Let a host marked for the relay actually be reached through it

The relay checkbox said it was not wired up, and the plan said the server
half was built and shipped. It was not: there was ADR 0004, the contract
records, RelayOptions and a CHECK constraint, and RelayOptions said in its
own remarks that there was no endpoint and no dialler. So both halves are
built here, as step 3 of docs/reaching-a-host-you-cannot-dial.md.

Server, in Features/Relay:

- POST /api/v1/relay/tickets makes every decision: the host, Connect on
  its vault, the relay switched on, and the stored name resolved once and
  judged by RelayAddressPolicy. Loopback, link-local, 0.0.0.0/8,
  multicast and the metadata endpoints are refused with no configuration
  escape, with IPv4-mapped and NAT64 spellings unwrapped first; then the
  operator's denials, allow-list and private-network switch. What passes
  goes into a Data Protection ticket, and nothing resolves the name again.
  Every target refusal is the same 403, so the endpoint cannot be used to
  map the operator's DNS; the real reason is logged.
- GET /api/v1/relay/connect takes the ticket as a second subprotocol,
  checks its expiry against TimeProvider and spends it with a unique
  insert into relay_ticket_use before accepting the upgrade. After the
  upgrade a refusal is a close code: 4502 for a host the server could not
  reach, 4429 for a limit, 4408 idle, 4409 lifetime, 1001 after a drain.
- relay_session is the audit row, written from short-lived scopes.

Client:

- RelayBridge is a loopback SOCKS5 listener that accepts once and pipes
  to the relay socket. It rides on the new SshLoopbackProxy.Lifetime, so
  the connection disposes it on every way a session ends and a failed
  connect disposes it before throwing.
- ServerRelay, exposed as IVaultServer.Relay from /api/v1/meta, asks for
  the ticket, opens the socket, starts the bridge and turns every refusal
  into a sentence.
- VaultViewModel and TransfersViewModel reach a relayed host through it,
  never fall back to dialling it directly, and replace SSH.NET's transport
  error with the relay's own reason when it gave one.

Neither side ever cancels a WebSocket operation. Cancelling one aborts
the socket, and an aborted socket sends no close frame, which is the only
way the client learns the server could not reach the host.
AnIdleSession_IsClosedWithTheIdleCode asserts that the code arrives.

Three departures from ADR 0004, recorded in it: awaited copy loops rather
than Pipelines (same backpressure, two 32 KiB buffers), no ?ticket=
fallback, and no device id in the ticket.

Tests: API 245, shell 582 (5 skipped), layout 170, SSH 110 including a
real login through the bridge, API client 30, contracts 25. Not run: the
Android build (its change is copy only) and the Alpine layout oracle.

src/DodoSSH.Client.Ssh/RelayBridge.cs

@@ -0,0 +1,369 @@
+using System.Buffers;
+using System.Net;
+using System.Net.Sockets;
+using System.Net.WebSockets;
+
+namespace DodoSSH.Client.Ssh;
+
+/// <summary>
+/// A loopback SOCKS5 proxy that carries exactly one connection over one relay WebSocket.
+/// </summary>
+/// <remarks>
+/// <para>
+/// SSH.NET cannot be handed a stream, only an address — so the relay, which is a WebSocket, is reached by
+/// giving SSH.NET an address that leads into one. This listens on <c>127.0.0.1</c> at a port the system
+/// chooses, answers one SOCKS5 CONNECT, and from then on moves bytes: what SSH.NET writes goes out as binary
+/// messages, and what the relay sends comes back as the stream SSH.NET reads. That is ADR 0004's loopback
+/// bridge, and step 3 of <c>docs/reaching-a-host-you-cannot-dial.md</c>.
+/// </para>
+/// <para>
+/// <b>SOCKS5 rather than a plain pipe, so the host stays the host.</b> The connection made through this is
+/// told the target's real name and port, and pins its host key under them; the CONNECT carrying that name
+/// arrives here and is answered without being acted on, because where the bytes go was decided by the
+/// server when it issued the ticket, and nothing this side says can change it.
+/// </para>
+/// <para>
+/// Three properties are the security of this, and each is held by a test. It binds loopback and nothing
+/// else — asserted on the bound address, because a bridge on every interface would be a proxy into the
+/// relay for anyone on the network and nothing would report it. It accepts once and stops listening, so a
+/// second process on this machine cannot take a turn. And it gives up if nobody arrives within
+/// <see cref="AcceptTimeout"/>, so a connect that failed before dialling cannot leave it waiting.
+/// </para>
+/// <para>
+/// <b>No operation on the WebSocket is cancelled.</b> Cancelling one aborts a .NET WebSocket, and an aborted
+/// one never reads the close frame that says why the relay ended — which is the one thing the user needs to
+/// be told when the server could not reach the host. The bridge stops a direction by closing, and waits a
+/// bounded time for the other side to agree, as the relay's own endpoint does.
+/// </para>
+/// </remarks>
+public sealed class RelayBridge : IAsyncDisposable
+{
+    /// <summary>How long the bridge waits for its one connection before closing the relay unused.</summary>
+    /// <remarks>
+    /// The connection is made the moment the bridge exists, so this bounds a caller that failed between the
+    /// two. Longer than any SSH connect timeout, so it never ends a connect that is merely slow.
+    /// </remarks>
+    public static readonly TimeSpan AcceptTimeout = TimeSpan.FromSeconds(60);
+
+    private const int BufferBytes = 32 * 1024;
+
+    private static readonly TimeSpan CloseTimeout = TimeSpan.FromSeconds(5);
+
+    private readonly TcpListener listener;
+    private readonly WebSocket socket;
+    private readonly CancellationTokenSource stopping = new();
+    private readonly Task running;
+    private int disposed;
+
+    private RelayBridge(WebSocket socket, TimeSpan acceptTimeout)
+    {
+        this.socket = socket;
+
+        // The address rather than "localhost", so there is no resolver and no hosts file between this and
+        // the interface it means. See the remarks on SshLoopbackProxy.
+        listener = new TcpListener(IPAddress.Loopback, 0);
+        listener.Start(backlog: 1);
+
+        Endpoint = (IPEndPoint)listener.LocalEndpoint;
+        running = RunAsync(acceptTimeout);
+    }
+
+    /// <summary>Where the bridge is listening. Loopback, always.</summary>
+    public IPEndPoint Endpoint { get; }
+
+    /// <summary>The proxy to hand the connection, which then owns this bridge.</summary>
+    public SshLoopbackProxy Proxy => new(Endpoint.Port, this);
+
+    /// <summary>
+    /// How the relay closed the socket, once it has; null while it is open or when it vanished without
+    /// saying.
+    /// </summary>
+    /// <remarks>
+    /// Read after a connect through the bridge fails. The relay says in a close code what went wrong on its
+    /// side — the server could not reach the host, or has too many sessions open — and that is the sentence
+    /// the user needs in place of the transport error SSH.NET reports for a stream that ended.
+    /// </remarks>
+    public WebSocketCloseStatus? CloseStatus { get; private set; }
+
+    /// <summary>The relay's own words about why it closed, alongside <see cref="CloseStatus"/>.</summary>
+    public string? CloseDescription { get; private set; }
+
+    /// <summary>Completes when the bridge has finished: its one connection is over, or nobody came.</summary>
+    public Task Completion => running;
+
+    /// <summary>Starts a bridge over an open relay socket, which it then owns.</summary>
+    public static RelayBridge Start(WebSocket socket) => Start(socket, AcceptTimeout);
+
+    /// <summary>Starts a bridge with a shorter wait for its connection, for tests.</summary>
+    internal static RelayBridge Start(WebSocket socket, TimeSpan acceptTimeout)
+    {
+        ArgumentNullException.ThrowIfNull(socket);
+
+        return new RelayBridge(socket, acceptTimeout);
+    }
+
+    /// <inheritdoc />
+    /// <remarks>
+    /// Idempotent, because two owners may reach it: the connection that was handed the proxy, and a caller
+    /// tidying up after a connect that never got that far.
+    /// </remarks>
+    public async ValueTask DisposeAsync()
+    {
+        if (Interlocked.Exchange(ref disposed, 1) != 0)
+        {
+            return;
+        }
+
+        await stopping.CancelAsync().ConfigureAwait(false);
+        listener.Stop();
+
+        if (!await SettlesAsync(running, CloseTimeout * 2).ConfigureAwait(false))
+        {
+            socket.Abort();
+            await SettlesAsync(running, CloseTimeout).ConfigureAwait(false);
+        }
+
+        socket.Dispose();
+        stopping.Dispose();
+    }
+
+    private async Task RunAsync(TimeSpan acceptTimeout)
+    {
+        TcpClient? client = null;
+
+        try
+        {
+            using (var accepting = CancellationTokenSource.CreateLinkedTokenSource(stopping.Token))
+            {
+                accepting.CancelAfter(acceptTimeout);
+                client = await listener.AcceptTcpClientAsync(accepting.Token).ConfigureAwait(false);
+            }
+
+            // Once. A second connection to this port is somebody else's, and there is nothing for it here.
+            listener.Stop();
+            client.NoDelay = true;
+
+            var stream = client.GetStream();
+
+            if (await Socks5.AcceptConnectAsync(stream, stopping.Token).ConfigureAwait(false))
+            {
+                await PipeAsync(stream).ConfigureAwait(false);
+            }
+        }
+        catch (Exception exception)
+            when (exception is OperationCanceledException or SocketException or IOException or ObjectDisposedException)
+        {
+            // Nobody came, the bridge was disposed, or the connection went away mid-handshake. The relay is
+            // closed below either way.
+        }
+        finally
+        {
+            listener.Stop();
+            client?.Dispose();
+            await CloseSocketAsync(WebSocketCloseStatus.NormalClosure).ConfigureAwait(false);
+        }
+    }
+
+    /// <summary>Moves bytes both ways until either side ends, then lets the other finish.</summary>
+    private async Task PipeAsync(NetworkStream stream)
+    {
+        var upstream = TcpToRelayAsync(stream);
+        var downstream = RelayToTcpAsync(stream);
+
+        var first = await Task.WhenAny(upstream, downstream).ConfigureAwait(false);
+
+        if (first == upstream)
+        {
+            // SSH.NET closed its end. Say so to the relay, then give it a moment to answer, which is what
+            // ends the receive in progress without aborting it.
+            await CloseSocketAsync(WebSocketCloseStatus.NormalClosure).ConfigureAwait(false);
+
+            if (!await SettlesAsync(downstream, CloseTimeout).ConfigureAwait(false))
+            {
+                socket.Abort();
+            }
+        }
+        else
+        {
+            // The relay ended. Closing the TCP side is how SSH.NET hears it, as the end of its stream.
+            stream.Socket.Shutdown(SocketShutdown.Both);
+            await SettlesAsync(upstream, CloseTimeout).ConfigureAwait(false);
+        }
+    }
+
+    private async Task TcpToRelayAsync(NetworkStream stream)
+    {
+        var buffer = ArrayPool<byte>.Shared.Rent(BufferBytes);
+
+        try
+        {
+            int read;
+
+            while ((read = await stream.ReadAsync(buffer.AsMemory(0, BufferBytes), stopping.Token).ConfigureAwait(false)) > 0)
+            {
+                // Not cancellable, deliberately. See the remarks on this class.
+                await socket
+                    .SendAsync(buffer.AsMemory(0, read), WebSocketMessageType.Binary, endOfMessage: true, CancellationToken.None)
+                    .ConfigureAwait(false);
+            }
+        }
+        catch (Exception exception)
+            when (exception is OperationCanceledException or IOException or WebSocketException or ObjectDisposedException)
+        {
+            // Either end went away. PipeAsync decides what that means.
+        }
+        finally
+        {
+            ArrayPool<byte>.Shared.Return(buffer);
+        }
+    }
+
+    private async Task RelayToTcpAsync(NetworkStream stream)
+    {
+        var buffer = ArrayPool<byte>.Shared.Rent(BufferBytes);
+
+        try
+        {
+            while (true)
+            {
+                // Not cancellable, deliberately. See the remarks on this class.
+                var received = await socket.ReceiveAsync(buffer.AsMemory(), CancellationToken.None)
+                    .ConfigureAwait(false);
+
+                if (received.MessageType == WebSocketMessageType.Close)
+                {
+                    CloseStatus = socket.CloseStatus;
+                    CloseDescription = socket.CloseStatusDescription;
+                    return;
+                }
+
+                await stream.WriteAsync(buffer.AsMemory(0, received.Count), stopping.Token).ConfigureAwait(false);
+            }
+        }
+        catch (Exception exception)
+            when (exception is OperationCanceledException or IOException or WebSocketException or ObjectDisposedException)
+        {
+            // Either end went away. PipeAsync decides what that means.
+        }
+        finally
+        {
+            ArrayPool<byte>.Shared.Return(buffer);
+        }
+    }
+
+    /// <summary>Sends a close, if one can still be sent, bounded so a vanished peer cannot hold it.</summary>
+    private async Task CloseSocketAsync(WebSocketCloseStatus status)
+    {
+        if (socket.State is not (WebSocketState.Open or WebSocketState.CloseReceived))
+        {
+            return;
+        }
+
+        using var timeout = new CancellationTokenSource(CloseTimeout);
+
+        try
+        {
+            await socket.CloseOutputAsync(status, string.Empty, timeout.Token).ConfigureAwait(false);
+        }
+        catch (Exception exception)
+            when (exception is OperationCanceledException or WebSocketException or ObjectDisposedException)
+        {
+            // Gone already.
+        }
+    }
+
+    private static async Task<bool> SettlesAsync(Task task, TimeSpan within)
+    {
+        try
+        {
+            await task.WaitAsync(within).ConfigureAwait(false);
+            return true;
+        }
+        catch (TimeoutException)
+        {
+            return false;
+        }
+    }
+
+    /// <summary>
+    /// The server half of SOCKS5 (RFC 1928), as much of it as SSH.NET speaks: no authentication, CONNECT.
+    /// </summary>
+    private static class Socks5
+    {
+        private const byte Version = 5;
+        private const byte NoAuthentication = 0;
+        private const byte NoAcceptableMethod = 0xFF;
+        private const byte Connect = 1;
+        private const byte Succeeded = 0;
+        private const byte CommandNotSupported = 7;
+        private const byte AddressTypeNotSupported = 8;
+
+        /// <summary>
+        /// Answers the greeting and one CONNECT, whatever it names.
+        /// </summary>
+        /// <returns>Whether the stream is now a tunnel; false when the client asked for something else.</returns>
+        /// <remarks>
+        /// The target in the request is read and discarded. It is the host the connection is <em>about</em>,
+        /// which is why SSH.NET sends it, and it is not where the bytes go, which the relay decided. The bound
+        /// address in the reply is all zeroes: nothing on the far side of a relay has an address the client
+        /// could use, and SSH.NET does not read it.
+        /// </remarks>
+        internal static async Task<bool> AcceptConnectAsync(Stream stream, CancellationToken cancellationToken)
+        {
+            var header = await ReadAsync(stream, 2, cancellationToken).ConfigureAwait(false);
+
+            if (header[0] != Version)
+            {
+                return false;
+            }
+
+            var methods = await ReadAsync(stream, header[1], cancellationToken).ConfigureAwait(false);
+
+            if (!methods.Contains(NoAuthentication))
+            {
+                await stream.WriteAsync(new byte[] { Version, NoAcceptableMethod }, cancellationToken).ConfigureAwait(false);
+                return false;
+            }
+
+            await stream.WriteAsync(new byte[] { Version, NoAuthentication }, cancellationToken).ConfigureAwait(false);
+
+            var request = await ReadAsync(stream, 4, cancellationToken).ConfigureAwait(false);
+
+            if (request[0] != Version || request[1] != Connect)
+            {
+                await ReplyAsync(stream, CommandNotSupported, cancellationToken).ConfigureAwait(false);
+                return false;
+            }
+
+            var addressLength = request[3] switch
+            {
+                1 => 4,
+                4 => 16,
+                3 => (await ReadAsync(stream, 1, cancellationToken).ConfigureAwait(false))[0],
+                _ => -1,
+            };
+
+            if (addressLength < 0)
+            {
+                await ReplyAsync(stream, AddressTypeNotSupported, cancellationToken).ConfigureAwait(false);
+                return false;
+            }
+
+            // The address, then the port. Neither is used; see the remarks above.
+            await ReadAsync(stream, addressLength + 2, cancellationToken).ConfigureAwait(false);
+
+            await ReplyAsync(stream, Succeeded, cancellationToken).ConfigureAwait(false);
+            return true;
+        }
+
+        private static Task ReplyAsync(Stream stream, byte status, CancellationToken cancellationToken) =>
+            stream.WriteAsync(new byte[] { Version, status, 0, 1, 0, 0, 0, 0, 0, 0 }, cancellationToken).AsTask();
+
+        private static async Task<byte[]> ReadAsync(Stream stream, int count, CancellationToken cancellationToken)
+        {
+            var buffer = new byte[count];
+            await stream.ReadExactlyAsync(buffer, cancellationToken).ConfigureAwait(false);
+            return buffer;
+        }
+    }
+}

src/DodoSSH.Client.Ssh/SshConnection.cs

@@ -38,12 +38,19 @@ public sealed record SshPrivateKeyCredential(byte[] PrivateKeyPem, string? Passp
 /// stand-in. See the gate in <c>SshNetConnectionFactory</c>, which pins what this request names.
 /// </para>
 /// <para>
-/// Nothing in this assembly opens one. The proxy is somebody else's — a forward on a bastion connection,
-/// or the relay bridge — and its lifetime belongs to whoever opened it, which must outlast the connection
-/// made through it. See <c>docs/reaching-a-host-you-cannot-dial.md</c>.
+/// <b>Whoever opens a proxy may hand it to the connection made through it.</b> A proxy with no
+/// <see cref="Lifetime"/> belongs to its opener, which must keep it listening for as long as the connection
+/// lives. One with a lifetime is given away: the connection disposes it after itself, and a connect that
+/// fails disposes it before throwing — so a proxy opened for one connection, which is what the relay bridge
+/// is, cannot outlive it on any of the paths a session ends by. See
+/// <c>docs/reaching-a-host-you-cannot-dial.md</c>.
 /// </para>
 /// </remarks>
-public sealed record SshLoopbackProxy(int Port);
+/// <param name="Port">The loopback port the proxy listens on.</param>
+/// <param name="Lifetime">
+/// What keeps the proxy listening, when the connection is to own it; null when its opener does.
+/// </param>
+public sealed record SshLoopbackProxy(int Port, IAsyncDisposable? Lifetime = null);
 
 /// <summary>Everything needed to reach one host.</summary>
 /// <param name="Host">Hostname or address.</param>

src/DodoSSH.Client.Ssh/SshNetConnectionFactory.cs

@@ -75,12 +75,22 @@ public sealed class SshNetConnectionFactory(IKnownHostStore knownHosts)
     {
         ArgumentNullException.ThrowIfNull(request);
 
-        var client = new SshClient(BuildConnectionInfo(request)) { KeepAliveInterval = KeepAliveInterval };
+        var lifetime = request.Proxy?.Lifetime;
 
-        var gate = await ConnectThroughHostKeyGateAsync(client, request, progress, cancellationToken)
-            .ConfigureAwait(false);
+        try
+        {
+            var client = new SshClient(BuildConnectionInfo(request)) { KeepAliveInterval = KeepAliveInterval };
 
-        return new SshNetConnection(client, gate.Presented!);
+            var gate = await ConnectThroughHostKeyGateAsync(client, request, progress, cancellationToken)
+                .ConfigureAwait(false);
+
+            return new SshNetConnection(client, gate.Presented!, lifetime);
+        }
+        catch
+        {
+            await DisposeAsync(lifetime).ConfigureAwait(false);
+            throw;
+        }
     }
 
     /// <inheritdoc />
@@ -95,6 +105,40 @@ public sealed class SshNetConnectionFactory(IKnownHostStore knownHosts)
     {
         ArgumentNullException.ThrowIfNull(request);
 
+        var lifetime = request.Proxy?.Lifetime;
+
+        try
+        {
+            return await OpenSftpCoreAsync(request, lifetime, cancellationToken).ConfigureAwait(false);
+        }
+        catch
+        {
+            await DisposeAsync(lifetime).ConfigureAwait(false);
+            throw;
+        }
+    }
+
+    /// <summary>
+    /// Disposes a proxy the connection was given, when the connection it was given for did not happen.
+    /// </summary>
+    /// <remarks>
+    /// Every failure path, including the ones before a socket exists — an unreadable key throws from
+    /// <see cref="BuildConnectionInfo"/> — because a relay bridge left listening after a failed connect is a
+    /// loopback SOCKS proxy with an open relay session behind it, and nothing would ever close it.
+    /// </remarks>
+    private static async ValueTask DisposeAsync(IAsyncDisposable? lifetime)
+    {
+        if (lifetime is not null)
+        {
+            await lifetime.DisposeAsync().ConfigureAwait(false);
+        }
+    }
+
+    private async Task<ISftpSession> OpenSftpCoreAsync(
+        SshConnectionRequest request,
+        IAsyncDisposable? lifetime,
+        CancellationToken cancellationToken)
+    {
         // Keep-alive here for the reason it is on the shell client: a transfer connection left open while
         // somebody browses is idle in exactly the same way, and a file listing against a machine that has
         // since rebooted would otherwise wait on a socket nothing will ever answer.
@@ -125,7 +169,7 @@ public sealed class SshNetConnectionFactory(IKnownHostStore knownHosts)
             throw;
         }
 
-        return new SshNetSftpSession(client, gate.Presented!, home);
+        return new SshNetSftpSession(client, gate.Presented!, home, lifetime);
     }
 
     /// <summary>
@@ -326,7 +370,15 @@ public sealed class SshNetConnectionFactory(IKnownHostStore knownHosts)
 }
 
 /// <summary>An SSH.NET-backed connection.</summary>
-internal sealed class SshNetConnection(SshClient client, HostKeyPresentation hostKey) : ISshConnection
+/// <remarks>
+/// Owns the proxy it was reached through when it was given one, and disposes it after the client — the
+/// order that lets SSH.NET close its end of the loopback socket before the thing on the other end goes.
+/// </remarks>
+internal sealed class SshNetConnection(
+    SshClient client,
+    HostKeyPresentation hostKey,
+    IAsyncDisposable? proxyLifetime = null)
+    : ISshConnection
 {
     /// <inheritdoc />
     public bool IsConnected => client.IsConnected;
@@ -366,10 +418,14 @@ internal sealed class SshNetConnection(SshClient client, HostKeyPresentation hos
     }
 
     /// <inheritdoc />
-    public ValueTask DisposeAsync()
+    public async ValueTask DisposeAsync()
     {
         client.Dispose();
-        return ValueTask.CompletedTask;
+
+        if (proxyLifetime is not null)
+        {
+            await proxyLifetime.DisposeAsync().ConfigureAwait(false);
+        }
     }
 }
 

src/DodoSSH.Client.Ssh/SshNetSftpSession.cs

@@ -14,13 +14,23 @@ namespace DodoSSH.Client.Ssh;
 internal sealed class SshNetSftpSession : ISftpSession
 {
     private readonly SftpClient client;
+    private readonly IAsyncDisposable? proxyLifetime;
 
     private string? transportFailure;
     private int announced;
 
-    internal SshNetSftpSession(SftpClient client, HostKeyPresentation hostKey, string homeDirectory)
+    /// <param name="client">The connected client.</param>
+    /// <param name="hostKey">What the server presented.</param>
+    /// <param name="homeDirectory">Where the session starts.</param>
+    /// <param name="proxyLifetime">The proxy this session was reached through, when it owns it.</param>
+    internal SshNetSftpSession(
+        SftpClient client,
+        HostKeyPresentation hostKey,
+        string homeDirectory,
+        IAsyncDisposable? proxyLifetime = null)
     {
         this.client = client;
+        this.proxyLifetime = proxyLifetime;
 
         HostKey = hostKey;
         HomeDirectory = homeDirectory;
@@ -243,7 +253,7 @@ internal sealed class SshNetSftpSession : ISftpSession
 
         client.Dispose();
 
-        return ValueTask.CompletedTask;
+        return proxyLifetime?.DisposeAsync() ?? ValueTask.CompletedTask;
     }
 
     /// <remarks>