microVM Metadata Service

The Firecracker microVM Metadata Service (MMDS) is a mutable data store which can be used for sharing information between host and guests, in a secure and easy at hand way.

Configuring and activating the microVM Metadata Service

By default, MMDS is not reachable from the guest operating system. At microVM runtime, MMDS is tightly coupled with a network interface, which allows MMDS requests. When configuring the microVM, if MMDS needs to be activated, a network interface has to be configured to allow MMDS requests. This can be achieved in two steps:

Attach one (or more) network interfaces through an HTTP PUT request to /network-interfaces/${MMDS_NET_IF}. The full network configuration API can be found in the firecracker swagger file.
Configure MMDS through an HTTP PUT request to /mmds/config resource and include the IDs of the network interfaces that should allow forwarding requests to MMDS in the network_interfaces list. The complete MMDS API is described in the firecracker swagger file.

Examples

Attaching a network device with ID MMDS_NET_IF:

bash

MMDS_NET_IF=eth0
curl --unix-socket /tmp/firecracker.socket -i                 \
  -X PUT 'http://localhost/network-interfaces/${MMDS_NET_IF}' \
  -H 'Accept: application/json'                               \
  -H 'Content-Type: application/json'                         \
  -d '{
      "iface_id": "${MMDS_NET_IF}",
      "guest_mac": "AA:FC:00:00:00:01",
      "host_dev_name": "tap0"
    }'

Configuring MMDS to receive requests through the MMDS_NET_IF network interface ID:

bash

MMDS_IPV4_ADDR=169.254.170.2
curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT "http://localhost/mmds/config"     \
    -H "Content-Type: application/json"       \
    -d '{
             "network_interfaces": ["${MMDS_NET_IF}"]
    }'

MMDS can be configured pre-boot only, using the Firecracker API server. Enabling MMDS without at least a network device attached will return an error.

The IPv4 address used by guest applications when issuing requests to MMDS can be customized through the same HTTP PUT request to /mmds/config resource, by specifying the IPv4 address to the ipv4_address field. If the IP configuration is not provided before booting up the guest, the MMDS IPv4 address defaults to 169.254.169.254.

bash

MMDS_IPV4_ADDR=169.254.170.2
curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT "http://localhost/mmds/config"     \
    -H "Content-Type: application/json"       \
    -d '{
             "network_interfaces": ["${MMDS_NET_IF}"],
             "ipv4_address": "${MMDS_IPV4_ADDR}"
    }'

MMDS is tightly coupled with a network interface which is used to route MMDS packets. To send MMDS intended packets, guest applications must insert a new rule into the routing table of the guest OS. This new rule must forward MMDS intended packets to a network interface which allows MMDS requests. For example:

bash

MMDS_IPV4_ADDR=169.254.170.2
MMDS_NET_IF=eth0
ip route add ${MMDS_IPV4_ADDR} dev ${MMDS_NET_IF}

MMDS supports two methods to access the contents of the metadata store from the guest operating system: V1 and V2. More about the particularities of the two mechanisms can be found in the Retrieving metadata in the guest operating system section. The MMDS version used can be specified when configuring MMDS, through the version field of the HTTP PUT request to /mmds/config resource. Accepted values are V1(deprecated) and V2 and the default MMDS version used in case the version field is missing is Version 1.

bash

MMDS_IPV4_ADDR=169.254.170.2
curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT "http://localhost/mmds/config"     \
    -H "Content-Type: application/json"       \
    -d '{
             "network_interfaces": ["${MMDS_NET_IF}"],
             "version": "V2",
             "ipv4_address": "${MMDS_IPV4_ADDR}"
    }'

Inserting and updating metadata

Inserting and updating metadata is possible through the Firecracker API server. The metadata inserted in MMDS must be any valid JSON. A user can create or update the MMDS data store before the microVM is started or during its operation. To insert metadata into MMDS, an HTTP PUT request to the /mmds resource has to be issued. This request must have a payload with metadata structured in JSON format. To replace existing metadata, a subsequent HTTP PUT request to the /mmds resource must be issued, using as a payload the new metadata. A complete description of metadata insertion firecracker API can be found in the firecracker swagger file.

An example of an API request for inserting metadata is provided below:

bash

curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT "http://localhost/mmds"            \
    -H "Content-Type: application/json"       \
    -d '{
            "latest": {
                  "meta-data": {
                       "ami-id": "ami-12345678",
                       "reservation-id": "r-fea54097",
                       "local-hostname": "ip-10-251-50-12.ec2.internal",
                       "public-hostname": "ec2-203-0-113-25.compute-1.amazonaws.com",
                       "network": {
                            "interfaces": {
                                 "macs": {
                                      "02:29:96:8f:6a:2d": {
                                           "device-number": "13345342",
                                           "local-hostname": "localhost",
                                           "subnet-id": "subnet-be9b61d"
                                      }
                                 }
                            }
                       }
                  }
            }
    }'

To partially update existing metadata, an HTTP PATCH request to the /mmds resource has to be issued, using as a payload the metadata patch, as JSON Merge Patch functionality describes. A complete description of updating metadata Firecracker API can be found in the firecracker swagger file.

An example API for how to update existing metadata is offered below:

bash

curl --unix-socket /tmp/firecracker.socket -i \
    -X PATCH "http://localhost/mmds"          \
    -H "Content-Type: application/json"       \
    -d '{
            "latest": {
                  "meta-data": {
                       "ami-id": "ami-87654321",
                       "reservation-id": "r-79054aef",
                  }
            }
    }'

Retrieving metadata

MicroVM metadata can be retrieved both from host and guest operating systems. For the scope of this chapter, let's assume the data store content is the JSON below:

json

{
    "latest": {
          "meta-data": {
               "ami-id": "ami-87654321",
               "reservation-id": "r-79054aef"
          }
    }
}

Retrieving metadata in the host operating system

To retrieve existing MMDS metadata from host operating system, an HTTP GET request to the /mmds resource must be issued. The HTTP response returns the existing metadata, as a JSON formatted text. A complete description of retrieving metadata Firecracker API can be found in the firecracker swagger file.

Below you can see how to retrieve metadata from the host:

bash

curl -s --unix-socket /tmp/firecracker.socket http://localhost/mmds

Output:

json

{
    "latest": {
          "meta-data": {
               "ami-id": "ami-87654321",
               "reservation-id": "r-79054aef"
          }
    }
}

Retrieving metadata in the guest operating system

Accessing the contents of the metadata store from the guest operating system can be done using one of the following methods:

V1: simple request/response method (deprecated)
V2: session-oriented method

Version 1 (Deprecated)

Version 1 is deprecated and will be removed in the next major version change. Version 2 should be used instead.

To retrieve existing MMDS metadata using MMDS version 1, an HTTP GET request must be issued. The requested resource can be referenced by its corresponding JSON Pointer, which is also the path of the MMDS request. The HTTP response content will contain the referenced metadata resource.

As in version 2, version 1 also supports a session oriented method in order to make the migration easier. See the next section for the session oriented method. Note that version 1 returns a successful response to a GET request even with an invalid token or no token not to break existing workloads. mmds.rx_invalid_token and mmds.rx_no_token metrics track the number of GET requests with invalid tokens and missing tokens respectively, helping users evaluate their readiness for migrating to MMDS version 2.

Requests containing any other HTTP methods than GET and PUT will receive 405 Method Not Allowed error.

bash

MMDS_IPV4_ADDR=169.254.170.2
RESOURCE_POINTER_OBJ=latest/meta-data
curl -s "http://${MMDS_IPV4_ADDR}/${RESOURCE_POINTER_OBJ}"

Version 2

Similar to IMDSv2, MMDS version 2 (V2) is a session oriented method, which makes use of a session token in order to allow fetching metadata contents.

The session must start with an HTTP PUT request that generates the session token. In order to be successful, the request must respect the following constraints:

must be directed towards /latest/api/token path
must contain a X-metadata-token-ttl-seconds or X-aws-ec2-metadata-token-ttl-seconds header specifying the token lifetime in seconds. The value cannot be lower than 1 or greater than 21600 (6 hours).
must not contain a X-Forwarded-For header.

bash

MMDS_IPV4_ADDR=169.254.170.2
TOKEN=`curl -X PUT "http://${MMDS_IPV4_ADDR}/latest/api/token" \
      -H "X-metadata-token-ttl-seconds: 21600"`

The HTTP response from MMDS is a plaintext containing the session token.

During the duration specified by the token's time to live value, all subsequent GET requests must specify the session token through the X-metadata-token or X-aws-ec2-metadata-token header in order to fetch data from MMDS.

bash

MMDS_IPV4_ADDR=169.254.170.2
RESOURCE_POINTER_OBJ=latest/meta-data
curl -s "http://${MMDS_IPV4_ADDR}/${RESOURCE_POINTER_OBJ}" \
    -H "X-metadata-token: ${TOKEN}"

After the token expires, it becomes unusable and a new session token must be issued.

Snapshotting considerations

The data store is not persisted across snapshots, in order to avoid leaking vm-specific information that may need to be reseeded into the data store for a new clone.

The MMDS version, network stack configuration and IP address used for accessing the service are persisted across snapshot-restore.

If the targeted snapshot version does not support Mmds Version 2, it will not be persisted in the snapshot (the clone will use the default, V1). Similarly, if a snapshotted Vm state contains the Mmds version but the Firecracker version used for restoring does not support persisting the version, the default will be used.

MMDS formats

The response format can be JSON or IMDS. The IMDS documentation can be found here. The output format can be selected by specifying the optional Accept header. Using Accept: application/json will format the output to JSON, while using Accept: plain/text or not specifying this optional header at all will format the output to IMDS. Setting imds_compat to true through PUT request to /mmds/config enforces MMDS to always respond in IMDS format regardless of the Accept header. This allows code written to work on EC2 IMDS to also work on Firecracker MMDS.

Retrieving MMDS resources in IMDS format, other than JSON string and object types, is not supported.

Below is an example on how to retrieve the latest/meta-data resource in JSON format:

bash

MMDS_IPV4_ADDR=169.254.170.2
RESOURCE_POINTER_OBJ=latest/meta-data
curl -s -H "Accept: application/json" "http://${MMDS_IPV4_ADDR}/${RESOURCE_POINTER_OBJ}"

Output:

json

{
    "ami-id": "ami-87654321",
    "reservation-id": "r-79054aef"
}

Retrieving the latest/meta-data/ami-id resource in JSON format:

bash

MMDS_IPV4_ADDR=169.254.170.2
RESOURCE_POINTER_STR=latest/meta-data/ami-id
curl -s -H "Accept: application/json" "http://${MMDS_IPV4_ADDR}/${RESOURCE_POINTER_STR}"

Output:

json

"ami-87654321"

Retrieving the latest resource in IMDS format:

bash

MMDS_IPV4_ADDR=169.254.170.2
RESOURCE_POINTER=latest
curl -s "http://${MMDS_IPV4_ADDR}/${RESOURCE_POINTER}"

Output:

text

meta-data/

Retrieving the latest/meta-data/ resource in IMDS format:

bash

MMDS_IPV4_ADDR=169.254.170.2
RESOURCE_POINTER=latest/meta-data
curl -s "http://${MMDS_IPV4_ADDR}/${RESOURCE_POINTER}"

Output:

text

ami-id
reservation-id

Retrieving the latest/meta-data/ami-id resource in IMDS format:

bash

MMDS_IPV4_ADDR=169.254.170.2
RESOURCE_POINTER=latest/meta-data/ami-id
curl -s "http://${MMDS_IPV4_ADDR}/${RESOURCE_POINTER}"

Output:

text

ami-87654321

Errors

200 - Ok

The request was successfully processed and a response was successfully formed.

400 - Bad Request

The request was malformed.

401 - Unauthorized

Only when using MMDS V2. The HTTP request either lacks the session token, or the token specified is invalid. A token is invalid if it was not generated using an HTTP PUT request or if it has expired.

404 - Not Found

The requested resource can not be found in the MMDS data store.

405 - Method Not Allowed

The HTTP request uses a not allowed HTTP method and a response with the Allow header was formed. When using MMDS V1, this is returned for any HTTP method other than GET. When MMDS V2 is configured, the only accepted HTTP methods are PUT and GET.

501 - Not Implemented

The requested HTTP functionality is not supported by MMDS or the requested resource is not supported in IMDS format.

Appendix

Example use case: credential rotation

For this example, the guest expects to find some sort of credentials (say, a secret access key) by issuing a GET request to http://169.254.169.254/latest/meta-data/credentials/secret-key. Most similar use cases will encompass the following sequence of steps:

Some agent running on the host sends a PUT request with the initial contents of the MMDS, using the Firecracker API. This most likely takes place before the microVM starts running, but may also happen at a later time. Guest MMDS requests which arrive prior to contents being available receive a NotFound response.
The contents are saved to MMDS.
The guest sends a GET request for the secret key, which is intercepted by MMDS.
MMDS processes the request and sends back an HTTP response with the ensembled secret key as a JSON string.

After a while, the host agent decides to rotate the secret key. It does so by updating the data store with a new value. This can be done via a PUT request to the /mmds API resource, which replaces everything, or with a PATCH request that only touches the desired key. This effectively triggers the first two steps again.

The guest reads the new secret key, going one more time through the last three steps. This can happen after a notification from the host agent, or discovered via periodic polling, or some other mechanism. Since access to the data store is thread safe, the guest can only receive either the old version, or the new version of the key, and not some intermediate state caused by the update.