cloud-init/doc/rtd/topics/format.rst

Formats

User data that will be acted upon by cloud-init must be in one of the following types.

Gzip Compressed Content

Content found to be gzip compressed will be uncompressed. The uncompressed data will then be used as if it were not compressed. This is typically is useful because user-data is limited to ~16384¹ bytes.

Mime Multi Part Archive

This list of rules is applied to each part of this multi-part file. Using a mime-multi part file, the user can specify more than one type of data.

For example, both a user data script and a cloud-config type could be specified.

Supported content-types:

• text/x-include-once-url
• text/x-include-url
• text/cloud-config-archive
• text/upstart-job
• text/cloud-config
• text/part-handler
• text/x-shellscript
• text/cloud-boothook

Helper script to generate mime messages ~~~~~~~~~~~~~~~~

#!/usr/bin/python

import sys

from email.mime.multipart import MIMEMultipart
from email.mime.text import MIMEText

if len(sys.argv) == 1:
    print("%s input-file:type ..." % (sys.argv[0]))
    sys.exit(1)

combined_message = MIMEMultipart()
for i in sys.argv[1:]:
    (filename, format_type) = i.split(":", 1)
    with open(filename) as fh:
        contents = fh.read()
    sub_message = MIMEText(contents, format_type, sys.getdefaultencoding())
    sub_message.add_header('Content-Disposition', 'attachment; filename="%s"' % (filename))
    combined_message.attach(sub_message)

print(combined_message)

User-Data Script

Typically used by those who just want to execute a shell script.

Begins with: #! or Content-Type: text/x-shellscript when using a MIME archive.

Example

$ cat myscript.sh

#!/bin/sh
echo "Hello World.  The time is now $(date -R)!" | tee /root/output.txt

$ euca-run-instances --key mykey --user-data-file myscript.sh ami-a07d95c9 

Include File

This content is a include file.

The file contains a list of urls, one per line. Each of the URLs will be read, and their content will be passed through this same set of rules. Ie, the content read from the URL can be gzipped, mime-multi-part, or plain text.

Begins with: #include or Content-Type: text/x-include-url when using a MIME archive.

Cloud Config Data

Cloud-config is the simplest way to accomplish some things via user-data. Using cloud-config syntax, the user can specify certain things in a human friendly format.

These things include:

• apt upgrade should be run on first boot
• a different apt mirror should be used
• additional apt sources should be added
• certain ssh keys should be imported
• and many more...

Note: The file must be valid yaml syntax.

See the yaml_examples section for a commented set of examples of supported cloud config formats.

Begins with: #cloud-config or Content-Type: text/cloud-config when using a MIME archive.

Upstart Job

Content is placed into a file in /etc/init, and will be consumed by upstart as any other upstart job.

Begins with: #upstart-job or Content-Type: text/upstart-job when using a MIME archive.

Cloud Boothook

This content is boothook data. It is stored in a file under /var/lib/cloud and then executed immediately. This is the earliest hook available. Note, that there is no mechanism provided for running only once. The boothook must take care of this itself. It is provided with the instance id in the environment variable INSTANCE_I. This could be made use of to provide a 'once-per-instance' type of functionality.

Begins with: #cloud-boothook or Content-Type: text/cloud-boothook when using a MIME archive.

Part Handler

This is a part-handler. It will be written to a file in /var/lib/cloud/data based on its filename (which is generated). This must be python code that contains a list_types method and a handle_type method. Once the section is read the list_types method will be called. It must return a list of mime-types that this part-handler handles.

The handle_type method must be like:

def handle_part(data, ctype, filename, payload):
  # data = the cloudinit object
  # ctype = "__begin__", "__end__", or the mime-type of the part that is being handled.
  # filename = the filename of the part (or a generated filename if none is present in mime data)
  # payload = the parts' content

Cloud-init will then call the handle_type method once at begin, once per part received, and once at end. The begin and end calls are to allow the part handler to do initialization or teardown.

Begins with: #part-handler or Content-Type: text/part-handler when using a MIME archive.

Example

../../examples/part-handler.txt

Also this blog post offers another example for more advanced usage.

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1. See your cloud provider for applicable user-data size limitations...↩︎