OpenID Connect

Fly Machines sometimes need to access 3rd-party services or cloud providers like AWS, Azure, or GCP. To authenticate against these 3rd parties, your Machines need to supply credentials such as a password or token to the cloud provider; usually these are stored as fly secrets and then passed into your Machine’s environment variables.

However this approach involves creating hardcoded long-lived credentials on the 3rd party platform and involves either managing a lot of tokens or sharing the same token across multiple apps.

Using OpenID Connect (OIDC) allows for a more secure approach by using short-lived access tokens directly from 3rd party platforms. The only caveat is that the 3rd party platform needs to support OIDC tokens and you need to establish a trust between Fly.io and the 3rd party. Providers which currently support OIDC include Amazon Web Services, Azure, Google Cloud Platform, and HashiCorp Vault, among others.

Benefits of OpenID Connect (OIDC)

Adopting OIDC to manage access to 3rd party services allows your apps to easily use good security practices like:

• No hardcoded credentials, these are often shared across Machines and apps. OIDC allows for every Machine to have its own credential.
• Granular control over Machines access to 3rd party resources through leveraging the authentication (authN) and authorization (authZ) tools of the 3rd party.
• Rotating credentials: credentials issued from 3rd parties are only valid for 15 minutes before they expire.

Understanding OpenID Connect

Every request to the token endpoint generates a unique Json Web Token (JWT). When you give this JWT to a 3rd party they validate the token against the OIDC configuration hosted at the endpoint in the issuer (iss) claim. Below is an example of an issued OIDC token. A useful feature here is that the subject (sub) claim references the Machine’s org name, app name and Machine name. This allows for regex checks to be built against the claim which can restrict accepted tokens to a specific app or Machine.

{
    "app_id": "11111111",
    "app_name": "example-app",
    "aud": "https://fly.io/exmaple-org",
    "exp": 1712099653,
    "iat": 1712099053,
    "image": "docker-hub-mirror.fly.io/you/image:latest",
    "image_digest": "sha256:2c1cdaded1b3820020c9dc9fdd1d6e798d6f6ca36861bb6ae64019fad6be9ee3",
    "iss": "https://oidc.fly.io/exmaple-org",
    "jti": "93ca09e1-70e0-477b-a260-1d8fcd4ef4f4",
    "machine_id": "148e21ea7e46e8",
    "machine_name": "example-machine",
    "machine_version": "01HTGGC1TZ2JHK83J4AC0R3VET",
    "nbf": 1712099053,
    "org_id": "11111111",
    "org_name": "example-org",
    "region": "sea",
    "sub": "example-org:example-app:example-machine"
}

Customizing OIDC token claims

Customizing tokens allows for powerful authorization clauses in the platform you’re authenticating against. For example, with AWS, you can require that a role can only be assumed by tokens which match certain aud and sub claims. This allows you to lock down and provide credentials only to a specific Fly.io app or Machine.

You can customize the audience (aud) claim of your tokens by providing a value for aud in the body of the POST request. This lets you specify the recipient of the token. This is what it looks like in a curl request with the aud claim set to sts.amazonaws.com.

curl --unix-socket /.fly/api -X POST "http://localhost/v1/tokens/oidc" --data '{"aud":"sts.amazonaws.com"}'

OpenID Connect Trust Policies in AWS

You can leverage the trust policy of an IAM role to restrict which Machines in your organisation can assume a role in AWS. For example the following policy would only allow Machines in the app foo-bar within your org to assume the role it’s attached to.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "Federated": "arn:aws:iam::123456123456:oidc-provider/oidc.fly.io/<org-name>"
            },
            "Action": "sts:AssumeRoleWithWebIdentity",
            "Condition": {
              "StringEquals": {
                "oidc.fly.io/<org-name>:aud": "sts.amazonaws.com",
              },
                "StringLike": {
                "oidc.fly.io/<org-name>:sub": "org-name:foo-bar:*"
              }
            }
        }
    ]
}

Reading from an S3 Bucket on a Fly Machine

To read from an S3 bucket using a Fly Machine you’ll first need to set up a trust between AWS and Fly.io by creating an OIDC provider in AWS and then attaching a trust policy to the role policy your Machine will assume. If you’ve already packaged up your app in a Dockerfile, then you can follow these steps to read from an S3 bucket:

1. Create a Fly.io app by running fly launch in the same directory as your Dockerfile.

2. Find the slug for the organisation you want to launch your app in.

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   fly orgs list
   

3. Create an OpenID Connect Identity provider in your AWS account with your org slug and the following settings:

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   Provider URL: https://oidc.fly.io/<org-slug>
   Audience: sts.amazonaws.com
   

4. Create an IAM Role with the following settings:

   • Trusted Identity: Web Identity -> Identity Provider -> oidc.fly.io
   • Select the AmazonS3ReadOnlyAccess policy.

5. Set the AWS_ROLE_ARN as an environment variable in your fly.toml.

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   [env]
     AWS_ROLE_ARN = "arn:aws:iam::<ACCOUNT_ID>:role/<ROLE_NAME>"
   

6. Deploy your app and let the AWS SDK will do the rest.

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   fly deploy