Metadata service

The Compute service uses a special metadata service to enable virtual machine instances to retrieve instance-specific data. Instances access the metadata service at http://169.254.169.254. The metadata service supports two sets of APIs: an OpenStack metadata API and an EC2-compatible API. Each of the APIs is versioned by date.

To retrieve a list of supported versions for the OpenStack metadata API, make a GET request to http://169.254.169.254/openstack For example:

$ curl http://169.254.169.254/openstack
2012-08-10
latest

To list supported versions for the EC2-compatible metadata API, make a GET request to http://169.254.169.254.

For example:

$ curl http://169.254.169.254
1.0
2007-01-19
2007-03-01
2007-08-29
2007-10-10
2007-12-15
2008-02-01
2008-09-01
2009-04-04
latest

If you write a consumer for one of these APIs, always attempt to access the most recent API version supported by your consumer first, then fall back to an earlier version if the most recent one is not available.

Metadata from the OpenStack API is distributed in JSON format. To retrieve the metadata, make a GET request to http://169.254.169.254/openstack/2012-08-10/meta_data.json.

For example:

$ curl http://169.254.169.254/openstack/2012-08-10/meta_data.json

{
   "uuid":"d8e02d56-2648-49a3-bf97-6be8f1204f38",
   "availability_zone":"nova",
   "hostname":"test.novalocal",
   "launch_index":0,
   "meta":{
      "priority":"low",
      "role":"webserver"
   },
   "public_keys":{
      "mykey":"ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQDYVEprvtYJXVOBN0XNKVVRNCRX6BlnNbI+USLGais1sUWPwtSg7z9K9vhbYAPUZcq8c/s5S9dg5vTHbsiyPCIDOKyeHba4MUJq8Oh5b2i71/3BISpyxTBH/uZDHdslW2a+SrPDCeuMMoss9NFhBdKtDkdG9zyi0ibmCP6yMdEX8Q== Generated by Nova\n"
   },
   "name":"test"
}

Instances also retrieve user data (passed as the user_data parameter in the API call or by the --user_data flag in the nova boot command) through the metadata service, by making a GET request to http://169.254.169.254/openstack/2012-08-10/user_data.

For example:

$ curl http://169.254.169.254/openstack/2012-08-10/user_data
                        #!/bin/bash
echo 'Extra user data here'

The metadata service has an API that is compatible with version 2009-04-04 of the Amazon EC2 metadata service; virtual machine images that are designed for EC2 work properly with OpenStack.

The EC2 API exposes a separate URL for each metadata. You can retrieve a listing of these elements by making a GET query to http://169.254.169.254/2009-04-04/meta-data/

For example:

$ curl http://169.254.169.254/2009-04-04/meta-data/
ami-id
ami-launch-index
ami-manifest-path
block-device-mapping/
hostname
instance-action
instance-id
instance-type
kernel-id
local-hostname
local-ipv4
placement/
public-hostname
public-ipv4
public-keys/
ramdisk-id
reservation-id
security-groups

$ curl http://169.254.169.254/2009-04-04/meta-data/block-device-mapping/
ami

$ curl http://169.254.169.254/2009-04-04/meta-data/placement/
availability-zone

$ curl http://169.254.169.254/2009-04-04/meta-data/public-keys/
0=mykey

Instances can retrieve the public SSH key (identified by keypair name when a user requests a new instance) by making a GET request to http://169.254.169.254/2009-04-04/meta-data/public-keys/0/openssh-key.

For example:

$ curl http://169.254.169.254/2009-04-04/meta-data/public-keys/0/openssh-key
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQDYVEprvtYJXVOBN0XNKVVRNCRX6BlnNbI+USLGais1sUWPwtSg7z9K9vhbYAPUZcq8c/s5S9dg5vTHbsiyPCIDOKyeHba4MUJq8Oh5b2i71/3BISpyxTBH/uZDHdslW2a+SrPDCeuMMoss9NFhBdKtDkdG9zyi0ibmCP6yMdEX8Q== Generated by Nova

Instances can retrieve user data by making a GET request to http://169.254.169.254/2009-04-04/user-data.

For example:

$ curl http://169.254.169.254/2009-04-04/user-data
#!/bin/bash
echo 'Extra user data here'

The metadata service is implemented by either the nova-api service or the nova-api-metadata service. (The nova-api-metadata service is generally only used when running in multi-host mode, it retrieves instance-specific metadata). If you are running the nova-api service, you must have metadata as one of the elements of the list of the enabled_apis configuration option in /etc/nova/nova.conf. The default enabled_apis configuration setting includes the metadata service, so you should not need to modify it.

Hosts access the service at 169.254.169.254:80, and this is translated to metadata_host:metadata_port by an iptables rule established by the nova-network servce. In multi-host mode, you can set metadata_host to 127.0.0.1.

To enable instances to reach the metadata service, the nova-network service configures iptables to NAT port 80 of the 169.254.169.254 address to the IP address specified in metadata_host (default $my_ip, which is the IP address of the nova-network service) and port specified in metadata_port (default 8775) in /etc/nova/nova.conf.

	Warning
	The metadata_host configuration option must be an IP address, not a host name.

	Note
	The default Compute service settings assume that the nova-network service and the nova-api service are running on the same host. If this is not the case, you must make this change in the /etc/nova/nova.conf file on the host running the nova-network service: Set the metadata_host configuration option to the IP address of the host where the nova-api service runs.