Add /_cat/master.

% curl localhost:9200/_cat/master
id                     ip             node
Zdumn8bkTuOGRLfr9JL8jQ 192.168.20.109 Petros, Dominic

src/main/java/org/elasticsearch/rest/action/RestActionModule.java

@@ -67,6 +67,7 @@ import org.elasticsearch.rest.action.admin.indices.warmer.delete.RestDeleteWarme
 import org.elasticsearch.rest.action.admin.indices.warmer.get.RestGetWarmerAction;
 import org.elasticsearch.rest.action.admin.indices.warmer.put.RestPutWarmerAction;
 import org.elasticsearch.rest.action.bulk.RestBulkAction;
+import org.elasticsearch.rest.action.cat.RestMasterAction;
 import org.elasticsearch.rest.action.cat.RestShardsAction;
 import org.elasticsearch.rest.action.count.RestCountAction;
 import org.elasticsearch.rest.action.delete.RestDeleteAction;
@@ -183,5 +184,6 @@ public class RestActionModule extends AbstractModule {
         bind(RestExplainAction.class).asEagerSingleton();
 
         bind(RestShardsAction.class).asEagerSingleton();
+        bind(RestMasterAction.class).asEagerSingleton();
     }
 }

src/main/java/org/elasticsearch/rest/action/cat/RestMasterAction.java

@@ -0,0 +1,95 @@
+/*
+ * Licensed to ElasticSearch and Shay Banon under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. ElasticSearch licenses this
+ * file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.rest.action.cat;
+
+import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.admin.cluster.state.ClusterStateRequest;
+import org.elasticsearch.action.admin.cluster.state.ClusterStateResponse;
+import org.elasticsearch.client.Client;
+import org.elasticsearch.common.Table;
+import org.elasticsearch.common.inject.Inject;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.transport.InetSocketTransportAddress;
+import org.elasticsearch.rest.*;
+import org.elasticsearch.rest.action.support.RestTable;
+
+import java.io.IOException;
+
+import static org.elasticsearch.rest.RestRequest.Method.GET;
+
+public class RestMasterAction extends BaseRestHandler {
+
+    @Inject
+    public RestMasterAction(Settings settings, Client client, RestController controller) {
+        super(settings, client);
+        controller.registerHandler(GET, "/_cat/master", this);
+    }
+
+    @Override
+    public void handleRequest(final RestRequest request, final RestChannel channel) {
+        final ClusterStateRequest clusterStateRequest = new ClusterStateRequest();
+        clusterStateRequest.filterMetaData(true);
+        clusterStateRequest.local(request.paramAsBoolean("local", clusterStateRequest.local()));
+        clusterStateRequest.masterNodeTimeout(request.paramAsTime("master_timeout", clusterStateRequest.masterNodeTimeout()));
+
+        client.admin().cluster().state(clusterStateRequest, new ActionListener<ClusterStateResponse>() {
+            @Override
+            public void onResponse(final ClusterStateResponse clusterStateResponse) {
+                try {
+                    channel.sendResponse(RestTable.buildResponse(buildTable(clusterStateResponse), request, channel));
+                } catch (Throwable e) {
+                    onFailure(e);
+                }
+            }
+
+            @Override
+            public void onFailure(Throwable e) {
+                try {
+                    channel.sendResponse(new XContentThrowableRestResponse(request, e));
+                } catch (IOException e1) {
+                    logger.error("Failed to send failure response", e1);
+                }
+            }
+        });
+    }
+
+    private Table buildTable(ClusterStateResponse state) {
+        Table table = new Table();
+        table.startHeaders()
+                .addCell("id")
+                .addCell("ip")
+                .addCell("node")
+                .endHeaders();
+
+        String masterId = state.getState().nodes().masterNodeId();
+        String masterIp = ((InetSocketTransportAddress) state.getState().nodes().get(masterId).address()).address().getAddress().getHostAddress();
+        String masterName = state.getState().nodes().masterNode().name();
+
+        table.startRow();
+
+        table.addCell(masterId);
+        table.addCell(masterIp);
+        table.addCell(masterName);
+
+        table.endRow();
+
+        return table;
+    }
+}