I am trying to use fly.io to deploy a simple Phoenix + Ecto app, but have problems connecting Ecto to the provisioned Postgres instance.

I created a postgres instance with flyctl postgres create and attached it to the application with flyctl postgres attach --postgres-app my-app-postgres. This adds a DATABASE_URL environment variable to the application context, which I read out in my releases.exs. I checked and my application receives the correct DATABASE_URL. Unfortunately, Postgresx cannot connect to the database and fails with the error:

Postgrex.Protocol (#PID<0.2945.0>) failed to connect: ** (DBConnection.ConnectionError) 
tcp connect (my-app-postgres.internal:5432): non-existing domain - :nxdomain

The problem seems to be Fly’s own DNS resolver, which should resolve the my-app-postgres.internal part in the DATABASE_URL to an IP address. I saw that the LiveView-Counter example project uses a custom DNS Strategy to resolve the APP_NAME.internal URLs. I wondered whether I can set a similar strategy for Ecto to use.

My assumption is that Ecto tries to resolve the my-app-postgres.internal URl with a public DNS instead of the Fly.io internal DNS, which is responsible for the .internal-URLs in the internal network of Fly.

My question is therefore: Do you know how I could configure Phoenix or Ecto to use the internal DNS for the .internal-URLs?

I saw a similar question, where the network_mode in the docker-compose.yml was used to let Phoenix discover other services through the host network, but with Fly.io, one can only use a Dockerfile and not a docker-compose.yml file to create the application. So, I wouldn’t know how to set the network_mode inside the Dockerfile.

Edit: I set the private_network=true flag in my fly.toml file, but it didn’t help:

app = "my-app"

kill_signal = "SIGINT"
kill_timeout = 5

[experimental]
  private_network=true

[[services]]
  internal_port = 4000
  protocol = "tcp"

  [services.concurrency]
    hard_limit = 25
    soft_limit = 20

  [[services.ports]]
    handlers = ["http"]
    port = "80"

  [[services.ports]]
    handlers = ["tls", "http"]
    port = "443"

  [[services.tcp_checks]]
    grace_period = "1s"
    interval = "15s"
    port = "4000"
    restart_limit = 6
    timeout = "2s"