From 0c5a23bbfca1c2abb1d7555eb1c5c0c0e420a156 Mon Sep 17 00:00:00 2001
From: Adien Akhmad <adien.akhmad@gmail.com>
Date: Mon, 24 Mar 2025 04:51:43 +0700
Subject: [PATCH] fix mdns crash on scoped IPv6 link-local

Before this change the library will always crash with "SocketException: no route to host" when used on IPv6 interface with a scoped address e.g.[FE80::1%1].

The root cause was because the multicast address for IPv6 didn't specify a scope ID for such cases.

This commit fixes it by first checking if the local endpoint is an IPv6 link local, if true, it will create and use a scoped multicast address for mdns. The resulting multicast address is then stored for reuse.
---
 TinyDNS/MDNS.cs | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/TinyDNS/MDNS.cs b/TinyDNS/MDNS.cs
index ffad172..998d83e 100644
--- a/TinyDNS/MDNS.cs
+++ b/TinyDNS/MDNS.cs
@@ -11,6 +11,7 @@
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 using System.Buffers;
+using System.Collections.Concurrent;
 using System.Net;
 using System.Net.NetworkInformation;
 using System.Net.Sockets;
@@ -32,6 +33,7 @@ public class MDNS : IDisposable
         private Socket? listenerV4;
         private Socket? listenerV6;
         private readonly List<Socket> senders = [];
+        private readonly ConcurrentDictionary<long, IPAddress> multicastAddressV6Scoped = [];
 
         public delegate Task MessageEventHandler(DNSMessageEvent e);
         public event MessageEventHandler? AnswerReceived;
@@ -441,7 +443,7 @@ private async Task SendMessage(Message msg)
                     if (sender.AddressFamily == AddressFamily.InterNetwork)
                         await sender.SendToAsync(buffer.Slice(0, len), SocketFlags.None, new IPEndPoint(MulticastAddress, PORT), stop.Token);
                     else
-                        await sender.SendToAsync(buffer.Slice(0, len), SocketFlags.None, new IPEndPoint(MulticastAddressV6, PORT), stop.Token);
+                        await sender.SendToAsync(buffer.Slice(0, len), SocketFlags.None, new IPEndPoint(GetMulticastAddressV6(sender.LocalEndPoint), PORT), stop.Token);
                     await Task.Delay(5);
                 }
             }finally
@@ -450,6 +452,15 @@ private async Task SendMessage(Message msg)
             }
         }
 
+        private IPAddress GetMulticastAddressV6(EndPoint? endPoint)
+        {
+            if (endPoint is not IPEndPoint { Address.IsIPv6LinkLocal: true } ip)
+                return MulticastAddressV6;
+
+            Func<long, IPAddress> factory = scope => new IPAddress(new byte[] { 0xFF, 0x02, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xFB }, scope);
+            return multicastAddressV6Scoped.GetOrAdd(ip.Address.ScopeId, factory); 
+        }
+
         public void Stop()
         {
             stop.Cancel();