Fix UdpClient.EnableBroadcast not preventing broadcast sends (#124482)

matantsach · claude · web-flow · commit e2322af414f5 · 2026-03-16T22:48:42.000+01:00
## Summary Fixes #118055 `UdpClient.CheckForBroadcast()` unconditionally auto-enables the `Broadcast` socket option when sending to `IPAddress.Broadcast`, even when the user has explicitly set `EnableBroadcast = false`. This PR makes `CheckForBroadcast()` respect the user's explicit choice. ## Changes - Added `_isBroadcastSetByUser` field to track whether `EnableBroadcast` has been explicitly set by the user - Modified `CheckForBroadcast()` to skip auto-enable when the user has explicitly set `EnableBroadcast` - Backward-compatible: auto-enable behavior is preserved when users have not explicitly set the property **Note:** The pre-existing `_isBroadcast` flag is not reset when the `Client` socket is replaced via the property setter. The new `_isBroadcastSetByUser` flag follows this same pattern for consistency. Resetting state on socket replacement could be a follow-up improvement. ## Testing - `EnableBroadcast_ExplicitlyDisabled_NotAutoEnabled` — verifies that explicitly disabling broadcast prevents auto-enable on broadcast send - `EnableBroadcast_ExplicitlyEnabledThenDisabled_NotAutoEnabled` — verifies the toggle case (enable → disable) also prevents auto-enable - `EnableBroadcast_NotExplicitlySet_AutoEnabled` — verifies backward compatibility: auto-enable still works when the user hasn't set the property --------- Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
diff --git a/src/libraries/System.Net.Sockets/src/System/Net/Sockets/UDPClient.cs b/src/libraries/System.Net.Sockets/src/System/Net/Sockets/UDPClient.cs
@@ -171,6 +171,7 @@ public bool EnableBroadcast
             set
             {
                 _clientSocket.EnableBroadcast = value;
+                _isBroadcastSetByUser = true;
             }
         }
 
@@ -247,14 +248,18 @@ protected virtual void Dispose(bool disposing)
         }
 
         private bool _isBroadcast;
+        private bool _isBroadcastSetByUser;
         private void CheckForBroadcast(IPAddress ipAddress)
         {
             // Here we check to see if the user is trying to use a Broadcast IP address
             // we only detect IPAddress.Broadcast (which is not the only Broadcast address)
             // and in that case we set SocketOptionName.Broadcast on the socket to allow its use.
             // if the user really wants complete control over Broadcast addresses they need to
             // inherit from UdpClient and gain control over the Socket and do whatever is appropriate.
-            if (_clientSocket != null && !_isBroadcast && IsBroadcast(ipAddress))
+            //
+            // If the user has explicitly set EnableBroadcast, we respect their choice
+            // and do not auto-enable broadcast.
+            if (_clientSocket != null && !_isBroadcast && !_isBroadcastSetByUser && IsBroadcast(ipAddress))
             {
                 // We need to set the Broadcast socket option.
                 // Note that once we set the option on the Socket we never reset it.
diff --git a/src/libraries/System.Net.Sockets/tests/FunctionalTests/UdpClientTest.cs b/src/libraries/System.Net.Sockets/tests/FunctionalTests/UdpClientTest.cs
@@ -320,6 +320,55 @@ public void EnableBroadcast_Roundtrips()
             }
         }
 
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
+        [InlineData(false)]
+        [InlineData(true)]
+        public void EnableBroadcast_ExplicitlyDisabled_NotAutoEnabled(bool setTrueFirst)
+        {
+            using (var udpClient = new UdpClient())
+            {
+                if (setTrueFirst) udpClient.EnableBroadcast = true;
+                udpClient.EnableBroadcast = false;
+
+                // Sending to a broadcast address should not auto-enable
+                // broadcast when the user explicitly disabled it.
+                // The send may throw SocketException on platforms that reject
+                // broadcast sends when the socket option is not set.
+                try
+                {
+                    udpClient.Send(new byte[1], 1, new IPEndPoint(IPAddress.Broadcast, UnusedPort));
+                }
+                catch (SocketException)
+                {
+                }
+
+                Assert.False(udpClient.EnableBroadcast);
+            }
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsMultithreadingSupported))]
+        public void EnableBroadcast_NotExplicitlySet_AutoEnabled()
+        {
+            using (var udpClient = new UdpClient())
+            {
+                Assert.False(udpClient.EnableBroadcast);
+
+                // When the user hasn't explicitly set EnableBroadcast,
+                // sending to a broadcast address should auto-enable it
+                // (preserving backward-compatible behavior).
+                try
+                {
+                    udpClient.Send(new byte[1], 1, new IPEndPoint(IPAddress.Broadcast, UnusedPort));
+                }
+                catch (SocketException)
+                {
+                    return; // Platform doesn't support broadcast sends; skip assertion.
+                }
+
+                Assert.True(udpClient.EnableBroadcast);
+            }
+        }
+
         [PlatformSpecific(TestPlatforms.Windows)] // ExclusiveAddressUse is Windows-specific
         [Fact]
         public void ExclusiveAddressUse_Roundtrips()

Original file line number	Diff line number	Diff line change
`@@ -171,6 +171,7 @@ public bool EnableBroadcast`
`171`	`171`	`set`
`172`	`172`	`{`
`173`	`173`	`_clientSocket.EnableBroadcast = value;`
	`174`	`+ _isBroadcastSetByUser = true;`
`174`	`175`	`}`
`175`	`176`	`}`
`176`	`177`
`@@ -247,14 +248,18 @@ protected virtual void Dispose(bool disposing)`
`247`	`248`	`}`
`248`	`249`
`249`	`250`	`private bool _isBroadcast;`
	`251`	`+ private bool _isBroadcastSetByUser;`
`250`	`252`	`private void CheckForBroadcast(IPAddress ipAddress)`
`251`	`253`	`{`
`252`	`254`	`// Here we check to see if the user is trying to use a Broadcast IP address`
`253`	`255`	`// we only detect IPAddress.Broadcast (which is not the only Broadcast address)`
`254`	`256`	`// and in that case we set SocketOptionName.Broadcast on the socket to allow its use.`
`255`	`257`	`// if the user really wants complete control over Broadcast addresses they need to`
`256`	`258`	`// inherit from UdpClient and gain control over the Socket and do whatever is appropriate.`
`257`		`- if (_clientSocket != null && !_isBroadcast && IsBroadcast(ipAddress))`
	`259`	`+ //`
	`260`	`+ // If the user has explicitly set EnableBroadcast, we respect their choice`
	`261`	`+ // and do not auto-enable broadcast.`
	`262`	`+ if (_clientSocket != null && !_isBroadcast && !_isBroadcastSetByUser && IsBroadcast(ipAddress))`
`258`	`263`	`{`
`259`	`264`	`// We need to set the Broadcast socket option.`
`260`	`265`	`// Note that once we set the option on the Socket we never reset it.`