Fix ICE failures by replacing mDNS .local addresses with real peer IPs

Modern browsers obfuscate local IP addresses with mDNS hostnames
(e.g. abcdef12-3456.local) for privacy. When mDNS resolution is
unavailable on the LAN, WebRTC ICE candidates cannot be resolved
and the peer connection fails.

This fix makes the signaling server extract each client's real IP
from the TCP connection and share it via Welcome/PeerJoined messages.
The client then replaces any .local mDNS addresses in SDP offers,
answers, and trickle ICE candidates with the peer's actual IP address,
enabling direct IP connectivity without relying on mDNS.

https://claude.ai/code/session_01EZrFdyAR35RLwRdHUA9zPJ

lanshare/src/main.rs

@@ -1,5 +1,6 @@
 use axum::{
     extract::{
+        connect_info::ConnectInfo,
         ws::{Message, WebSocket, WebSocketUpgrade},
         State,
     },
@@ -63,6 +64,7 @@ enum SignalMessage {
     /// Server assigns an ID to a new peer
     Welcome {
         peer_id: String,
+        ip: String,
         peers: Vec<PeerInfo>,
     },
     /// Peer announces themselves (with optional name)
@@ -73,6 +75,7 @@ enum SignalMessage {
     PeerJoined {
         peer_id: String,
         name: String,
+        ip: String,
     },
     /// Broadcast: a peer left
     PeerLeft {
@@ -120,6 +123,7 @@ enum SignalMessage {
 struct PeerInfo {
     peer_id: String,
     name: String,
+    ip: String,
     is_sharing: bool,
     has_audio: bool,
 }
@@ -127,6 +131,7 @@ struct PeerInfo {
 /// State for a connected peer
 struct PeerState {
     name: String,
+    ip: String,
     is_sharing: bool,
     has_audio: bool,
     tx: broadcast::Sender<SignalMessage>,
@@ -150,6 +155,7 @@ impl AppState {
             .map(|entry| PeerInfo {
                 peer_id: entry.key().clone(),
                 name: entry.value().name.clone(),
+                ip: entry.value().ip.clone(),
                 is_sharing: entry.value().is_sharing,
                 has_audio: entry.value().has_audio,
             })
@@ -216,19 +222,27 @@ async fn main() {
 
     let config = RustlsConfig::from_pem_file(args.certificate, args.key).await.unwrap();
 
-    axum_server::bind_rustls(addr, config).serve(app.into_make_service()).await.unwrap();
+    axum_server::bind_rustls(addr, config)
+        .serve(app.into_make_service_with_connect_info::<SocketAddr>())
+        .await
+        .unwrap();
 }
 
 async fn serve_index() -> impl IntoResponse {
     Html(include_str!("../static/index.html"))
 }
 
-async fn ws_handler(ws: WebSocketUpgrade, State(state): State<Arc<AppState>>) -> impl IntoResponse {
+async fn ws_handler(
-    ws.on_upgrade(move |socket| handle_socket(socket, state))
+    ws: WebSocketUpgrade,
+    ConnectInfo(addr): ConnectInfo<SocketAddr>,
+    State(state): State<Arc<AppState>>,
+) -> impl IntoResponse {
+    ws.on_upgrade(move |socket| handle_socket(socket, state, addr))
 }
 
-async fn handle_socket(socket: WebSocket, state: Arc<AppState>) {
+async fn handle_socket(socket: WebSocket, state: Arc<AppState>, addr: SocketAddr) {
     let peer_id = Uuid::new_v4().to_string();
+    let peer_ip = addr.ip().to_string();
     let (tx, _) = broadcast::channel::<SignalMessage>(64);
 
     // Add peer to state with default name
@@ -236,6 +250,7 @@ async fn handle_socket(socket: WebSocket, state: Arc<AppState>) {
         peer_id.clone(),
         PeerState {
             name: format!("Peer {}", &peer_id[..8]),
+            ip: peer_ip.clone(),
             is_sharing: false,
             has_audio: false,
             tx: tx.clone(),
@@ -250,6 +265,7 @@ async fn handle_socket(socket: WebSocket, state: Arc<AppState>) {
     // Send welcome message with current peer list
     let welcome = SignalMessage::Welcome {
         peer_id: peer_id.clone(),
+        ip: peer_ip,
         peers: peers.into_iter().filter(|p| p.peer_id != peer_id).collect(),
     };
 
@@ -319,6 +335,11 @@ async fn handle_socket(socket: WebSocket, state: Arc<AppState>) {
 async fn handle_signal(state: &AppState, from_peer: &str, signal: SignalMessage) {
     match signal {
         SignalMessage::Announce { name } => {
+            let ip = state
+                .peers
+                .get(from_peer)
+                .map(|p| p.ip.clone())
+                .unwrap_or_default();
             if let Some(mut peer) = state.peers.get_mut(from_peer) {
                 peer.name = name.clone();
             }
@@ -326,6 +347,7 @@ async fn handle_signal(state: &AppState, from_peer: &str, signal: SignalMessage)
                 SignalMessage::PeerJoined {
                     peer_id: from_peer.to_string(),
                     name,
+                    ip,
                 },
                 Some(from_peer),
             );

lanshare/static/index.html

@@ -428,8 +428,9 @@
     // ==================== State ====================
     const state = {
         peerId: null,
+        peerIp: null,               // Our own IP as seen by the server
         peerName: localStorage.getItem('lanshare_name') || null,
-        peers: new Map(),           // peer_id -> { name, isSharing, hasAudio, stream, peerConnection }
+        peers: new Map(),           // peer_id -> { name, ip, isSharing, hasAudio, stream, peerConnection }
         localStream: null,
         isSharing: false,
         includeAudio: true,
@@ -456,6 +457,14 @@
         return candidates;
     }
     
+    // Replace mDNS .local addresses with a real IP.
+    // Modern browsers obfuscate local IPs as UUIDs ending in .local for privacy,
+    // which breaks direct LAN connections when mDNS resolution isn't available.
+    function replaceMdnsWithIp(str, realIp) {
+        if (!str || !realIp) return str;
+        return str.replace(/[a-f0-9-]+\.local/gi, realIp);
+    }
+
     console.log(`ICE mode: ${USE_TRICKLE_ICE ? 'TRICKLE' : 'BUNDLED'}`);
 
     // ==================== DOM Elements ====================
@@ -537,20 +546,22 @@
         switch (msg.type) {
             case 'welcome':
                 state.peerId = msg.peer_id;
+                state.peerIp = msg.ip;
+                console.log(`Our IP as seen by server: ${msg.ip}`);
                 connectingOverlay.style.display = 'none';
                 updateConnectionStatus(true);
-                
+
                 // Set name
                 if (!state.peerName) {
                     state.peerName = prompt('Enter your name:', `User ${msg.peer_id.slice(0, 4)}`) || `User ${msg.peer_id.slice(0, 4)}`;
                     localStorage.setItem('lanshare_name', state.peerName);
                 }
-                
+
                 send({ type: 'announce', name: state.peerName });
-                
+
                 // Add existing peers
                 for (const peer of msg.peers) {
-                    addPeer(peer.peer_id, peer.name, peer.is_sharing, peer.has_audio);
+                    addPeer(peer.peer_id, peer.name, peer.ip, peer.is_sharing, peer.has_audio);
                     // Request stream from peers that are already sharing
                     if (peer.is_sharing) {
                         requestStreamFrom(peer.peer_id);
@@ -560,7 +571,7 @@
                 break;
                 
             case 'peer_joined':
-                addPeer(msg.peer_id, msg.name, false, false);
+                addPeer(msg.peer_id, msg.name, msg.ip, false, false);
                 // If we're sharing, proactively send an offer to the new peer
                 if (state.isSharing && state.localStream) {
                     await sendOfferTo(msg.peer_id);
@@ -607,7 +618,7 @@
                 if (state.isSharing && state.localStream) {
                     // Ensure we have this peer in our state (handles race condition)
                     if (!state.peers.has(msg.from)) {
-                        addPeer(msg.from, `Peer ${msg.from.slice(0, 4)}`, false, false);
+                        addPeer(msg.from, `Peer ${msg.from.slice(0, 4)}`, null, false, false);
                     }
                     await sendOfferTo(msg.from);
                 }
@@ -628,11 +639,12 @@
     }
 
     // ==================== Peer Management ====================
-    function addPeer(peerId, name, isSharing, hasAudio) {
+    function addPeer(peerId, name, ip, isSharing, hasAudio) {
         if (peerId === state.peerId) return;
-        
+
         state.peers.set(peerId, {
             name,
+            ip,
             isSharing,
             hasAudio,
             stream: null,
@@ -681,13 +693,16 @@
                 const c = event.candidate;
                 console.log(`[${peerId.slice(0,8)}] Local candidate: ${c.candidate}`);
                 console.log(`  → type: ${c.type}, protocol: ${c.protocol}, address: ${c.address}, port: ${c.port}`);
-                
+
                 if (USE_TRICKLE_ICE) {
+                    // Replace mDNS .local addresses with our real IP for direct LAN connectivity
+                    const candidateObj = event.candidate.toJSON();
+                    candidateObj.candidate = replaceMdnsWithIp(candidateObj.candidate, state.peerIp);
                     send({
                         type: 'ice_candidate',
                         from: state.peerId,
                         to: peerId,
-                        candidate: JSON.stringify(event.candidate),
+                        candidate: JSON.stringify(candidateObj),
                     });
                 }
             } else {
@@ -784,10 +799,11 @@
             await waitForIceGathering(pc, peerId);
         }
         
-        const sdpToSend = pc.localDescription;
+        // Replace mDNS .local addresses with our real IP for direct LAN connectivity
+        const sdpToSend = { type: pc.localDescription.type, sdp: replaceMdnsWithIp(pc.localDescription.sdp, state.peerIp) };
         const candidates = extractCandidatesFromSDP(sdpToSend.sdp);
         console.log(`[${peerId.slice(0,8)}] Sending offer with ${candidates.length} candidates in SDP`);
-        
+
         send({
             type: 'offer',
             from: state.peerId,
@@ -799,39 +815,42 @@
     async function handleOffer(fromPeerId, sdpJson) {
         // Ensure we have this peer in state
         if (!state.peers.has(fromPeerId)) {
-            addPeer(fromPeerId, `Peer ${fromPeerId.slice(0, 4)}`, true, false);
+            addPeer(fromPeerId, `Peer ${fromPeerId.slice(0, 4)}`, null, true, false);
         }
-        
+
         const peer = state.peers.get(fromPeerId);
-        
+
         // Close existing connection if any (new offer = renegotiation)
         if (peer.peerConnection) {
             peer.peerConnection.close();
         }
-        
+
         console.log(`[${fromPeerId.slice(0,8)}] handleOffer: creating answer (${USE_TRICKLE_ICE ? 'trickle' : 'bundled'})`);
         const pc = createPeerConnection(fromPeerId);
         peer.peerConnection = pc;
-        
+
+        // Replace any mDNS .local addresses in the remote SDP with the peer's real IP
         const sdp = JSON.parse(sdpJson);
+        sdp.sdp = replaceMdnsWithIp(sdp.sdp, peer.ip);
         const remoteCandidates = extractCandidatesFromSDP(sdp.sdp);
         console.log(`[${fromPeerId.slice(0,8)}] Received offer with ${remoteCandidates.length} candidates in SDP:`);
         remoteCandidates.forEach(c => console.log(`  ${c}`));
-        
+
         await pc.setRemoteDescription(new RTCSessionDescription(sdp));
-        
+
         const answer = await pc.createAnswer();
         await pc.setLocalDescription(answer);
-        
+
         if (!USE_TRICKLE_ICE) {
             // Bundled mode: wait for all candidates to be in SDP
             await waitForIceGathering(pc, fromPeerId);
         }
-        
+
-        const sdpToSend = pc.localDescription;
+        // Replace mDNS .local addresses with our real IP for direct LAN connectivity
+        const sdpToSend = { type: pc.localDescription.type, sdp: replaceMdnsWithIp(pc.localDescription.sdp, state.peerIp) };
         const localCandidates = extractCandidatesFromSDP(sdpToSend.sdp);
         console.log(`[${fromPeerId.slice(0,8)}] Sending answer with ${localCandidates.length} candidates in SDP`);
-        
+
         send({
             type: 'answer',
             from: state.peerId,
@@ -846,18 +865,20 @@
             console.warn(`[${fromPeerId.slice(0,8)}] handleAnswer: no peer connection`);
             return;
         }
-        
+
         // Only set remote description if we're in the right state
         if (peer.peerConnection.signalingState !== 'have-local-offer') {
             console.warn(`[${fromPeerId.slice(0,8)}] handleAnswer: wrong state: ${peer.peerConnection.signalingState}`);
             return;
         }
-        
+
+        // Replace any mDNS .local addresses in the remote SDP with the peer's real IP
         const sdp = JSON.parse(sdpJson);
+        sdp.sdp = replaceMdnsWithIp(sdp.sdp, peer.ip);
         const remoteCandidates = extractCandidatesFromSDP(sdp.sdp);
         console.log(`[${fromPeerId.slice(0,8)}] Received answer with ${remoteCandidates.length} candidates in SDP:`);
         remoteCandidates.forEach(c => console.log(`  ${c}`));
-        
+
         console.log(`[${fromPeerId.slice(0,8)}] handleAnswer: setting remote description`);
         await peer.peerConnection.setRemoteDescription(new RTCSessionDescription(sdp));
     }
@@ -868,10 +889,12 @@
             console.warn(`[${fromPeerId.slice(0,8)}] handleIceCandidate: no peer connection`);
             return;
         }
-        
+
+        // Replace any mDNS .local addresses with the peer's real IP
         const candidate = JSON.parse(candidateJson);
+        candidate.candidate = replaceMdnsWithIp(candidate.candidate, peer.ip);
         console.log(`[${fromPeerId.slice(0,8)}] Remote candidate: ${candidate.candidate}`);
-        
+
         peer.peerConnection.addIceCandidate(new RTCIceCandidate(candidate))
             .then(() => console.log(`[${fromPeerId.slice(0,8)}] Successfully added remote candidate`))
             .catch(err => console.error(`[${fromPeerId.slice(0,8)}] Error adding ICE candidate:`, err));