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.Api/Setup/Configuration.cs

@@ -54,6 +54,13 @@ internal static class Configuration
                 + "setting exists to provide.")
             .ValidateOnStart();
 
+        services.AddRelayOptions();
+
+        return services;
+    }
+
+    /// <summary>The relay's options, which carry more rules than any other section. See ADR 0004.</summary>
+    private static void AddRelayOptions(this IServiceCollection services) =>
         services.AddOptions<RelayOptions>()
             .BindConfiguration(RelayOptions.SectionName)
             .ValidateDataAnnotations()
@@ -63,11 +70,28 @@ internal static class Configuration
             .Validate(
                 options => options.IdleTimeout < options.MaxSessionDuration,
                 "Relay:IdleTimeout must be shorter than Relay:MaxSessionDuration.")
+            .Validate(
+                options => options.TicketLifetime > TimeSpan.Zero
+                    && options.TicketLifetime <= TimeSpan.FromSeconds(30),
+                "Relay:TicketLifetime must be positive and at most 30 seconds. A ticket is a bearer "
+                + "credential in a header; ADR 0004 bounds how long one may be worth stealing.")
+            .Validate(
+                options => options.ConnectTimeout > TimeSpan.Zero && options.DrainTimeout >= TimeSpan.Zero,
+                "Relay:ConnectTimeout must be positive and Relay:DrainTimeout must not be negative.")
+            .Validate(
+                options => Features.Relay.RelayAddressPolicy.AreNetworks(options.AllowedNetworks)
+                    && Features.Relay.RelayAddressPolicy.AreNetworks(options.DeniedNetworks),
+                "Relay:AllowedNetworks and Relay:DeniedNetworks must hold CIDR networks such as "
+                + "10.20.0.0/16, with no host bits set. An entry this server cannot read would be a gap "
+                + "the operator believes is closed.")
+            .Validate(
+                options => !options.Enabled
+                    || (Uri.TryCreate(options.WebSocketUrl, UriKind.Absolute, out var url)
+                        && (string.Equals(url.Scheme, Uri.UriSchemeWs, StringComparison.Ordinal)
+                            || string.Equals(url.Scheme, Uri.UriSchemeWss, StringComparison.Ordinal))),
+                "Relay:WebSocketUrl must be an absolute ws:// or wss:// URL.")
             .ValidateOnStart();
 
-        return services;
-    }
-
     /// <summary>
     /// Warns about configuration that is individually valid but dangerous in combination.
     /// </summary>

src/DodoSSH.Api/Setup/DodoOptions.cs

@@ -121,17 +121,44 @@ public sealed class RelayOptions
     /// the ACL: only hosts in a vault the caller holds Connect on can be dialled at all.
     /// </para>
     /// <para>
-    /// <b>Nothing reads this setting yet, and that is the point of saying so here.</b> The relay is
-    /// an ADR and a contract; there is no endpoint and no dialler. This option exists so the shape
-    /// is settled before the code is, which makes it exactly the kind of comment that gets read as a
-    /// description of an implemented control. It is not one. Whoever builds the dialler owes ADR 0004
-    /// §3 in full — loopback, link-local, <c>0.0.0.0/8</c>, multicast and the cloud metadata addresses
-    /// denied with no configuration escape, and IPv4-mapped IPv6 normalised and re-checked — and this
-    /// flag governs RFC1918 only, never that list.
+    /// Governs the private ranges only — RFC1918, carrier-grade NAT (which is where a Tailnet lives) and
+    /// IPv6 unique-local — and never the list <c>RelayAddressPolicy</c> refuses with no configuration
+    /// escape: loopback, link-local, <c>0.0.0.0/8</c>, multicast and the cloud metadata addresses.
+    /// Nothing here can re-open those, which is what ADR 0004 §3 means by non-overridable.
     /// </para>
     /// </remarks>
     public bool AllowPrivateNetworks { get; set; } = true;
 
+    /// <summary>
+    /// Networks, in CIDR form, that a relay target must fall inside. Empty means any network the other
+    /// rules permit.
+    /// </summary>
+    /// <remarks>
+    /// When set, this is the whole answer for the ranges it names: a target inside one is permitted even
+    /// with <see cref="AllowPrivateNetworks"/> off, because "only 10.20.0.0/16" is the precise thing an
+    /// operator means by setting both. It still cannot reach the non-overridable list, and
+    /// <see cref="DeniedNetworks"/> still wins over it.
+    /// </remarks>
+    public IList<string> AllowedNetworks { get; } = [];
+
+    /// <summary>Networks, in CIDR form, that a relay target may never fall inside.</summary>
+    /// <remarks>
+    /// Checked before <see cref="AllowedNetworks"/>, so a deny inside an allow is a hole in it — the way
+    /// round that lets an operator carve the database subnet out of an otherwise permitted range.
+    /// </remarks>
+    public IList<string> DeniedNetworks { get; } = [];
+
+    /// <summary>
+    /// A directory to keep the ticket key ring in. Unset means the platform default.
+    /// </summary>
+    /// <remarks>
+    /// Losing the key ring is harmless in the way losing <c>Sync:CursorSigningKey</c> is: a ticket lives
+    /// thirty seconds, so a restart invalidates at most the connects in flight and the client asks
+    /// again. What does need it is more than one node — every node must be able to read every other's
+    /// tickets for round-robin to work, so a multi-node deployment points this at shared storage.
+    /// </remarks>
+    public string? KeyRingDirectory { get; set; }
+
     /// <summary>Maximum concurrent sessions per user.</summary>
     [Range(1, 1000)]
     public int MaxConcurrentSessionsPerUser { get; set; } = 10;

src/DodoSSH.Api/Setup/EndpointRegistration.cs

@@ -1,6 +1,7 @@
 using DodoSSH.Api.Features.Events;
 using DodoSSH.Api.Features.Identity;
 using DodoSSH.Api.Features.Meta;
+using DodoSSH.Api.Features.Relay;
 using DodoSSH.Api.Features.Sync;
 using DodoSSH.Api.Features.Teams;
 using DodoSSH.Contracts;
@@ -45,6 +46,8 @@ internal static class EndpointRegistration
             typeof(SyncPullEndpoint),
             typeof(SyncPushEndpoint),
             typeof(VaultEventsEndpoint),
+            typeof(RelayTicketEndpoint),
+            typeof(RelayConnectEndpoint),
             typeof(CreateTeamEndpoint),
             typeof(ListTeamsEndpoint),
             typeof(UpdateTeamEndpoint),
@@ -65,7 +68,6 @@ internal static class EndpointRegistration
             // Registered as each feature lands:
             //   Identity   — key rotation, passphrase change
             //   Vaults     — per-item ACLs
-            //   Relay      — tickets and the WebSocket
             //   Audit, Admin
         });
 

src/DodoSSH.Api/Setup/RelaySetup.cs

@@ -0,0 +1,59 @@
+using DodoSSH.Api.Features.Relay;
+using Microsoft.AspNetCore.DataProtection;
+using Microsoft.AspNetCore.DataProtection.KeyManagement;
+using Microsoft.AspNetCore.DataProtection.Repositories;
+using Microsoft.Extensions.Options;
+
+namespace DodoSSH.Api.Setup;
+
+/// <summary>The relay's services. See ADR 0004.</summary>
+internal static class RelaySetup
+{
+    /// <summary>
+    /// How much longer than the drain the host waits before it stops regardless.
+    /// </summary>
+    /// <remarks>
+    /// Room for the close handshakes and the audit rows' final writes once the drain has run out, which
+    /// is the part of a shutdown an operator would otherwise lose without noticing.
+    /// </remarks>
+    private static readonly TimeSpan ShutdownMargin = TimeSpan.FromSeconds(10);
+
+    internal static IServiceCollection AddDodoRelay(this IServiceCollection services)
+    {
+        // Singletons: a session outlives the request that opened it, a protector is safe to share, and
+        // the network seam holds no state.
+        services.AddSingleton<RelayTicketCodec>();
+        services.AddSingleton<RelaySessionRegistry>();
+        services.AddSingleton<IRelayNetwork, SystemRelayNetwork>();
+
+        // The application name isolates this deployment's payloads from any other ASP.NET application
+        // sharing a key directory, and — unlike the default, which is derived from the content root —
+        // is the same on every node, which is what lets any node verify any node's ticket.
+        services.AddDataProtection().SetApplicationName("DodoSSH");
+
+        services.AddOptions<KeyManagementOptions>()
+            .Configure<IOptions<RelayOptions>, ILoggerFactory>((keys, relay, loggers) =>
+            {
+                if (relay.Value.KeyRingDirectory is { Length: > 0 } directory)
+                {
+                    keys.XmlRepository = new FileSystemXmlRepository(new DirectoryInfo(directory), loggers);
+                }
+            });
+
+        // Longer than the drain, or the host would stop waiting for live sessions before the drain ADR 0004
+        // promises them had run. Harmless with the relay off: nothing else holds a request open that long,
+        // and the host stops as soon as the last request has.
+        services.AddOptions<HostOptions>()
+            .Configure<IOptions<RelayOptions>>((host, relay) =>
+            {
+                var wanted = relay.Value.DrainTimeout + ShutdownMargin;
+
+                if (wanted > host.ShutdownTimeout)
+                {
+                    host.ShutdownTimeout = wanted;
+                }
+            });
+
+        return services;
+    }
+}