iOS: file-only :inet_db lookup by default in Mob.App.start/0

CHANGELOG.md

@@ -8,6 +8,11 @@ Full module documentation: [hexdocs.pm/mob](https://hexdocs.pm/mob).
 
 ---
 
+## [Unreleased]
+
+### Fixed
+- iOS: `Mob.App.start/0` now switches `:inet_db` to file-only lookup and seeds `localhost` before any user code runs — BEAM's default `:native` lookup tries to `execve` the `inet_gethost` port program, which the iOS sandbox refuses, crashing the first `Node.connect` / `:erpc.call` / `gen_tcp.connect/3` with `:badarg`. Apps no longer need to set the lookup chain themselves; `Mob.DNS.configure_pure_beam/1` still composes on top for outbound DNS. See `guides/dns_on_ios.md`.
+
 ## [0.6.7]
 
 ### Added

guides/dns_on_ios.md

@@ -34,6 +34,25 @@ first in the chain.
 
 ---
 
+## What mob already does for you
+
+Before any of this matters, `Mob.App.start/0` runs
+`Mob.App.configure_ios_inet_db/0` on iOS (both simulator and device),
+which switches the lookup chain to `[:file]` and seeds `localhost`.
+That's the minimum needed so `Node.connect`, `:erpc.call`, and
+`gen_tcp.connect/3` with a binary host don't crash the calling
+process on the first lookup. Apps don't have to do anything for
+distribution and local-loopback TCP to work.
+
+`Mob.DNS.configure_pure_beam/1` is the next step *on top of that* —
+it upgrades the chain to `[:file, :dns]` and seeds fallback
+nameservers so outbound HTTP to public hosts works. The framework
+default doesn't enable `:dns` because apps that don't talk to the
+public internet shouldn't pay for fallback-nameserver state they
+won't use.
+
+---
+
 ## Why this exists
 
 BEAM resolves hostnames the same way it always has: it spawns an

guides/troubleshooting.md

@@ -342,6 +342,11 @@ app sandbox forbids. Every hostname lookup through `:inet` fails immediately.
 Android works because its OTP helpers ship as `lib*.so` in `jniLibs/`, which
 SELinux allows to exec; iOS has no equivalent escape hatch.
 
+`Mob.App.start/0` already switches the lookup chain to `[:file]` on iOS so
+distribution and local-loopback TCP work without setup. That doesn't help
+public-internet hostnames though — you still need to opt into one of the
+DNS strategies below to talk to Req / Finch / Mint endpoints.
+
 **Fix:** Call `Mob.DNS.resolve/1` once per backend before your first request,
 typically in your app's `on_start/0`:
 

lib/mob/app.ex

@@ -66,6 +66,17 @@ defmodule Mob.App do
       Do not override — implement `on_start/0` instead.
       """
       def start do
+        # iOS-only: BEAM's default :native hostname lookup spawns the
+        # `inet_gethost` port program via execve, which the iOS app
+        # sandbox refuses. Any subsequent code path that resolves a
+        # hostname — Node.connect, :erpc.call, gen_tcp.connect with a
+        # binary host, Logger forwarding to a named node — crashes the
+        # calling process with badarg before this is fixed. Switch to
+        # file-only lookup and seed `localhost` so distribution and
+        # local TCP work out of the box. Apps that need real outbound
+        # DNS layer Mob.DNS.configure_pure_beam/1 on top in on_start/0.
+        Mob.App.configure_ios_inet_db()
+
         Mob.NativeLogger.install()
 
         # Compile theme from options passed to `use Mob.App, theme: [...]`
@@ -123,6 +134,58 @@ defmodule Mob.App do
     end
   end
 
+  @doc """
+  Apply the iOS-only `:inet_db` workaround so distribution, RPC, and
+  TCP-by-hostname don't crash on the first lookup.
+
+  iOS sandboxes any app that isn't Apple's own and refuses `execve` of
+  binaries the app didn't get a special pass for. BEAM's default
+  `:native` hostname-resolution path spawns the `inet_gethost` port
+  program — exactly the kind of `execve` iOS rejects — so the very
+  first `:inet.getaddr/2` call (transitively reached by `Node.connect`,
+  `:erpc.call`, `gen_tcp.connect/3` with a binary host, etc.) crashes
+  the calling process with `:badarg`. The simulator hits the same
+  failure for a related but distinct reason: `inet_gethost` doesn't
+  live at the path BEAM expects under the mob iOS sim OTP layout.
+  Either way, the fix is the same.
+
+  Switching the lookup chain to `[:file]` keeps everything in BEAM's
+  in-process name table — no port program, no fork, no `execve`. We
+  also seed `localhost` so apps using `@localhost` node names (or any
+  `gen_tcp` call that resolves `"localhost"`) work without further
+  setup.
+
+  Called automatically by the macro-generated `start/0` before
+  anything else, so app `on_start/0` code never has to think about it.
+  Apps that need outbound DNS (Req / Finch / Mint to arbitrary hosts)
+  can layer `Mob.DNS.configure_pure_beam/1` on top — it upgrades the
+  chain to `[:file, :dns]` and seeds fallback nameservers, while the
+  file-table entries we add here keep winning.
+
+  Other platforms (`:android`, `:host`) are unaffected — BEAM's native
+  resolver works there. Safe to call on host BEAM where the NIF isn't
+  loaded; rescues the `UndefinedFunctionError` / `ErlangError` and
+  returns `:ok`.
+  """
+  @spec configure_ios_inet_db() :: :ok
+  def configure_ios_inet_db do
+    case safe_platform() do
+      :ios ->
+        :inet_db.set_lookup([:file])
+        :inet_db.add_host({127, 0, 0, 1}, [~c"localhost"])
+        :ok
+
+      _ ->
+        :ok
+    end
+  end
+
+  defp safe_platform do
+    :mob_nif.platform()
+  rescue
+    _ in [UndefinedFunctionError, ErlangError] -> :host
+  end
+
   # ── Navigation helpers ─────────────────────────────────────────────────────
 
   @doc """

test/mob/app_test.exs

@@ -0,0 +1,19 @@
+defmodule Mob.AppTest do
+  use ExUnit.Case, async: false
+
+  describe "configure_ios_inet_db/0" do
+    test "is a no-op on host BEAM where the NIF isn't loaded" do
+      lookup_before = :inet_db.res_option(:lookup)
+
+      assert :ok = Mob.App.configure_ios_inet_db()
+
+      assert :inet_db.res_option(:lookup) == lookup_before
+    end
+
+    test "idempotent — repeated calls don't crash or stack state" do
+      assert :ok = Mob.App.configure_ios_inet_db()
+      assert :ok = Mob.App.configure_ios_inet_db()
+      assert :ok = Mob.App.configure_ios_inet_db()
+    end
+  end
+end