TNS-12502 Error Thrown Repeatedly in Listener Log


Oracle Net Services - Version to [Release 9.2 to 11.2]
Information in this document applies to any platform.
***Checked for relevance on 6-JUL-2016***


Entries like the following appear regularly in the listener log:

TNS-12502: TNS:listener received no CONNECT_DATA from client


This message is often caused by devices or processes that probe a range of TCP ports. When the probe hits the listener, the listener throws this error.

Check for a pattern by examining the timestamps in the listener log where these errors are thrown. For example, every 3 minutes or every 5 minutes.

If a pattern exists, this is likely a port monitoring tool.

1. Probing the TCP port means that the client is closing the connection as soon as it was opened (he just wants to see whether the port is open for access). This breaks the SQL*Net protocol which is waiting for the client request data upon the connection is opened.

2. Other potentials:

Look for any application that is trying to open a socket against the IP Address and port of the listener.

For example:

a) ftp "host" "port"; or

b) the execution of the following "class" can cause this message:

public class Test

  public static void main (String args [])
    System.out.println("Before Connection"); connection;
    try {
    connection = new"",1521);
    catch( e ) {System.out.println("Error");}

3. Other causes may lie with dropped connections due to lack of resources or defective client applications which are failing to correctly connect with the listener, per the SQL*Net protocol.


Interrupt monitoring tool activity or restrict access from that client.
Be aware that TCP probes may also indicate hostile activity with regards to network security. Please review this matter with your system/network administrator.