DodoSSH/docs/platform-flags.md

# Platform flags

Things known or suspected to behave differently outside Windows, plus deployment gotchas that
have already cost time once. **Development and testing are currently Windows-only**, so anything
here marked *unverified* has not run on the platform in question and must not be assumed to work.

Each entry says what the risk is, why it matters, and what to do about it. Delete an entry when it
has been verified or made moot — not when it merely stops being convenient.

## Cryptography

**`ChaCha20Poly1305.IsSupported` is false on macOS**, and on Windows builds before 10.0.20142.
This is why the client uses NSec (libsodium) rather than the BCL for content encryption; see
docs/crypto.md §1. *Already mitigated* — but if a BCL AEAD path is ever added as a fallback it
**must** gate on `IsSupported` rather than assuming availability, or the client will fail to open
any vault on macOS.

**Argon2id timings are measured on one Windows machine only.** 256 MiB with t=4 took 323 ms here.
The floor and ceiling in `EnrollmentLimits` were chosen against that number. *Unverified
elsewhere:* recalibrate on the slowest target platform before recommending a default profile,
because a cost that is comfortable on a desktop can make unlock unusable on a low-power laptop —
and the parameters are stored per user at enrollment, so a bad default is a per-user migration.

**libsodium ships native binaries per RID.** This complicates single-file and AOT publishing, and
on macOS every native library (`libsodium`, `libSkiaSharp`, `libHarfBuzzSharp`, `libe_sqlite3`)
must be signed **individually** with `--options runtime --timestamp` before the bundle is signed,
or notarization fails with an error that does not name the offending file.

## Desktop client

**The WebView runs on Windows.** `Avalonia.Controls.WebView` 12.0.1 (MIT, no licence key) hosts the
terminal page: WebView2 launches, navigates to the loopback page, runs its JavaScript and completes the
WebSocket handshake. Verified by observing an established TCP connection from `msedgewebview2` to the data
plane port.

Note precisely what that evidence covers, because it was once stretched to cover more: every clause above
is about the process and the socket. It says nothing about how the control **composites** with
Avalonia-drawn content, which is the axis on which it does not behave like an ordinary control — see the
next entry.

**A native child window cannot be covered by Avalonia content, on any platform that hosts it windowed.**
`NativeWebView` attaches a real Win32 child HWND through `NativeControlHost`, and a child window paints
above everything its parent draws, whatever the visual tree's z-order says. Layering a screen over the
terminal therefore does nothing: the WebView's rectangle stays on top. In this shell that sliced the setup
and unlock cards at the terminal column's left edge, put every one of their buttons inside the WebView's
rectangle at the window's default width — so the flow could only be completed by keyboard — and handed
Win32 focus to WebView2 on any click in that region, which makes a text box stop accepting keystrokes with
no visible cause.

The fix is to collapse the control, not to cover it: `IsVisible="{Binding IsUnlocked}"` on the
`NativeWebView`. That is safe, and this is the part worth recording, because the opposite was asserted here
for a while:

- `NativeControlHost` creates the native attachment from **attach to the visual tree**, not from layout and
  not from visibility. Its `UpdateHost` never reads `IsEffectivelyVisible`; only
  `TryUpdateNativeControlPosition` does, choosing `HideWithSize` over `ShowInBounds`.
- `NativeWebView` stashes a `Source` assigned before its adapter exists and replays it once created, so
  navigation is never lost to ordering. The shell already depends on that replay.
- So a collapsed WebView still starts WebView2, still loads the page and still lets the renderer attach its
  socket. Confirmed on Windows: 35 `msedgewebview2` processes with the control collapsed behind the setup
  screen.

The previous version of this entry claimed the reverse — that hiding it would mean never realising it — and
cited the `msedgewebview2` connection as verification. That observation was made while the overlay was
showing but, because of the airspace behaviour above, the WebView was in fact uncovered and in plain view.
It confirmed only that a *visible* WebView is realised, which nobody disputed, and could not discriminate
the case it was attached to. A process-level check cannot verify a rendering claim; that needs a
screenshot, and this defect shipped because one was never taken.

**What the first connection after unlocking actually depends on** is the `await
workspace.WaitForRendererAsync()` in `VaultViewModel.ConnectAsync`, because `TerminalDataPlane.SendAsync`
drops frames when no renderer is attached rather than queueing them. That await is the invariant; the
control's visibility is not. It currently has no timeout, so a WebView2 that fails to initialise hangs
Connect with the busy flag stuck — worth fixing on its own merits.

**The Windows app manifest must declare a `supportedOS` list.** Without it the process reports a
downlevel Windows version and Avalonia's native control host fails outright — *"Unable to create child
window for native control host"* — so the WebView, and therefore the terminal, does not start at all.
`[STAThread]` on `Main` is equally mandatory: WebView2 checks the apartment state and refuses to
initialise on an MTA thread.

**WebView2 spawns a process tree, not a process.** Around 35 processes were observed for one embedded
view. That is the concrete reason the design uses one WebView hosting N terminals rather than one per
tab: twenty tabs would mean twenty of those trees.

**The Avalonia WebView on Linux remains unproven, and is still the largest risk in the plan.** The
package's own release notes say `NativeWebView` gained Linux support via a **WPE** backend
(`libwpewebkit-2.0`), which is much less widely installed than WebKitGTK — and it ships a separate
`NativeWebDialog` described as *"particularly useful for platforms like Linux where embedded WebView
controls might not be available"*, which is the vendor confirming the concern. *Unverified:* a spike
must cover Ubuntu on both Wayland and X11, Fedora KDE, and macOS 15.

`ITerminalHost` was supposed to be the seam that keeps a backend swap cheap, and it is **declared but not
implemented** — nothing in the application uses it, and the view navigates `NativeWebView.Source` directly.
Swapping backends today means editing `MainWindow.axaml` and its code-behind. That is a small job, but do
not plan around a seam that is currently only a file.

One more reason the Linux picture may be better than this entry assumes: the package also ships
`NativeWebViewCompositorHost`, a non-windowed host drawn through Avalonia's compositor. A compositor host
would not have the airspace problem described below at all. Whether it can be selected deliberately is
unknown and worth establishing during the spike, because it would change how overlays can be built.

**`Avalonia.Diagnostics` has no 12.x release** (latest is 11.3.18), so the developer tools overlay is
unavailable on Avalonia 12. Development-only, so nothing ships differently — but debugging a layout
problem currently means reasoning rather than inspecting.

**The xterm bundles are vendored, not built.** `@xterm/xterm` 6.0.0 with the fit and webgl addons, all
MIT, committed as UMD bundles under `WebAssets/vendor` and embedded as Avalonia resources. No npm or
esbuild step, so a clean clone builds with the .NET SDK alone. The cost is that upgrades are a manual
re-download; the licence and versions are recorded here so that stays visible.

**SSH.NET's `window-change` is verified working** as of 2025.1.0 — resolved, not a flag.
`ShellStream.ChangeWindowSize(columns, rows, width, height)` exists and the remote genuinely
observes it: `PtyAndResizeSpikeTests` reads `stty size` back from a real sshd after resizing, and
repeated resizes each take effect. The `IChannelSession` fallback is not needed. That suite stays
in place as a regression guard, because an upgrade that silently stopped sending the request would
present as wrapped output only after a resize — easy to misattribute to the terminal emulator.

**`ShellStream.Write` buffers and requires an explicit `Flush`.** Without one a keystroke is accepted,
reported as written, and never reaches the remote — the terminal displays output perfectly and simply
stops responding to input. SSH.NET's own `WriteLine` flushes, which is why a spike that used it never
hit this. `SshNetShellSession.WriteAsync` now flushes per write; batching would be wrong anyway, since
a terminal has to put a keystroke on the wire immediately.

**`ShellStream` does not override `ReadAsync`.** The base `Stream` implementation therefore runs
the blocking `Read` on a thread-pool thread, so every open session parks one thread for as long as
it is idle. Fine for the handful of tabs M1 targets; revisit before advertising many concurrent
sessions, since the fix is either an upstream change or driving `IChannelSession` directly.

**SSH.NET cannot share one connection between `SshClient` and `SftpClient`.** A shell plus SFTP to
the same host means two TCP connections, two authentications and — later — two relay sockets.
Connect SFTP lazily and reuse the cached decrypted credential so the user is not prompted twice.

**Agent forwarding is de-scoped from v1.** It needs an upstream SSH.NET change. A vault-backed
agent of our own plus ProxyJump covers the real use cases.

**The SSH suite pulls `linuxserver/openssh-server` from Docker Hub**, which is rate-limited for
unauthenticated pulls. If CI starts failing on image pulls rather than on tests, that is why.

**MSIX packaging is ruled out, not merely deprioritised.** A packaged app runs WebView2 in an
AppContainer where loopback connections are blocked without a `CheckNetIsolation` exemption. The
terminal data plane *is* a loopback WebSocket, so MSIX would break the product outright. Velopack
for Windows/macOS/AppImage; Flatpak and deb/rpm defer updates to the package manager.

**Linux ships AppImage and Flatpak first**, specifically so the WebKit runtime is bundled rather
than assumed present on the user's machine.

**Opening the system browser depends on the platform handler.** `SystemBrowserLauncher` uses
`UseShellExecute`, which delegates to `ShellExecute` on Windows, `open` on macOS and `xdg-open` on
Linux. *Unverified off Windows:* `xdg-open` comes from `xdg-utils`, which is not guaranteed on a
minimal desktop or inside a Flatpak sandbox — where the portal is the correct route instead. If
sign-in silently does nothing on Linux, this is the first thing to check. `IBrowserLauncher` exists
so a platform-specific opener can be substituted without touching the flow.

## Identity provider

**A loopback redirect URI must be registered without a port, not with a wildcard port.** Keycloak — and
providers implementing RFC 8252 §7.3 generally — ignores the port when the registered redirect URI's host
is a loopback literal, which is what lets a native client bind an ephemeral port. Registering
`http://127.0.0.1:*/callback` looks more explicit and is *broken*: the `*` is parsed as a literal port and
every real authorization request comes back `400 Invalid parameter: redirect_uri`. Keycloak's wildcard
support is trailing-only, so a `*` in the middle of a URI never means what it looks like.

Register `http://127.0.0.1/callback`. Keep the path — it is the part that stops another process on the
machine having an authorization code delivered to a different endpoint. `Oidc:LoopbackRedirectPattern`,
which the server advertises through `/.well-known/dodossh-configuration`, says the same thing so an
operator configuring a different provider copies something that works.

Found by running the sign-in against a real Keycloak; every test until then used a stub that accepted
whatever it was given.

**Keycloak marks its session cookies `Secure` even over plain HTTP**, because `SameSite=None` is only
legal alongside `Secure`. A spec-conformant HTTP client therefore refuses to store them from an `http://`
origin — .NET's `CookieContainer` drops every one silently — and the login form POST then comes back
`400` with no explanation at all. Browsers complete the flow because they treat loopback as a trustworthy
origin and make the exception.

This does not affect the product: the client uses the system browser, which makes that exception. It does
affect any non-browser automation against a development Keycloak, which has to carry the cookies by hand
(see `ScriptedBrowser`) or be given HTTPS. Two hours of "the credentials must be wrong".

**`--import-realm` skips a realm that already exists.** Editing `deploy/keycloak/realm-dodossh.json` and
running `docker compose restart keycloak` therefore changes nothing, and the stale configuration keeps
being served — which reads exactly like the edit being wrong. `start-dev` keeps its state in an H2
database inside the container, so the realm has to be recreated along with it:
`docker compose rm -sf keycloak && docker compose up -d keycloak`. Cost an otherwise inexplicable
debugging detour.

`DodoSSH.SystemTests` is immune to this by construction — its Keycloak is created and destroyed per run —
which is a second reason the end-to-end suite starts its own containers rather than reusing the developer's
stack. Editing the realm file and rerunning the suite always tests the edit.

## Local cache

**The cache location is per-OS and must stay non-roaming.** `ClientPaths` chooses it:
`%LOCALAPPDATA%\DodoSSH` on Windows, `~/Library/Application Support/DodoSSH` on macOS,
`$XDG_DATA_HOME/dodossh` or `~/.local/share/dodossh` on Linux. It must **not** land anywhere that syncs
to a cloud drive or roams: two machines writing one SQLite file through a file-sync client corrupts it,
and the whole point of the outbox is that each machine has its own. That is also why Windows uses
`%LOCALAPPDATA%` and not `%APPDATA%`, which roams in a domain environment.

The platform branches are explicit rather than delegating to
`Environment.SpecialFolder.LocalApplicationData` everywhere, because on macOS the runtime maps that to
`~/.local/share` rather than to `~/Library/Application Support`. *Verified on Windows only* — the client
created `%LOCALAPPDATA%\DodoSSH\cache.db` and migrated it on first launch. The macOS and Linux branches
are reasoned, not run.

**SQLite timestamps are stored as integers, deliberately.** EF's default `DateTimeOffset` mapping for
SQLite is a text form it then refuses to order or compare, so any query that sorts or filters by time
throws at execution rather than at model build. `UnixMillisecondsConverter` is applied as a convention
so a timestamp added later cannot be the one left unconverted. This is provider behaviour, not
platform behaviour, but it cost a debugging session and will again if the converter is removed.

**The cache is three files, not one.** EF Core's SQLite provider puts the database in WAL mode, which is
the right mode here — a background sync pass writes while the interface reads, and under the default
rollback journal those reads would fail busy — but it means `cache.db` is accompanied by `cache.db-wal`
and `cache.db-shm`. Any backup, export or uninstall routine that touches only `cache.db` is wrong.
Verified by launching the client and reading `PRAGMA journal_mode`, after a comment in the code claimed
the opposite.

**Pooled SQLite connections keep the file open after the last context is disposed.** On Windows that
means locked, so the application cannot delete or replace its own cache and a test cannot clean up after
itself. `ClientCacheFactory.Dispose` clears the pool for exactly this reason; removing that line makes
the failure appear only on Windows.

**No SQLCipher, on any platform.** The rows are already ciphertext from the server, so an encrypted
database file would protect bytes that are protected already at the cost of a native dependency and a
licence obligation — and `bundle_e_sqlcipher` was deprecated in SQLitePCLRaw 3.0. The consequence to
be honest about: the cache offers no protection against another process running as the same user. See
`LocalCacheProtector` for what it does and does not defend against.

## Build and CI

**Integration tests need a Docker daemon** (Testcontainers). They run on `ubuntu-latest` in CI.
macOS runners have no Docker daemon, and the Windows CI job is deliberately build-only. So
anything proved by an integration test is proved on Linux only — which is the right place for
server code, and no coverage at all for client platform behaviour.

**The end-to-end suite launches the API's own launcher executable**, falling back to `dotnet exec` on the
assembly. The fallback exists for one reason: a checkout or artefact copy that lost the execute bit
produces a `Win32Exception` on Linux and nothing whatsoever on Windows. *Verified on Windows only* — the
launcher path is what runs here, so the fallback itself is reasoned rather than exercised. If the suite
fails in CI with a permission error before any container work, that is the path to look at.

**It also depends on `Server:PublicBaseUrl` being knowable before startup.** The port is chosen by binding
a loopback socket and releasing it, because the API reads that URL at startup and advertises it to clients,
so it cannot be discovered from Kestrel afterwards. The window for another process to take the port is a
few milliseconds; if the suite ever fails with an address-in-use, this is why, and a retry is the fix
rather than a redesign.

**`[CallerFilePath]` is rewritten to `/_/...` under `ContinuousIntegrationBuild`.** Any test that
locates a fixture by source path passes locally and fails in CI. Copy fixtures to the output
directory and read them via `AppContext.BaseDirectory` instead; `GoldenVectorTests` shows the
pattern.

**`dotnet format --verify-no-changes` is part of the CI gate** and exits non-zero on style
warnings, not just whitespace. Run it before pushing; a build with zero warnings can still fail
that step.

## Deployment

**PostgreSQL 18 moved its data directory** to `/var/lib/postgresql`, not `/var/lib/postgresql/data`
as in 17 and earlier. A compose file carried over from an older version silently gets an empty
volume — the database appears to work and loses everything on restart. Relevant to any compose
file other than `deploy/docker-compose.dev.yml`, which is already correct.

**Keycloak in the dev stack listens on host port 18080, not 8080.** On this machine an unrelated
Apache Tomcat holds `127.0.0.1:8080`, and a loopback-specific bind wins over Docker's `0.0.0.0`
publish when resolving `localhost` — so every realm request returned 404 while the container
looked healthy. If discovery fails against a locally-published container, check for another
process bound specifically to loopback before suspecting the container.

**`Sync:CursorSigningKey` generates an ephemeral per-process key when unset.** Fine for a single
node; on a multi-node deployment cursors issued by one node are rejected by another, so clients
resync from the beginning repeatedly. Must be configured explicitly before running more than one
instance. `WarnOnRiskyConfiguration` logs this at startup.

**Rate limiting is not implemented yet** (M2). `POST /api/v1/me/enrollment` and the sync endpoints
are reachable by any authenticated caller at any rate. Enrollment requires a valid access token
and is idempotent, so the exposure is resource consumption rather than a credential-guessing
surface — but it is still an unmetered write path.

**`/api/v1/me` does not update `last_seen_at_utc`.** Deliberate: a GET that writes on every call is
a smell, and nothing depends on the value yet. Revisit when device management lands, since that is
the first feature that needs it.