flytrap

Flytrap is a Rust crate for reading the Fly.io runtime environment.

• Read Fly.io environment variables like $FLY_PUBLIC_IP into a struct
• Query Fly.io internal DNS addresses like top3.nearest.of.<app>.internal
• Parse Fly.io request headers like Fly-Client-IP (into an IpAddr)
• Turn Fly.io region codes like ord into names like ”Chicago” and lat/long coordinates

A demo app is available at flytrap.fly.dev which shows this crate’s capabilities.

Usage

Flytrap can be added to your project with cargo:

cargo add flytrap

Most of the crate’s features are enabled by default, but at your option, you can enable only what you need by setting default-features = false.

cargo add flytrap --no-default-features --features 'dns environment http serde'

Placement

The Placement type gives access to Fly.io runtime environment variables like $FLY_PUBLIC_IP and $FLY_REGION.

use flytrap::{Placement, Machine}

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let runtime = Placement::current()?;

    println!("Fly.io app: {}", runtime.app);
    println!("    region: {}", runtime.location);

    if let Some(Machine{ id, memory: Some(memory), image: Some(image), .. }) = runtime.machine {
        println!("   machine: {id} ({memory} MB) running {image}");
    }

    if let Some(public_ip) = runtime.public_ip {
        println!(" public IP: {}", public_ip);
    }
    println!("private IP: {}", runtime.private_ip);

    Ok(())
}

Regions

Flytrap models Fly.io regions as an enum:

use flytrap::{City, Placement, Region};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let runtime = Placement::current()?;
    let region = runtime.region().unwrap_or(Region::Guadalajara);

    show(region);
    Ok(())
}

fn show(region: Region) {
    let City { name, country, geo } = region.city;
    println!("Running in {name} ({country}) @ {}, {}", geo.x(), geo.y());
}

Regions implement Ord, and sort geographically:

#[test]
fn longitude_latitude() {
    use flytrap::Region::*;

    let mut regions = [Bucharest, Chicago, HongKong, Johannesburg,
                       LosAngeles, Madrid, Santiago, Tokyo];
    regions.sort();

    assert_eq!(regions, [LosAngeles, Chicago, Santiago, Madrid,
                         Bucharest, Johannesburg, HongKong, Tokyo]);
}

If Flytrap receives a region code it doesn’t recognize (i.e., a region which didn’t exist when your version of Flytrap was built), the raw code like ord will be stored as a RegionCode.

(If you build Flytrap without the regions feature, flytrap::Region simply becomes an alias for String.)

HTTP headers

The http module contains typed Header implementations of the HTTP request headers added by Fly.io edge proxies, like Fly-Client-IP.

use axum::{response::Html, TypedHeader};
use flytrap::http::{FlyClientIp, FlyRegion};

async fn ip(
    TypedHeader(ip): TypedHeader<FlyClientIp>,
    TypedHeader(edge): TypedHeader<FlyRegion>,
) -> Html<String> {
    Html(format!("Your IP: <code>{ip}</code> (via {edge})"))
}

DNS queries

Create a Resolver in order to query the Fly.io .internal DNS zone.

use flytrap::Resolver;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let resolver = Resolver::new()?;

    // Discover all instances of the currently-running app
    let peers = resolver.current()?.peers().await?;

    for peer in peers {
        println!("peer {} in {} @ {}", peer.id, peer.location, peer.private_ip);
    }
}

Features

Flytrap’s compilation can be controlled through a number of Cargo features.

• dns: Enable Resolver for querying Fly.io internal DNS, via hickory-dns ⭐
• detect: Enable automatic Resolver setup for Wireguard VPN clients, via if-addrs ⭐️
• environment: Enable code which reads Fly.io environment variables like $FLY_PUBLIC_IP ⭐️
• http: Enable types for HTTP headers like Fly-Client-IP ⭐️
• nightly: Enable code which is only accepted by nightly Rust toolchains
• regions: Enable the Region type and RegionDetails structures ⭐️
• serde: Enable Serde #[derive(Deserialize, Serialize)] on this crate’s types
• system-resolver: Enable the Resolver::system() constructor, which reads /etc/resolv.conf

(Features marked with a ⭐️ are enabled by default.)