-Dhudson.udp=-1 to disable UDP broadcast.

changelog.html

@@ -55,7 +55,8 @@ Upcoming changes</a>
 <!-- Record your changes in the trunk here. -->
 <div id="trunk" style="display:none"><!--=TRUNK-BEGIN=-->
 <ul class=image>
-  <li class=>
+  <li class=rfe>
+    Can use <code>-Dhudson.udp=-1</code> to disable UDP broadcast without showing an ugly exception.
 </ul>
 </div><!--=TRUNK-END=-->
 

core/src/main/java/jenkins/model/Jenkins.java

@@ -820,11 +820,13 @@ public class Jenkins extends AbstractCIBase implements ModifiableTopLevelItemGro
             } else
                 tcpSlaveAgentListener = null;
 
-            try {
-                udpBroadcastThread = new UDPBroadcastThread(this);
-                udpBroadcastThread.start();
-            } catch (IOException e) {
-                LOGGER.log(Level.WARNING, "Failed to broadcast over UDP",e);
+            if (UDPBroadcastThread.PORT != -1) {
+                try {
+                    udpBroadcastThread = new UDPBroadcastThread(this);
+                    udpBroadcastThread.start();
+                } catch (IOException e) {
+                    LOGGER.log(Level.WARNING, "Failed to broadcast over UDP (use -Dhudson.udp=-1 to disable)", e);
+                }
             }
             dnsMultiCast = new DNSMultiCast(this);