Project: Firecracker (Runtime / Container Runtime)
File: mmds-user-guide.md

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. 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/${MMDSNETIF}`. The full network configuration API can be found in the . Configure MMDS through an HTTP `PUT` request to `/mmds/config` resource and include the IDs of the network interfaces that should allow forwarding requests to MMDS in the `network_interfaces` list. The complete MMDS API is described in the . Attaching a network device with ID `MMDSNETIF`: ```bash MMDSNETIF=eth0 curl --unix-socket /tmp/firecracker.socket -i \ -X PUT 'http://localhost/network-interfaces/${MMDSNETIF}' \ -H 'Accept: application/json' \ -H 'Content-Type: application/json' \ -d '{ "ifaceid": "${MMDSNET_IF}", "guest_mac": "AA:FC:00:00:00:01", "hostdevname": "tap0" }' ``` Configuring MMDS to receive requests through the `MMDSNETIF` network interface ID: ```bash MMDSIPV4ADDR=169.254.170.2 curl --unix-socket /tmp/firecracker.socket -i \ -X PUT "http://localhost/mmds/config" \ -H "Content-Type: application/json" \ -d '{ "networkinterfaces": ["${MMDSNET_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 MMDSIPV4ADDR=169.254.170.2 curl --unix-socket /tmp/firecracker.socket -i \ -X PUT "http://localhost/mmds/config" \ -H "Content-Type: application/json" \ -d '{ "networkinterfaces": ["${MMDSNET_IF}"], "ipv4address": "${MMDSIPV4_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 MMDSIPV4ADDR=169.254.170.2 MMDSNETIF=eth0 ip route add ${MMDSIPV4ADDR} dev ${MMDSNETIF} ``` 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 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 . ```bash MMDSIPV4ADDR=169.254.170.2 curl --unix-socket

-i \ -X PUT "http://localhost/mmds/config" \ -H "Content-Type: application/json" \ -d '{ "networkinterfaces": ["${MMDSNET_IF}"], "version": "V2", "ipv4address": "${MMDSIPV4_ADDR}" }' ``` 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 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 . 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 functionality describes. A complete description of updating metadata Firecracker API can be found in the . 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", } } }' ``` 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" } } } ``` 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 . 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" } } } ``` 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 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

The requested resource can be referenced by its corresponding , which is also the path of the MMDS request. The HTTP response content will contain the referenced metadata resource. The only HTTP method supported by MMDS version 1 is `GET`. Requests containing any other HTTP method will receive 405 Method Not Allowed error. ```bash MMDSIPV4ADDR=169.254.170.2 RESOURCEPOINTEROBJ=latest/meta-data curl -s "http://${MMDSIPV4ADDR}/${RESOURCEPOINTEROBJ}" ``` Similar to , 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` 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 MMDSIPV4ADDR=169.254.170.2 TOKEN=`curl -X PUT "http://${MMDSIPV4ADDR}/latest/api/token" \ -H "X-metadata-token-ttl-seconds: 21600"` ``` The HTTP response from MMDS is a plaintext containing the session token. During the duration specified by the token's time to live value, all subsequent `GET` requests must specify the session token through the `X-metadata-token` header in order to fetch data from MMDS. ```bash MMDSIPV4ADDR=169.254.170.2 RESOURCEPOINTEROBJ=latest/meta-data curl -s "http://${MMDSIPV4ADDR}/${RESOURCEPOINTEROBJ}" \ -H "X-metadata-token: ${TOKEN}" ``` After the token expires, it becomes unusable and a new session token must be issued. The data store is not persisted across snapshots, in order to avoid leaking vm-specific information that may need to be reseeded into the data store for a new clone. The MMDS version, network stack configuration and IP address used for accessing the service are persisted across snapshot-restore. If the targeted snapshot version does not support Mmds Version 2, it will not be persisted in the snapshot (the clone will use the default, V1). Similarly, if a snapshotted Vm state contains the Mmds version but the Firecracker version used for restoring does not support persisting the version, the default will be used. The response format can be JSON or IMDS. The IMDS documentation can be found . The output format can be selected by specifying the optional `Accept` header. Using `Accept: application/json` will format the output to JSON, while using `Accept: plain/text` or not specifying this optional header at all will format the output to IMDS. Retrieving MMDS resources in IMDS format, other than JSON `string` and `object` types, is not supported. Below is an example on how to retrieve the `latest/meta-data` resource in JSON format: ```bash MMDSIPV4ADDR=169.254.170.2 RESOURCEPOINTEROBJ=latest/meta-data curl -s -H "Accept: application/json" "http://${MMDSIPV4ADDR}/${RESOURCEPOINTEROBJ}" ``` Output: ```json { "ami-id": "ami-87654321", "reservation-id": "r-79054aef" } ``` Retrieving the `latest/meta-data/ami-id` resource in JSON format: ```bash

RESOURCEPOINTERSTR=latest/meta-data/ami-id curl -s -H "Accept: application/json" "http://${MMDSIPV4ADDR}/${RESOURCEPOINTERSTR}" ``` Output: ```json "ami-87654321" ``` Retrieving the `latest` resource in IMDS format: ```bash MMDSIPV4ADDR=169.254.170.2 RESOURCE_POINTER=latest curl -s "http://${MMDSIPV4ADDR}/${RESOURCE_POINTER}" ``` Output: ```text meta-data/ ``` Retrieving the `latest/meta-data/` resource in IMDS format: ```bash MMDSIPV4ADDR=169.254.170.2 RESOURCE_POINTER=latest/meta-data curl -s "http://${MMDSIPV4ADDR}/${RESOURCE_POINTER}" ``` Output: ```text ami-id reservation-id ``` Retrieving the `latest/meta-data/ami-id` resource in IMDS format: ```bash MMDSIPV4ADDR=169.254.170.2 RESOURCE_POINTER=latest/meta-data/ami-id curl -s "http://${MMDSIPV4ADDR}/${RESOURCE_POINTER}" ``` Output: ```text ami-87654321 ``` 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. 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.