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.

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


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.


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.


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.


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


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. is an example of the Docker Image Reference’s format.


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


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.


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


Process Group: The process group associated with the Machine.


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


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


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.


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.


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.


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, “,” contains the client and the one proxy it passed through.

MDN has full documentation for this header.


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


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


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

Request and Response Headers


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 in the field, denoting that version 2 of HTTP was used by the connection as it passed through the proxy.