added a new extension to manipulate the connection

core/src/main/java/hudson/ProxyConfiguration.java

@@ -156,6 +156,10 @@ public final class ProxyConfiguration implements Saveable {
                 }
             });
         }
+
+        for (URLConnectionDecorator d : URLConnectionDecorator.all())
+            d.decorate(con);
+
         return con;
     }
 

core/src/main/java/hudson/URLConnectionDecorator.java

+/*
+ * The MIT License
+ *
+ * Copyright (c) 2004-2009, Sun Microsystems, Inc., Kohsuke Kawaguchi
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package hudson;
+
+import jenkins.model.Jenkins;
+
+import java.io.IOException;
+import java.net.URL;
+import java.net.URLConnection;
+
+/**
+ * Decorates the connections that Jenkins open to access external resources.
+ *
+ * @see ProxyConfiguration#open(URL)
+ * @author Kohsuke Kawaguchi
+ * @since 1.426
+ */
+public abstract class URLConnectionDecorator implements ExtensionPoint {
+    /**
+     * Called before it gets connected. Can be used to tweak parameters.
+     */
+    public abstract void decorate(URLConnection con) throws IOException;
+
+    /**
+     * Returns all the registered {@link URLConnectionDecorator}s.
+     */
+    public static ExtensionList<URLConnectionDecorator> all() {
+        return Jenkins.getInstance().getExtensionList(URLConnectionDecorator.class);
+    }
+}