announce server hostname in MANIFEST, display in client

Server reads its hostname via gethostname(2) and includes it as a
length-prefixed UTF-8 string at the start of the MANIFEST payload.

Client displays 'Server: <hostname> — <MAC>' in a label above the
upstream list. Both sides updated for the new MANIFEST format:
  [hostname_len:1][hostname:N][entries...]

Protocol version unchanged (still 1); MANIFEST payload layout change
is backward-incompatible but both sides ship together.

win/gatuna-client/Frame.cs

@@ -40,7 +40,7 @@ readonly record struct UpstreamEntry(
 abstract record Frame
 {
     internal record Discover : Frame;
-    internal record Manifest(UpstreamEntry[] Entries) : Frame;
+    internal record Manifest(string Hostname, UpstreamEntry[] Entries) : Frame;
     internal record Open(byte UpstreamId) : Frame;
     internal record OpenAck(uint SessionId, byte UpstreamId) : Frame;
     internal record OpenNak(byte UpstreamId, byte Reason) : Frame;
@@ -74,7 +74,7 @@ static class FrameCodec
             Frame.Discover =>
                 Build(Proto.TypeDiscover, 0, []),
             Frame.Manifest manifest =>
-                Build(Proto.TypeManifest, 0, BuildManifestPayload(manifest.Entries)),
+                Build(Proto.TypeManifest, 0, BuildManifestPayload(manifest.Hostname, manifest.Entries)),
             Frame.Open open =>
                 Build(Proto.TypeOpen, 0, [open.UpstreamId]),
             Frame.OpenAck ack =>
@@ -90,9 +90,14 @@ static class FrameCodec
         };
     }
 
-    static byte[] BuildManifestPayload(UpstreamEntry[] entries)
+    static byte[] BuildManifestPayload(string hostname, UpstreamEntry[] entries)
     {
         using var ms = new MemoryStream();
+        var hnBytes = Encoding.UTF8.GetBytes(hostname);
+        var hnLen = (byte)Math.Min(hnBytes.Length, 255);
+        ms.WriteByte(hnLen);
+        if (hnLen > 0)
+            ms.Write(hnBytes, 0, hnLen);
         foreach (var e in entries)
         {
             var labelBytes = Encoding.UTF8.GetBytes(e.Label ?? "");
@@ -141,25 +146,35 @@ static class FrameCodec
 
     static Frame.Manifest? ParseManifest(ReadOnlySpan<byte> payload)
     {
+        if (payload.Length < 1)
+            return null;
+        var hnLen = payload[0];
+        if (1 + hnLen > payload.Length)
+            return null;
+        var hostname = hnLen == 0
+            ? ""
+            : Encoding.UTF8.GetString(payload[1..(1 + hnLen)]);
+        var rest = payload[(1 + hnLen)..];
+
         var entries = new List<UpstreamEntry>();
         int i = 0;
-        while (i < payload.Length)
+        while (i < rest.Length)
         {
-            if (i + 5 > payload.Length)
+            if (i + 5 > rest.Length)
                 return null;
-            var id = payload[i];
-            var proto = payload[i + 1];
-            var port = (ushort)(payload[i + 2] << 8 | payload[i + 3]);
-            var labelLen = payload[i + 4];
+            var id = rest[i];
+            var proto = rest[i + 1];
+            var port = (ushort)(rest[i + 2] << 8 | rest[i + 3]);
+            var labelLen = rest[i + 4];
             i += 5;
-            if (i + labelLen > payload.Length)
+            if (i + labelLen > rest.Length)
                 return null;
             string? label = labelLen == 0
                 ? null
-                : Encoding.UTF8.GetString(payload[i..(i + labelLen)]);
+                : Encoding.UTF8.GetString(rest[i..(i + labelLen)]);
             i += labelLen;
             entries.Add(new UpstreamEntry(id, proto, port, label));
         }
-        return new Frame.Manifest(entries.ToArray());
+        return new Frame.Manifest(hostname, entries.ToArray());
     }
 }

win/gatuna-client/MainForm.cs

@@ -7,6 +7,7 @@ public partial class MainForm : Form
     readonly SessionManager _sessions = new();
     readonly ComboBox _deviceBox = new();
     readonly Button _discoverBtn = new();
+    readonly Label _serverLabel = new();
     readonly ListView _listView = new();
     readonly Label _statusLabel = new();
     TunnelLink? _link;
@@ -23,7 +24,8 @@ public partial class MainForm : Form
         InitializeComponents();
 
         _sessions.Log += msg => this.Invoke(() => _statusLabel.Text = msg);
-        _sessions.ManifestReceived += entries => this.Invoke(() => PopulateList(entries));
+        _sessions.ManifestReceived += (hostname, mac, entries) =>
+            this.Invoke(() => PopulateList(hostname, mac, entries));
 
         foreach (var d in TunnelLink.ListDevices())
         {
@@ -54,9 +56,15 @@ public partial class MainForm : Form
         _discoverBtn.Click += OnDiscover;
         Controls.Add(_discoverBtn);
 
-        _listView.Left = pad; _listView.Top = _discoverBtn.Bottom + 8;
+        _serverLabel.Left = pad; _serverLabel.Top = _discoverBtn.Bottom + 8;
+        _serverLabel.Width = ClientSize.Width - pad * 2;
+        _serverLabel.AutoEllipsis = true;
+        _serverLabel.Text = "Server: not connected";
+        Controls.Add(_serverLabel);
+
+        _listView.Left = pad; _listView.Top = _serverLabel.Bottom + 8;
         _listView.Width = ClientSize.Width - pad * 2;
-        _listView.Height = 220;
+        _listView.Height = 200;
         _listView.View = View.Details;
         _listView.FullRowSelect = true;
         _listView.CheckBoxes = true;
@@ -96,11 +104,15 @@ public partial class MainForm : Form
         _sessions.AttachLink(_link);
         _link.Open();
         _sessions.Discover();
+        _serverLabel.Text = "Server: discovering...";
         _statusLabel.Text = "discovering...";
     }
 
-    void PopulateList(UpstreamEntry[] entries)
+    void PopulateList(string hostname, byte[] mac, UpstreamEntry[] entries)
     {
+        var macStr = string.Join(":", mac.Select(b => b.ToString("X2")));
+        _serverLabel.Text = $"Server: {hostname} — {macStr}";
+
         _listView.BeginUpdate();
         _listView.Items.Clear();
         foreach (var up in entries)

win/gatuna-client/SessionManager.cs

@@ -12,6 +12,7 @@ sealed class SessionManager : IDisposable
     readonly ConcurrentDictionary<int, ListenerState> _listeners = new();
 
     byte[]? _serverMac;
+    string _serverHostname = "";
     UpstreamEntry[] _upstreams = [];
 
     // Serialized OPEN: only one outstanding at a time.
@@ -20,10 +21,11 @@ sealed class SessionManager : IDisposable
     readonly Queue<PendingOpen> _openQueue = new();
 
     public event Action<string>? Log;
-    public event Action<UpstreamEntry[]>? ManifestReceived;
+    public event Action<string, byte[], UpstreamEntry[]>? ManifestReceived;
 
     public UpstreamEntry[] Upstreams => _upstreams;
     public byte[]? ServerMac => _serverMac;
+    public string ServerHostname => _serverHostname;
     public TunnelLink? Link => _link;
 
     public void AttachLink(TunnelLink link)
@@ -51,9 +53,10 @@ sealed class SessionManager : IDisposable
         {
             case Frame.Manifest manifest:
                 _serverMac = srcMac;
+                _serverHostname = manifest.Hostname;
                 _upstreams = manifest.Entries;
-                Log?.Invoke($"manifest: {manifest.Entries.Length} upstreams from {BitConverter.ToString(srcMac)}");
-                ManifestReceived?.Invoke(manifest.Entries);
+                Log?.Invoke($"manifest: {manifest.Entries.Length} upstreams from {manifest.Hostname} ({BitConverter.ToString(srcMac)})");
+                ManifestReceived?.Invoke(manifest.Hostname, srcMac, manifest.Entries);
                 break;
 
             case Frame.OpenAck ack: