wso2 esb proxy cannot return response sometimes

Question

I am working on esb4.8.1, and uses proxy as communication bridge between client and server.

To validate the esb proxy stability, I create a dummy proxy as below: In the monitor log, some times I got http response like "Permission denied: connect". Most time the proxy works pretty well.

<?xml version="1.0" encoding="UTF-8"?>
   <proxy xmlns="http://ws.apache.org/ns/synapse"
   name="EchoProxy"
   transports="http"
   startOnLoad="true"
   trace="enable"
   statistics="enable">   
    <description/>
    <target>
     <inSequence>
         <script language="js">
        mc.setPayloadXML(
        &lt;greeting&gt;Hello World&lt;/greeting&gt;
        );
    </script>
    <header name="To" action="remove"/>
    <property name="NO_ENTITY_BODY" scope="axis2" action="remove"/>
    <property name="RESPONSE" value="true"/>
    <send/>
    <log level="full"/>
     </inSequence>  
    </target>
</proxy>