title: zookeeper server STARTED but ruok no output
body: <p>I have set in zoo.cfg : <code>clientPort=2181</code></p>
  
  <blockquote>
    <p>cloudera@cloudera-vm:/$ sudo /usr/lib/zookeeper/bin/zkServer.sh start</p>
  </blockquote>
  
  <p>I get the following response:</p>
  
  <pre><code>JMX enabled by default
  Using config: /usr/lib/zookeeper/bin/../conf/zoo.cfg
  Starting zookeeper ... STARTED
  </code></pre>
  
  <blockquote>
    <p>cloudera@cloudera-vm:/$ sudo /usr/lib/zookeeper/bin/zkServer.sh status</p>
  </blockquote>
  
  <pre><code>JMX enabled by default
  Using config: /usr/lib/zookeeper/bin/../conf/zoo.cfg
  Mode: standalone
  </code></pre>
  
  <p>But </p>
  
  <pre><code>$ echo ruok | nc localhost 2181
  $
  </code></pre>
  
  <p>is not giving any output. where is the problem?</p>
  
  <p>Log file:</p>
  
  <pre><code>2014-10-03 08:40:10,912 - INFO  [main:QuorumPeerConfig@90] - Reading configuration from: /usr/lib/zookeeper/bin/../conf/zoo.cfg
  2014-10-03 08:40:10,916 - ERROR [main:QuorumPeerConfig@259] - Invalid configuration, only one server specified (ignoring)
  2014-10-03 08:40:10,916 - WARN  [main:QuorumPeerMain@105] - Either no config or no quorum defined in config, running  in standalone mode
  2014-10-03 08:40:10,927 - INFO  [main:QuorumPeerConfig@90] - Reading configuration from: /usr/lib/zookeeper/bin/../conf/zoo.cfg
  2014-10-03 08:40:10,928 - ERROR [main:QuorumPeerConfig@259] - Invalid configuration, only one server specified (ignoring)
  2014-10-03 08:40:10,928 - INFO  [main:ZooKeeperServerMain@94] - Starting server
  2014-10-03 08:40:10,933 - INFO  [main:Environment@97] - Server environment:zookeeper.version=3.3.5-cdh3u6--1, built on 03/20/2013 20:16 GMT
  2014-10-03 08:40:10,933 - INFO  [main:Environment@97] - Server environment:host.name=cloudera-vm
  2014-10-03 08:40:10,933 - INFO  [main:Environment@97] - Server environment:java.version=1.6.0_24
  2014-10-03 08:40:10,934 - INFO  [main:Environment@97] - Server environment:java.vendor=Sun Microsystems Inc.
  2014-10-03 08:40:10,934 - INFO  [main:Environment@97] - Server environment:java.home=/usr/lib/jvm/java-6-sun-1.6.0.24/jre
  2014-10-03 08:40:10,934 - INFO  [main:Environment@97] - Server environment:java.class.path=/usr/lib/zookeeper/bin/../build/classes:/usr/lib/zookeeper/bin/../build/lib/*.jar:/usr/lib/zookeeper/bin/../zookeeper-3.3.5-cdh3u6.jar:/usr/lib/zookeeper/bin/../lib/log4j-1.2.15.jar:/usr/lib/zookeeper/bin/../lib/jline-0.9.94.jar:/usr/lib/zookeeper/bin/../src/java/lib/*.jar:/usr/lib/zookeeper/bin/../conf:
  2014-10-03 08:40:10,934 - INFO  [main:Environment@97] - Server environment:java.library.path=/usr/lib/jvm/java-6-sun-1.6.0.24/jre/lib/i386/client:/usr/lib/jvm/java-6-sun-1.6.0.24/jre/lib/i386:/usr/lib/jvm/java-6-sun-1.6.0.24/jre/../lib/i386:/usr/java/packages/lib/i386:/lib:/usr/lib
  2014-10-03 08:40:10,934 - INFO  [main:Environment@97] - Server environment:java.io.tmpdir=/tmp
  2014-10-03 08:40:10,934 - INFO  [main:Environment@97] - Server environment:java.compiler=&lt;NA&gt;
  2014-10-03 08:40:10,934 - INFO  [main:Environment@97] - Server environment:os.name=Linux
  2014-10-03 08:40:10,934 - INFO  [main:Environment@97] - Server environment:os.arch=i386
  2014-10-03 08:40:10,935 - INFO  [main:Environment@97] - Server environment:os.version=2.6.35-28-generic
  2014-10-03 08:40:10,935 - INFO  [main:Environment@97] - Server environment:user.name=root
  2014-10-03 08:40:10,935 - INFO  [main:Environment@97] - Server environment:user.home=/root
  2014-10-03 08:40:10,935 - INFO  [main:Environment@97] - Server environment:user.dir=/
  2014-10-03 08:40:10,940 - INFO  [main:ZooKeeperServer@673] - tickTime set to 2000
  2014-10-03 08:40:10,940 - INFO  [main:ZooKeeperServer@682] - minSessionTimeout set to -1
  2014-10-03 08:40:10,941 - INFO  [main:ZooKeeperServer@691] - maxSessionTimeout set to -1
  2014-10-03 08:40:10,953 - INFO  [main:NIOServerCnxn$Factory@143] - binding to port 0.0.0.0/0.0.0.0:2181
  2014-10-03 08:40:10,963 - INFO  [main:FileSnap@82] - Reading snapshot /var/zookeeper/version-2/snapshot.0
  2014-10-03 08:40:10,972 - INFO  [main:FileTxnSnapLog@256] - Snapshotting: 0
  2014-10-03 08:45:42,225 - INFO  [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxn$Factory@251] - Accepted socket connection from /127.0.0.1:39879
  2014-10-03 08:45:42,231 - INFO  [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxn@1237] - Processing srvr command from /127.0.0.1:39879
  2014-10-03 08:45:42,239 - INFO  [Thread-1:NIOServerCnxn@1435] - Closed socket connection for client /127.0.0.1:39879 (no session established for client)
  </code></pre>