Fix UDP Auto-Discovery returning IPv6 for cross-subnet IPv4 requests

Fixes #15898

When a UDP discovery request is relayed from a different IPv4 subnet,
GetBindAddress() now correctly returns an IPv4 address instead of
incorrectly falling back to ::1.

Changes:
- Loopback fallback now prefers address family matching the source IP
- Interface fallback now prefers interfaces matching source address family
- Added test case for cross-subnet IPv4 request scenario

CONTRIBUTORS.md

@@ -164,6 +164,7 @@
  - [XVicarious](https://github.com/XVicarious)
  - [YouKnowBlom](https://github.com/YouKnowBlom)
  - [ZachPhelan](https://github.com/ZachPhelan)
+ - [ZeusCraft10](https://github.com/ZeusCraft10)
  - [KristupasSavickas](https://github.com/KristupasSavickas)
  - [Pusta](https://github.com/pusta)
  - [nielsvanvelzen](https://github.com/nielsvanvelzen)

src/Jellyfin.Networking/Manager/NetworkManager.cs

@@ -875,7 +875,20 @@ public class NetworkManager : INetworkManager, IDisposable
         if (availableInterfaces.Count == 0)
         {
             // There isn't any others, so we'll use the loopback.
-            result = IsIPv4Enabled && !IsIPv6Enabled ? "127.0.0.1" : "::1";
+            // Prefer loopback address matching the source's address family
+            if (source is not null && source.AddressFamily == AddressFamily.InterNetwork && IsIPv4Enabled)
+            {
+                result = "127.0.0.1";
+            }
+            else if (source is not null && source.AddressFamily == AddressFamily.InterNetworkV6 && IsIPv6Enabled)
+            {
+                result = "::1";
+            }
+            else
+            {
+                result = IsIPv4Enabled ? "127.0.0.1" : "::1";
+            }
+
             _logger.LogWarning("{Source}: Only loopback {Result} returned, using that as bind address.", source, result);
             return result;
         }
@@ -900,9 +913,19 @@ public class NetworkManager : INetworkManager, IDisposable
             }
         }
 
-        // Fallback to first available interface
+        // Fallback to an interface matching the source's address family, or first available
+        var preferredInterface = availableInterfaces
+            .FirstOrDefault(x => x.Address.AddressFamily == source.AddressFamily);
+
+        if (preferredInterface is not null)
+        {
+            result = NetworkUtils.FormatIPString(preferredInterface.Address);
+            _logger.LogDebug("{Source}: No matching subnet found, using interface with matching address family: {Result}", source, result);
+            return result;
+        }
+
         result = NetworkUtils.FormatIPString(availableInterfaces[0].Address);
-        _logger.LogDebug("{Source}: No matching interfaces found, using preferred interface as bind address: {Result}", source, result);
+        _logger.LogDebug("{Source}: No matching interfaces found, using first available interface as bind address: {Result}", source, result);
         return result;
     }
 

tests/Jellyfin.Networking.Tests/NetworkParseTests.cs

@@ -377,6 +377,8 @@ namespace Jellyfin.Networking.Tests
         [InlineData("192.168.1.208/24,-16,eth16|10.0.0.1/24,10,eth7", "192.168.1.0/24", "10.0.0.1", "192.168.1.209", "10.0.0.1")] // LAN not bound, so return external.
         [InlineData("192.168.1.208/24,-16,eth16|10.0.0.1/24,10,eth7", "192.168.1.0/24", "192.168.1.208,10.0.0.1", "8.8.8.8", "10.0.0.1")] // return external bind address
         [InlineData("192.168.1.208/24,-16,eth16|10.0.0.1/24,10,eth7", "192.168.1.0/24", "192.168.1.208,10.0.0.1", "192.168.1.210", "192.168.1.208")] // return LAN bind address
+        // Cross-subnet IPv4 request should return IPv4, not IPv6 (Issue #15898)
+        [InlineData("192.168.1.208/24,-16,eth16|fd00::1/64,10,eth7", "192.168.1.0/24", "", "192.168.2.100", "192.168.1.208")]
         public void GetBindInterface_ValidSourceGiven_Success(string interfaces, string lan, string bind, string source, string result)
         {
             var conf = new NetworkConfiguration