.
ResourceDisk.Format=y
ResourceDisk.MountPoint=/mnt
Userdata
Userdata is provided to cloud-init inside the ovf-env.xml file. Cloud-init expects that user-data will be provided as base64 encoded value inside the text child of a element named UserData or CustomData which is a direct child of the LinuxProvisioningConfigurationSet (a sibling to UserName) If both UserData and CustomData are provided behavior is undefined on which will be selected.

In the example below, user-data provided is ‘this is my userdata’, and the datasource config provided is {“agent_command”: [“start”, “walinuxagent”]}. That agent command will take affect as if it were specified in system config.

Example:

<wa:ProvisioningSection>
 <wa:Version>1.0</wa:Version>
 <LinuxProvisioningConfigurationSet
    xmlns="http://schemas.microsoft.com/windowsazure"
    xmlns:i="http://www.w3.org/2001/XMLSchema-instance">
  <ConfigurationSetType>LinuxProvisioningConfiguration</ConfigurationSetType>
  <HostName>myHost</HostName>
  <UserName>myuser</UserName>
  <UserPassword/>
  <CustomData>dGhpcyBpcyBteSB1c2VyZGF0YQ===</CustomData>
  <dscfg>eyJhZ2VudF9jb21tYW5kIjogWyJzdGFydCIsICJ3YWxpbnV4YWdlbnQiXX0=</dscfg>
  <DisableSshPasswordAuthentication>true</DisableSshPasswordAuthentication>
  <SSH>
   <PublicKeys>
    <PublicKey>
     <Fingerprint>6BE7A7C3C8A8F4B123CCA5D0C2F1BE4CA7B63ED7</Fingerprint>
     <Path>this-value-unused</Path>
    </PublicKey>
   </PublicKeys>
  </SSH>
  </LinuxProvisioningConfigurationSet>
</wa:ProvisioningSection>

Configuration for the datasource can be read from the system config’s or set via the dscfg entry in the LinuxProvisioningConfigurationSet. Content in dscfg node is expected to be base64 encoded yaml content, and it will be merged into the ‘datasource: Azure’ entry.

The ‘hostname_bounce: command‘ entry can be either the literal string ‘builtin’ or a command to execute. The command will be invoked after the hostname is set, and will have the ‘interface’ in its environment. If set_hostname is not true, then hostname_bounce will be ignored.

An example might be:
command: [“sh”, “-c”, “killall dhclient; dhclient $interface”]
datasource:
agent_command
Azure:
agent_command: [service, walinuxagent, start]
set_hostname: True
hostname_bounce:
# the name of the interface to bounce
interface: eth0
# policy can be ‘on’, ‘off’ or ‘force’
policy: on
# the method ‘bounce’ command.
command: “builtin”
hostname_command: “hostname”
hostname
When the user launches an instance, they provide a hostname for that instance. The hostname is provided to the instance in the ovf-env.xml file as HostName.

Whatever value the instance provides in its dhcp request will resolve in the domain returned in the ‘search’ request.

The interesting issue is that a generic image will already have a hostname configured. The ubuntu cloud images have ‘ubuntu’ as the hostname of the system, and the initial dhcp request on eth0 is not guaranteed to occur after the datasource code has been run. So, on first boot, that initial value will be sent in the dhcp request and that value will resolve.

In order to make the HostName provided in the ovf-env.xml resolve, a dhcp request must be made with the new value. Walinuxagent (in its current version) handles this by polling the state of hostname and bouncing (‘ifdown eth0; ifup eth0‘ the network interface if it sees that a change has been made.

cloud-init handles this by setting the hostname in the DataSource’s ‘get_data’ method via ‘hostname $HostName‘, and then bouncing the interface. This behavior can be configured or disabled in the datasource config. See ‘Configuration’ above.

Questions