The Fly Runtime Environment

When a Fly application is running, various information about the runtime environment is made available to it. Environment variables carry information that is generally applicable to the instance. These values come from three sources.

• Secrets
• The application’s env variable settings in fly.toml
• The Fly infrastructure, as detailed below.

Request headers carry information that is specific to the incoming request and its path taken to the application. Request headers are added by the HTTP handler service.

Environment Variables

FLY_APP_NAME

Application Name: Each application running on Fly has a unique Application Name. This identifies the application for the user and can also identify instances of the application running on Fly’s 6PN networking. For example, syd.$FLY_APP_NAME.internal can refer to an instance of the app running. Read more about 6PN Naming in the Network Services section.

FLY_ALLOC_ID

Allocation ID: Each instance of an application running on Fly has a unique Allocation ID. This can be used, for example, to distinguish between instances running in the same region. b996131a-5bae-215b-d0f1-2d75d1a8812b is an example of the Allocation ID’s format.

FLY_REGION

Region name: The three-letter name of the region the application instance is running in. Details of current regions are listed in the Regions page. As an example, “ams” is the region name for Amsterdam.

Not to be confused with the HTTP header Fly-Region, which is where the connection was accepted from.

FLY_PUBLIC_IP

IPV6 Public IP: The full public IPV6 for this instance. Read more in the Network Services section.

FLY_IMAGE_REF

Docker Image Reference: The name of the Docker image running this container. Useful if your application needs to launch Machine instances of itself to scale up-or-down. Read about how a Rails app runs Machines for scale-to-zero background workers. registry.fly.io/my-app-name:deployment-01H9RK9EYO9PGNBYAKGXSHV0PH is an example of the Docker Image Reference’s format.

FLY_MACHINE_ID

Machine ID: The unique ID that flyctl commands and the Machines API use to identify a Machine. You can find it in the logs.

FLY_MACHINE_VERSION

Machine Configuration Version: The version associated to a specific Machine configuration. When you update a Machine’s configuration (including when you update its Docker image), it gets a new FLY_MACHINE_VERSION. You can also find this value under the name Instance ID in the output of fly machine status. Changing the Machine’s metadata however doesn’t trigger a new version.

FLY_PRIVATE_IP

Private IPv6 Address: The IPv6 address of the Machine on its 6PN private network.

FLY_PROCESS_GROUP

Process Group: The process group associated with the Machine.

FLY_VM_MEMORY_MB

Machine Memory: The memory allocated to the Machine, in MB. It’s the same value you’ll find under https://fly.io/dashboard/personal/machines and VM Memory in the output of fly machine status. Learn more about Machine sizing.

PRIMARY_REGION

Primary Region: This is set in your fly.toml or with the --region flag during deploys. Learn more about configuring the primary region.

Request Headers

Fly-Client-IP

Client IP Address: The IP address Fly accepted a connection from. This will be the client making the initial request and as such, will also appear at the start of the X-Forwarded-For addresses.

Fly-Forwarded-Port

Original connection port: This header is always set by Fly and denotes the actual port that the client connected to the Fly edge node which is then forwarded to the application instance.

Fly-Region

Edge Node Region: This header is a three letter region code which represents the region that the connection was accepted in and routed from.

Not to be confused with the environment variable FLY_REGION, which is where the application is running.

X-Forwarded-For

Client and Proxy List: This is a comma separated list comprising of the client that originated the request and the proxy servers the request passed through. For example, “77.97.0.98, 77.83.142.33” contains the client and the one proxy it passed through.

MDN has full documentation for this header.

X-Forwarded-Proto

Original client protocol: The protocol which the client used to make the request. Either http or https.

X-Forwarded-Port

Original connection port: This header may be set by the client and should denote the port that the client set out to connect to.

X-Forwarded-SSL

SSL Status: This indicates if the client connected over SSL. Its value can be either on or off.

Request and Response Headers

Via

Proxy Route: This header, added by proxies, shows the path taken, and protocols used, by the connection. MDN has full documentation for this header. For example, a connection through the Fly edge may show 2 fly.io in the field, denoting that version 2 of HTTP was used by the connection as it passed through the fly.io proxy.