wso2 esb proxy cannot return response sometimes - wso2

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(
<greeting>Hello World</greeting>
);
</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>