CloudFormation - Instance Metadata & UserData

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Instance meta-data

Instance metadata is data about our instance that we can use to configure or manage the running instance.

ubuntu@ip-172-31-11-56:~$ curl http://169.254.169.254/latest/meta-data/
ami-id
ami-launch-index
ami-manifest-path
block-device-mapping/
hostname
instance-action
instance-id
instance-type
local-hostname
local-ipv4
mac
metrics/
network/
placement/
profile
public-hostname
public-ipv4
public-keys/
reservation-id
security-groups

We can get the value of some of the metadata items from the preceding example:

ubuntu@ip-172-31-11-56:~$ curl http://169.254.169.254/latest/meta-data/ami-id
ami-6e165d0e

On Ubuntu, you can use the ec2metadata script to query the EC2 instance Metadata from within a running EC2 instance:

ubuntu@ip-172-31-11-56:~$ ec2metadata
ami-id: ami-6e165d0e
...
user-data: unavailable

Userdata

We can also use instance metadata to access user data that we specified when launching our instance. For example, we can specify parameters for configuring our instance, or attach a simple script. We can also use this data to build more generic AMIs that can be modified by configuration files supplied at launch time.

To retrieve user data from within a running instance, we can use the following URI:

ubuntu@ip-172-31-11-56:~$ curl http://169.254.169.254/latest/user-data
<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
         "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
 <head>
  <title>404 - Not Found</title>
 </head>
 <body>
  <h1>404 - Not Found</h1>
 </body>
</html>

But the instance we're querying the info has no user data.

Note that we can only access instance metadata and user data from within the instance itself.

Here is our new userdata with Amazon Linux AMI instance:

Now, we can log in to the instance and see the user data:

[ec2-user@ip-172-31-39-104 ~]$ ec2-metadata
ami-id: ami-8c1be5f6
...
security-groups: einsteinish
user-data: User data. We can pass info into an instance via user data.

cloud-init

cloud-init handles early initialization of a cloud instance. Some of the things it configures are CloudInit:

setting a default locale
setting hostname
generate ssh private keys
adding ssh keys to user's .ssh/authorized_keys so they can log in
setting up ephemeral mount points
cloud-init's behavior can be configured via user-data. User-data can be given by the user at instance launch time. This is done via the --user-data or --user-data-file argument to ec2-run-instances

user-data scripts are a convenient way to do something on first boot of a launched instance. This input format is accepted to cloud-init and handled as we would expect.

Now we want to launch a new Ubuntu instance with the following user-data:

#!/bin/bash
apt-get update
apt-get upgrade -y
apt-get install apache2 -y
echo "<html><body><h1>Hello userdata</h1>" > /var/www/html/index.html
echo "Created from user-data and cloud-init"  >> /var/www/html/index.html
echo "</body></html>" >> /var/www/html/index.html

The script will be invoked at an "rc.local" like point in the boot sequence.

We can see the user data when we go into the instance:

ubuntu@ip-34-229-159-104:~$ ec2metadata
ami-id: ami-cd0f5cb6
...
user-data: b'#!/bin/bash\napt-get update\napt-get upgrade -y\napt-get install apache2 -y\necho "<html><body><h1>Hello userdata</h1>" > /var/www/html/index.html\necho "Created from user-data and cloud-init"  >> /var/www/html/index.html\necho "</body></html>" >> /var/www/html/index.html'

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