AWS Instance Metadata                       

Many circumstances exist in which an instance needs to know the IP address associated with itself. The instance may want to insert its IP address into a database that is used by a content management system to store information about the application it’s running or, upon initial launch, the instance may want to publish its IP address to other instances so that they can communicate with it. The first issue is how an instance can find out its own IP address when the address isn’t persistent.                              

 

Fortunately, AWS offers a convenient mechanism for instance self-discovery of the instance metadata, as it’s referred to. AWS provides instance metadata at the IP address 169.254.169.254. If you issue an HTTP GET command from within the instance, it will retrieve its own metadata. (HTTP refers tothe protocol used by the Web, and GET is a command that can be transmitted across HTTP to instruct a remote resource to execute the GET command against the resource.) A large amount of instance-specific data is available via the metadata IP address, such as

✓ The instance’s private IP address: 10.1.2.3, for example.

✓ The instance’s public IP address: 70.1.2.3, for example.                               

✓ The instance’s instance ID

✓ The instance’s security groups: Used to control network traffic access to instances. 

✓ The instance’s user data: Supplied to the instance when it’s launchedand reflects information that the account owner wants the instance to have during its operation. User data, which is somewhat analogous to command-line parameters, can be used to “pass in” information necessary for the instance to do its work. An example is a URL from which the instance should get data. User data can take the form of text (a string ofinformation, in other words) or a 16 kilobyte or less file from which textcan be read.