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.

PROTOCOL.md

@@ -59,18 +59,27 @@ Reserved (unimplemented in v1; parse returns Err, encode unimplemented):
 
 ### MANIFEST payload
 
-Variable-length entries, parsed sequentially until the payload is consumed.
+A hostname prefix followed by variable-length entries, parsed sequentially
+until the payload is consumed.
 
 ```
  0                   1                   2                   3
  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
-+---------------+---------------+-------------------------------+
++---------------+-----------------------------------------------+
+| hostname_len  | hostname (UTF-8, hostname_len bytes) ...       |
++---------------+-----------------------------------------------+
 | id            | proto         | port (big-endian)             |
 +---------------+---------------+-------------------------------+
 | label_len     | label (UTF-8, label_len bytes) ...           |
 +---------------+-----------------------------------------------+
+| ... repeated ...                                             |
++---------------------------------------------------------------+
 ```
 
+- **hostname_len** (u8): length in bytes of the server's hostname. `0` is valid
+  (unknown hostname).
+- **hostname** (`hostname_len` bytes, UTF-8): the server's hostname, read via
+  `gethostname(2)` at startup. Maximum 255 bytes.
 - **id** (u8): upstream identifier (1-based positional index from `gatunad`
   cmdline).
 - **proto** (u8): `1 = TCP`, `2 = UDP` (reserved; not emitted in v1).
@@ -80,7 +89,8 @@ Variable-length entries, parsed sequentially until the payload is consumed.
 - **label** (`label_len` bytes, UTF-8): human-readable name for the upstream,
   taken from the `PORT[:label]` cmdline argument. Maximum 255 bytes.
 
-To parse: read the 5-byte fixed prefix, then `label_len` bytes, and repeat until
+To parse: read `hostname_len`, then `hostname_len` bytes of hostname, then read
+5-byte fixed entry prefixes + `label_len` bytes of label each, repeating until
 the payload is exhausted. The number of entries is not carried explicitly.
 
 ### OPEN payload

gatunad/src/frame.rs

@@ -39,7 +39,7 @@ pub struct UpstreamEntry {
 #[derive(Clone, Debug)]
 pub enum Frame {
     Discover,
-    Manifest(Vec<UpstreamEntry>),
+    Manifest { hostname: String, entries: Vec<UpstreamEntry> },
     Open { upstream_id: u8 },
     OpenAck { session_id: u32, upstream_id: u8 },
     OpenNak { upstream_id: u8, reason: u8 },
@@ -94,8 +94,12 @@ impl Frame {
     pub fn encode(&self) -> Vec<u8> {
         match self {
             Frame::Discover => build(TYPE_DISCOVER, 0, Vec::new()),
-            Frame::Manifest(entries) => {
+            Frame::Manifest { hostname, entries } => {
                 let mut payload = Vec::new();
+                let hn_bytes = hostname.as_bytes();
+                let hn_len = hn_bytes.len().min(255) as u8;
+                payload.push(hn_len);
+                payload.extend_from_slice(&hn_bytes[..hn_len as usize]);
                 for e in entries {
                     encode_entry(&mut payload, e);
                 }
@@ -143,29 +147,38 @@ impl Frame {
                 Ok(Frame::Discover)
             }
             TYPE_MANIFEST => {
+                if payload.is_empty() {
+                    return Err(DecodeError::BadPayload("MANIFEST missing hostname prefix"));
+                }
+                let hn_len = payload[0] as usize;
+                if 1 + hn_len > payload.len() {
+                    return Err(DecodeError::BadPayload("MANIFEST hostname truncated"));
+                }
+                let hostname = String::from_utf8_lossy(&payload[1..1 + hn_len]).into_owned();
+                let rest = &payload[1 + hn_len..];
                 let mut entries = Vec::new();
                 let mut i = 0;
-                while i < payload.len() {
+                while i < rest.len() {
-                    if i + 5 > payload.len() {
+                    if i + 5 > rest.len() {
                         return Err(DecodeError::BadPayload("MANIFEST entry truncated"));
                     }
-                    let id = payload[i];
+                    let id = rest[i];
-                    let proto = payload[i + 1];
+                    let proto = rest[i + 1];
-                    let port = u16::from_be_bytes([payload[i + 2], payload[i + 3]]);
+                    let port = u16::from_be_bytes([rest[i + 2], rest[i + 3]]);
-                    let label_len = payload[i + 4] as usize;
+                    let label_len = rest[i + 4] as usize;
                     i += 5;
-                    if i + label_len > payload.len() {
+                    if i + label_len > rest.len() {
                         return Err(DecodeError::BadPayload("MANIFEST label truncated"));
                     }
                     let label = if label_len == 0 {
                         None
                     } else {
-                        Some(String::from_utf8_lossy(&payload[i..i + label_len]).into_owned())
+                        Some(String::from_utf8_lossy(&rest[i..i + label_len]).into_owned())
                     };
                     i += label_len;
                     entries.push(UpstreamEntry { id, proto, port, label });
                 }
-                Ok(Frame::Manifest(entries))
+                Ok(Frame::Manifest { hostname, entries })
             }
             TYPE_OPEN => {
                 if payload.len() != 1 {

gatunad/src/main.rs

@@ -52,6 +52,8 @@ async fn main() -> ExitCode {
         }
     };
 
+    let hostname = Arc::new(upstream::get_hostname());
+
     let link = match Link::open(&args.iface) {
         Ok(l) => Arc::new(l),
         Err(e) => {
@@ -112,7 +114,7 @@ async fn main() -> ExitCode {
                         continue;
                     }
                 };
-                handle_frame(frame, src, &tx, &store, &next_id, &table).await;
+                handle_frame(frame, src, &tx, &store, &next_id, &table, &hostname).await;
             }
             Ok(None) => {
                 // Ignorable frame (outgoing/short/mismatch) or transient; do not
@@ -135,10 +137,14 @@ async fn handle_frame(
     store: &SessionStore,
     next_id: &Arc<AtomicU32>,
     table: &Arc<crate::upstream::UpstreamTable>,
+    hostname: &Arc<String>,
 ) {
     match frame {
         Frame::Discover => {
-            let manifest = Frame::Manifest(table.entries());
+            let manifest = Frame::Manifest {
+                hostname: (**hostname).clone(),
+                entries: table.entries(),
+            };
             let _ = tx.send((src, manifest.encode())).await;
         }
         Frame::Open { upstream_id } => {
@@ -202,6 +208,6 @@ async fn handle_frame(
             store.lock().expect("store poisoned").remove(&session_id);
         }
         // Not expected from a client; ignore.
-        Frame::Manifest(_) | Frame::OpenAck { .. } | Frame::OpenNak { .. } => {}
+        Frame::Manifest { .. } | Frame::OpenAck { .. } | Frame::OpenNak { .. } => {}
     }
 }

gatunad/src/upstream.rs

@@ -1,6 +1,20 @@
 //! Upstream table: cmdline parsing and MANIFEST entry construction.
 
-use crate::frame::{UpstreamEntry, PROTO_TCP, PROTO_UDP};
+use crate::frame::{Frame, UpstreamEntry, PROTO_TCP, PROTO_UDP};
+
+/// Read the system hostname via `gethostname(2)`.
+pub fn get_hostname() -> String {
+    let mut buf = [0u8; 256];
+    let ret = unsafe {
+        libc::gethostname(buf.as_mut_ptr() as *mut libc::c_char, buf.len())
+    };
+    if ret == 0 {
+        let len = buf.iter().position(|&b| b == 0).unwrap_or(buf.len());
+        String::from_utf8_lossy(&buf[..len]).into_owned()
+    } else {
+        String::new()
+    }
+}
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 #[allow(dead_code)]

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 id = rest[i];
-            var proto = payload[i + 1];
+            var proto = rest[i + 1];
-            var port = (ushort)(payload[i + 2] << 8 | payload[i + 3]);
+            var port = (ushort)(rest[i + 2] << 8 | rest[i + 3]);
-            var labelLen = payload[i + 4];
+            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($"manifest: {manifest.Entries.Length} upstreams from {BitConverter.ToString(srcMac)}");
+                Log?.Invoke($"manifest: {manifest.Entries.Length} upstreams from {manifest.Hostname} ({BitConverter.ToString(srcMac)})");
-                ManifestReceived?.Invoke(manifest.Entries);
+                ManifestReceived?.Invoke(manifest.Hostname, srcMac, manifest.Entries);
                 break;
 
             case Frame.OpenAck ack: