Disabling external network access

You can configure the inet dns resolver to never resolve anything. That way only ip addresses will be possible to use.

For example:

echo "{lookup,[]}." > ./erl_inetrc
ERL_INETRC=./erl_inetrc iex
Erlang/OTP 27 [RELEASE CANDIDATE 2] [erts-14.2.3] [source-f7fa414e3e] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit:ns]

Interactive Elixir (1.17.0-dev) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> :gen_tcp.connect(~c"localhost",80,[])
{:error, :nxdomain}

You can read more about how to configure inetrc here: Inet Configuration — OTP 29.0.2 (erts 17.0.2)

You can also try Overview — extrace v0.6.0 with some of the lower-level functions in :inet but I am not sure that would work at all.

Thanks!

I forgot about the amazing dbg - I’ve put this into test/test_helper.ex

require Logger

:dbg.start()

:dbg.tracer(
  :process,
  {fn {:trace, _pid, :call, {:gen_tcp, :connect, [host, port | _]}}, n ->
     case {host, port} do
       {~c"localhost", 5432} -> :ok
       {~c"localhost", 6379} -> :ok
       _ -> Logger.error("\n\n****\n\nConnection to #{host}:#{port} not allowed\n\n****\n")
     end

     n + 1
   end, 0}
)

:dbg.tp(:gen_tcp, :connect, [])
:dbg.p(:all, :c)

and it will log unwanted connections during tests.