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.