title: Zookeeper 3.6.0 TCP requirements changed?
body: <p>After upgrading 1 of the existing v3.5.7 servers too V3.6.0 I found that the new node would not join the existing Zookeeper cluster, instead it would enter an endless loop of the following.</p>
  
  <p>Logs on the Existing node in cluster reported</p>
  
  <pre><code> [myid:1] - WARN  [QuorumPeer[myid=1](plain=[0:0:0:0:0:0:0:0]:2181).    (secure=disabled):QuorumCnxManager@685] - Cannot open channel to 3 at election address service-    zk-x.xxxxxxx.xxxxxxx/172.26.47.92:3888
  java.net.ConnectException: Connection refused (Connection refused)
      at java.net.PlainSocketImpl.socketConnect(Native Method)
      at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
      at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
      at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
      at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
      at java.net.Socket.connect(Socket.java:607)
      at      org.apache.zookeeper.server.quorum.QuorumCnxManager.connectOne(QuorumCnxManager.java:656)
      at org.apache.zookeeper.server.quorum.QuorumCnxManager.connectOne(QuorumCnxManager.java:720)
      at     org.apache.zookeeper.server.quorum.QuorumCnxManager.connectAll(QuorumCnxManager.java:741)
      at     org.apache.zookeeper.server.quorum.FastLeaderElection.lookForLeader(FastLeaderElection.java:910)
      at org.apache.zookeeper.server.quorum.QuorumPeer.run(QuorumPeer.java:1229)
  </code></pre>
  
  <p>Running a tcpdump on an existing node the following </p>
  
  <pre><code>IP 172.26.47.18.7 &gt; 172.26.47.92.36638: Flags [R.], seq 0, ack 3465718752, win 0, length 0
  </code></pre>
  
  <p>.92 is the new server (V3.6.0)</p>
  
  <p>After enabling port 7 on the firewall the cluster election was successful and the new server joined the cluster as normal.   I might have missed this in the release notes for v3.6.  Can anyone confirm this change?</p>