Thank you for your suggestion.
I tried to enable IPv6 explicitly but it didn’t work. It returned the same error. Other HTTP clients (hackney, HTTPoison) also returned the same error.
> req = Req.new(url: "https://private-app.internal:8080/messages/new", connect_options: [transport_opts: [inet6: true]])
> Req.post!(req)
** (Mint.TransportError) non-existing domain
(req 0.3.5) lib/req.ex:329: Req.post!/2
And I also made sure the services block fly.toml is empty and redeployed the app.
app = "private-app"
kill_signal = "SIGINT"
kill_timeout = 5
processes = []
[env]
[experimental]
auto_rollback = true
[[services]]