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2025-09-12 22:43:53 -04:00

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React Native SSH — iOS simulator limitations & paths forward

Last updated: 2025-09-12

TL;DR

  • Our current stack (react-native-ssh-sftp → NMSSH (aanah0 fork) → prebuilt OpenSSL/libssh2) fails on the iOS Simulator because the vendored native libraries are compiled for iOS device only (platform: iphoneos), not iOS Simulator (platform: iphonesimulator).

  • The proper modern fix is to use XCFrameworks that include both device and simulator slices, and have NMSSH link against those instead of raw static .a libraries.

  • We have two main paths:

    1. Keep using the existing RN package, but swap NMSSH’s underlying crypto/SSH libs to an XCFramework (e.g., from DimaRU/Libssh2Prebuild), via a tiny CocoaPods change.
    2. Build our own RN native module (TurboModule/JSI) that links libssh2 and OpenSSL directly (using or adapting the Libssh2Prebuild scripts), fixing iOS Simulator support and also addressing Android reliability issues (like double “disconnect” callbacks).

Current stack & limitation

Stack

  • App → react-native-ssh-sftp
  • iOS native → NMSSH (via the aanah0/NMSSH fork for a newer libssh2 baseline)
  • NMSSH bundles precompiled OpenSSL + libssh2 as static libraries.

Why iOS Simulator builds fail

  • Xcode treats device and simulator as different platforms. Since Apple Silicon, both may be arm64, but they are not interchangeable:

    • Device slice: ios-arm64 (platform: iphoneos)
    • Simulator slices: ios-arm64 and/or ios-x86_64 (platform: iphonesimulator)

  • NMSSH’s vendored libs are device-only (iphoneos). When the Simulator build links those, the linker throws:

    ld: building for 'iOS-simulator', but linking in object file .../libcrypto.a[arm64] built for 'iOS'

  • XCFrameworks solve this: they are a bundle containing the correct slices for both platforms. Xcode picks the proper slice automatically.

Temporary hack (not recommended): exclude arm64 for the simulator (EXCLUDED_ARCHS[sdk=iphonesimulator*]=arm64) to force an x86_64 sim under Rosetta. This “works” but loses native-sim performance and is brittle.

Path 1 — Keep current RN package; replace NMSSH’s vendored libs with an XCFramework

Goal: Do not change react-native-ssh-sftp or the NMSSH API. Only change how NMSSH links to OpenSSL/libssh2.

What we’ll use

  • DimaRU/Libssh2Prebuild: ships a ready-made XCFramework (“CSSH”) that bundles libssh2 + OpenSSL with both device and simulator slices.
  • CocoaPods can consume XCFrameworks via a binary pod (vendored_frameworks), but Libssh2Prebuild is published for SwiftPM. So we add a tiny CSSH-Binary podspec that points to its zipped XCFramework.

Minimal changes

  1. Fork NMSSH (from aanah0/NMSSH) and edit NMSSH.podspec

    • Remove the vendored static libs (the .a files).
    • Add a dependency on CSSH-Binary (our binary pod that vends CSSH.xcframework).

    Podspec diff (conceptual):

    @@
 Pod::Spec.new do |s|
   s.name     = 'NMSSH'
   # ... (other metadata unchanged)

-  s.vendored_libraries = [
-    'NMSSH-iOS/Libraries/lib/libssh2.a',
-    'NMSSH-iOS/Libraries/lib/libssl.a',
-    'NMSSH-iOS/Libraries/lib/libcrypto.a'
-  ]
+  s.dependency 'CSSH-Binary', '~> 1.11'
 end

  2. Create a tiny binary pod for CSSH (one-file repo): CSSH-Binary.podspec

    Pod::Spec.new do |s|
  s.name     = 'CSSH-Binary'
  s.version  = '1.11.0' # match the Libssh2/OpenSSL combo you choose
  s.summary  = 'libssh2 + OpenSSL as XCFramework (from DimaRU/Libssh2Prebuild)'
  s.license  = { :type => 'MIT' }
  s.homepage = 'https://github.com/your-org/cssh-binary'
  s.authors  = { 'Your Name' => 'you@example.com' }
  s.platform = :ios, '12.0'
  s.source   = {
    :http => 'https://github.com/DimaRU/Libssh2Prebuild/releases/download/1.11.0-OpenSSL-1-1-1w/CSSH.xcframework.zip'
  }
  s.vendored_frameworks = 'CSSH.xcframework'
end
    • You can host the zip yourself if you want deterministic supply (recommended for CI).
    • Ensure license attributions (OpenSSL 3.x: Apache-2.0; libssh2: BSD-3-Clause) are included in our OSS notices.

  3. Wire via Expo (expo-build-properties) in app.config.ts

    [
  'expo-build-properties',
  {
    ios: {
      extraPods: [
        // Binary pod that vends the XCFramework
        { name: 'CSSH-Binary', podspec: 'https://raw.githubusercontent.com/your-org/cssh-binary/main/CSSH-Binary.podspec' },
        // Our NMSSH fork that depends on CSSH-Binary
        { name: 'NMSSH', git: 'https://github.com/your-org/NMSSH.git', branch: 'cssh-xcframework' },
      ],
    },
    android: {
      packagingOptions: {
        pickFirst: ['META-INF/versions/9/OSGI-INF/MANIFEST.MF'],
      },
    },
  },
]

  4. Rebuild

    npx expo prebuild --platform ios --clean
npx pod-install
npx expo run:ios   # Simulator should now link correctly

Pros

  • Small surface area; we keep the RN package API intact.
  • Correct, future-proof device/simulator packaging via XCFrameworks.
  • Easy to maintain once the podspecs are set.

Cons/Risks

  • We maintain a fork of NMSSH (podspec only).
  • Potential conflicts with tools like Flipper if they also touch OpenSSL (rare; we can disable Flipper if needed).
  • We rely on Libssh2Prebuild versions/tags (or build our own artifacts using their scripts).

Path 2 — Build our own RN module (TurboModule/JSI) with libssh2/OpenSSL

Goal: Replace react-native-ssh-sftp + NMSSH with a modern RN Native Module that links libssh2 directly and ships correct device/simulator binaries out of the box.

Why consider this

  • Full control over the native surface (events, error handling, cancellation, reconnects).
  • Fix Android reliability issues we’ve observed (e.g., “disconnect” callback firing multiple times, causing dev crashes).
  • Avoid legacy Objective-C wrapper constraints; use TurboModules/JSI.

High-level plan

iOS

  • Use (or adapt) Libssh2Prebuild build scripts to produce our own libssh2.xcframework and OpenSSL.xcframework (or one combined framework).
  • Publish them as a binary CocoaPod (like CSSH-Binary) or vendor the XCFrameworks in our module pod.
  • Write a thin Obj-C++/C++ wrapper exposing the SSH API needed by JS (connect/auth/exec/sftp/streaming).
  • Export via TurboModule (codegen) or JSI (C++), and provide a typed TS API in the RN package.

Android

  • Build libssh2 against OpenSSL (or MbedTLS) with the NDK.
  • Package .so libs per ABI (arm64-v8a, x86_64, etc.).
  • Implement JNI layer with strict once-only callbacks (disconnect must be idempotent), and make the public JS API promise/observable based.
  • Add tests that assert no multiple “disconnect” emissions.

Common

  • Define a stable JS API:

    • Connection lifecycle (connect, ready, disconnect),
    • Auth variants (password, key, agent if supported),
    • Exec/PTY streaming,
    • SFTP (get/put/mkdir/list/stat),
    • Structured errors (error codes, host key checks, timeouts),
    • Events (onData, onExit, onError, onDisconnect).

  • Testing/CI: Simulated hosts (Docker) for integration tests; E2E with detox where feasible; CI matrix includes iOS Simulator and Android emulators.

Pros

  • Clean slate, better DX, fewer legacy constraints.
  • We can ensure simulator support is first-class.
  • Fixes Android issues definitively.

Cons

  • More initial engineering time.
  • We own native maintenance across platforms.

Known Android issue in current package

We’ve observed that disconnect can trigger the callback more than once, which crashes the app in dev (double resolve/reject or repeated event emission). If we keep the current package for now:

  • Add a JS-side guard to make disconnect() idempotent (ignore subsequent calls/notifications after the first).
  • If we fork, fix the native layer so it emits exactly once and cleans up listeners predictably.

(If we pursue Path 2, we’ll design the native layer so all callbacks/promises are strictly single-shot and lifecycle-scoped.)

Recommendation

  • Short term: Implement Path 1. It’s the smallest change to unblock Simulator builds:

    • Add a CSSH-Binary binary pod (vend CSSH.xcframework).
    • Fork NMSSH podspec to depend on it.
    • Wire both via expo-build-properties ios.extraPods.

  • Medium term: Plan Path 2 to remove legacy NMSSH constraints and resolve Android issues thoroughly using a Nitro/TurboModule with direct libssh2 bindings.

Appendix

A. CSSH-Binary.podspec (example)

Pod::Spec.new do |s|
  s.name     = 'CSSH-Binary'
  s.version  = '1.11.0'
  s.summary  = 'libssh2 + OpenSSL as XCFramework (from DimaRU/Libssh2Prebuild)'
  s.license  = { :type => 'MIT' }
  s.homepage = 'https://github.com/your-org/cssh-binary'
  s.authors  = { 'Your Name' => 'you@example.com' }
  s.platform = :ios, '12.0'
  s.source   = {
    :http => 'https://github.com/DimaRU/Libssh2Prebuild/releases/download/1.11.0-OpenSSL-1-1-1w/CSSH.xcframework.zip'
  }
  s.vendored_frameworks = 'CSSH.xcframework'
end

B. NMSSH.podspec diff (replace vendored .a libs)

-  s.vendored_libraries = [
-    'NMSSH-iOS/Libraries/lib/libssh2.a',
-    'NMSSH-iOS/Libraries/lib/libssl.a',
-    'NMSSH-iOS/Libraries/lib/libcrypto.a'
-  ]
+  s.dependency 'CSSH-Binary', '~> 1.11'

C. app.config.ts (Expo)

plugins: [
  // ...
  [
    'expo-build-properties',
    {
      ios: {
        extraPods: [
          { name: 'CSSH-Binary', podspec: 'https://raw.githubusercontent.com/your-org/cssh-binary/main/CSSH-Binary.podspec' },
          { name: 'NMSSH', git: 'https://github.com/your-org/NMSSH.git', branch: 'cssh-xcframework' },
        ],
      },
      android: {
        packagingOptions: {
          pickFirst: ['META-INF/versions/9/OSGI-INF/MANIFEST.MF'],
        },
      },
    },
  ],
]

D. Build/Release notes

  • Rebuild steps

    npx expo prebuild --platform ios --clean
npx pod-install
npx expo run:ios

  • Licenses: Include OpenSSL and libssh2 license texts in our OSS notice.

  • Flipper: If linking conflicts appear, disable Flipper in iOS debug builds.

E. Temporary simulator workaround (not recommended long-term)

In both the app target and Pods project:

EXCLUDED_ARCHS[sdk=iphonesimulator*] = arm64

This forces an x86_64 Simulator under Rosetta, avoiding the immediate link error but losing native-sim performance.

Decision log

  • ✅ Adopt Path 1 now (XCFramework via CSSH + NMSSH podspec tweak).
  • 🕒 Plan Path 2 (custom RN module) to address Android bugs and own the full stack.

Updates

    1. Implemented option 1. Seems to be working in simulator
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