Hostname → IP resolution that works around BEAM's broken DNS path on iOS.

Why this exists

BEAM resolves hostnames by spawning an external helper called inet_gethost (a port program). On macOS, Linux, Windows that works fine. On iOS it doesn't — the iOS app sandbox forbids execve of any binary the app didn't get a special pass for, and there's no equivalent of Android's lib*.so escape hatch. Result: :inet.getaddr/2 (and therefore Req, Finch, Mint, ReqLLM, and basically every Elixir HTTP library) fails on iOS the moment a request hits a hostname rather than a literal IP.

This module side-steps the problem by calling Darwin's getaddrinfo directly via a NIF, then seeding :inet_db with the result so subsequent BEAM-level lookups for the same host succeed from the in-process file table.

Android isn't affected — mob_beam.zig ships inet_gethost as libinet_gethost.so in jniLibs/, which the SELinux policy allows to execve. The NIF here would work on Android too but isn't wired up by default; the BEAM path is already functional there.

How to use it

Resolve each hostname your app talks to before the first Req / Finch / Mint call to that host. Once resolved, :inet_db retains the mapping for the lifetime of the BEAM, so subsequent HTTP calls go through without you doing anything else.

# At app startup, or before the first call:
{:ok, _ip} = Mob.DNS.resolve("api.example.com")

# Now this just works on iOS:
Req.get!("https://api.example.com/v1/things")

For a small fixed set of hosts, the convenience helper preresolve/1 does the whole list at once:

Mob.DNS.preresolve([
  "api.example.com",
  "auth.example.com"
])

Scope and limitations

• IPv4 only. Most cloud endpoints serve A records; IPv6 is a follow-up if it becomes useful.
• One IP per host. If the hostname has multiple A records, the first one is used. BEAM caches the result; failover isn't automatic. If your endpoint cycles IPs frequently you may need to re-resolve.
• No automatic refresh. Mappings stay in :inet_db until the BEAM exits. If a backend's IP changes mid-session, the cached entry will be stale — call resolve/1 again to refresh.
• Doesn't help raw NIF networking. If a third-party NIF calls libc getaddrinfo itself, it never goes through BEAM's DNS layer and doesn't need (or benefit from) this fix — it already works on iOS. Only :inet-mediated lookups (which covers almost all Elixir HTTP libraries) need our help.
• iOS only effectively. On Android and host (dev, macOS, Linux) the NIF works but is unnecessary; BEAM's built-in path is fine there.

Errors

{:ok, {a, b, c, d}}              # success
{:error, :badarg}                # host arg invalid
{:error, :nxdomain}              # no such hostname
{:error, :timeout}               # resolver TRY_AGAIN
{:error, :no_address}            # resolved but no IPv4
{:error, {:gai, code}}           # raw getaddrinfo error code
{:error, :nif_not_loaded}        # called off-device (host tests)