feat!: pre-handshake knock on the agent TCP ports; probes get the agent certificate public key [CI]

backend/fleet/src/Fleet.Link/Peer.cs

@@ -22,6 +22,9 @@ public sealed class PeerListenerOptions
     /// <summary>按对端 ID 查找允许连接的公钥（SPKI）。不在列表里的对端一律拒绝（期望状态里的 peers）。</summary>
     public required Func<string, byte[]?> LookupPeerKey { get; init; }
 
+    /// <summary>预握手敲门：每个连接先过这一关（名册里的 Probe 才算得出敲门包），不通过就直接断开，什么都不写。</summary>
+    public required KnockGate Knock { get; init; }
+
     public int MaxFrameBytes { get; init; } = 8 << 20;
 
     public int MaxSessions { get; init; } = 8;
@@ -98,7 +101,13 @@ public sealed class PeerListener
             PeerSockets.EnableKeepAlive(client.Client);
             using var timeout = new CancellationTokenSource(_options.HandshakeTimeout, _time);
             using CancellationTokenSource linked = CancellationTokenSource.CreateLinkedTokenSource(ct, timeout.Token);
-            var tls = new SslStream(client.GetStream(), leaveInnerStreamOpen: false);
+            Stream stream = client.GetStream();
+            if (!await _options.Knock.AdmitAsync(stream, (client.Client.RemoteEndPoint as IPEndPoint)?.Address, linked.Token))
+            {
+                return;
+            }
+
+            var tls = new SslStream(stream, leaveInnerStreamOpen: false);
             await tls.AuthenticateAsServerAsync(
                 new SslServerAuthenticationOptions { ServerCertificate = _options.Certificate, ClientCertificateRequired = false, EnabledSslProtocols = System.Security.Authentication.SslProtocols.Tls12 | System.Security.Authentication.SslProtocols.Tls13 },
                 linked.Token);
@@ -222,8 +231,10 @@ internal static class PeerSockets
 public static class PeerConnector
 {
     public static async Task<(PeerSessionInfo Info, TlsFrameTransport Transport)> ConnectAsync(
-        string host, int port, byte[] expectedSpkiSha256, NodeIdentity identity, string peerId, string? version, int maxFrameBytes, TimeProvider time, CancellationToken ct)
+        string host, int port, byte[] agentPublicKey, NodeIdentity identity, string peerId, string? version, int maxFrameBytes, TimeProvider time, CancellationToken ct)
     {
+        // 证书按公钥的指纹固定；敲门密钥由双方身份密钥派生
+        byte[] expectedSpkiSha256 = NodeIdentity.Fingerprint(agentPublicKey);
         TcpClient client = IPAddress.TryParse(host, out IPAddress? address) ? new TcpClient(address.AddressFamily) : new TcpClient();
         client.NoDelay = true;
         PeerSockets.EnableKeepAlive(client.Client);
@@ -232,8 +243,10 @@ public static class PeerConnector
             using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(10), time);
             using CancellationTokenSource linked = CancellationTokenSource.CreateLinkedTokenSource(ct, timeout.Token);
             await client.ConnectAsync(host, port, linked.Token);
+            Stream stream = client.GetStream();
+            await stream.WriteAsync(Knock.Build(Knock.DeriveKey(identity, agentPublicKey), time.GetUtcNow().ToUnixTimeMilliseconds()), linked.Token);
             var tls = new SslStream(
-                client.GetStream(), leaveInnerStreamOpen: false,
+                stream, leaveInnerStreamOpen: false,
                 (_, certificate, _, _) => certificate is not null && SpkiMatches(certificate, expectedSpkiSha256));
             await tls.AuthenticateAsClientAsync(new SslClientAuthenticationOptions { TargetHost = host }, linked.Token);
             var transport = new TlsFrameTransport(tls, maxFrameBytes);