netstat -an connection

The CLOSE_WAIT state you see when running the netstat -an | grep 9011 command refers to a state in the TCP connection lifecycle. Specifically, CLOSE_WAIT means that the local side (your application or server) has received a request from the remote side (client) to close the connection, but the local application has not yet fully closed the connection.

TCP Connection Lifecycle Overview

In a TCP connection, there are several states that a connection can go through. Here’s a quick overview:

1. ESTABLISHED: The connection is open, and data can be exchanged.
2. FIN_WAIT_1: The local side has initiated a connection termination (sent FIN), and is waiting for the remote side to acknowledge it.
3. FIN_WAIT_2: The remote side has acknowledged the FIN and is now waiting for the local side to acknowledge the connection termination.
4. CLOSE_WAIT: The remote side has sent a FIN to initiate the termination of the connection, and the local side has acknowledged it. Now, the local side is waiting for the application to close the socket.
5. LAST_ACK: The local side has sent a FIN to the remote side but is waiting for an acknowledgment from the remote side.
6. TIME_WAIT: The connection is fully closed, and the system is waiting to ensure that all packets are transmitted and received before completely removing the connection.
7. CLOSED: The connection is completely closed and no longer exists.

What Does CLOSE_WAIT Mean?

In the CLOSE_WAIT state:

• Remote side (client) has sent a FIN to the local side (server).
• Local side (server) has acknowledged the FIN and is waiting for the local application to close the connection.

The key thing is that the local application (your service or program) has not yet called close() on the socket, which is necessary to initiate the next state (LAST_ACK) and eventually close the connection properly.

Why Do You See Many CLOSE_WAIT States?

Here are a few reasons why you might see many CLOSE_WAIT connections:

1. Application Not Closing Sockets Properly:

   • The most common cause of a large number of CLOSE_WAIT states is that your application is not properly closing the sockets after it is done communicating. If your service receives a FIN (connection termination request) but does not call socket.close() or its equivalent, the connection stays in the CLOSE_WAIT state.

2. Delayed Socket Cleanup:

   • If your application has a large number of open connections or if it is slow to process requests, it may take a while to clean up all the sockets properly, leaving many connections in the CLOSE_WAIT state for a longer time.

3. Server/Application Configuration:

   • If the server or application is not correctly handling connection terminations, such as not properly reading from the socket or not calling shutdown() or close() on the socket, this can lead to many CLOSE_WAIT states.

4. Network or Client Behavior:

   • If clients are not gracefully closing the connection or if there are issues on the client side that prevent the connection from being fully closed, the local server may end up with connections in the CLOSE_WAIT state.

How to Resolve or Investigate Further

• Check Application Code: Ensure that your application is correctly closing sockets after completing communication. In Java, for example, you should always close sockets in a finally block to guarantee they get closed regardless of errors.

  Socket socket = new Socket(); try { // Use socket } finally { if (socket != null) { socket.close(); // Ensure the socket is closed } }

• Review Server Logs: Check your application and server logs for any errors or warnings related to socket connections that may indicate why connections are not being properly closed.

• Increase Socket Timeouts: If connections are stuck in CLOSE_WAIT for too long, it might help to adjust socket timeouts on both the server and client sides to ensure connections are closed more promptly.

• Check Client Behavior: Ensure that the clients are properly closing the connections after finishing communication.

• System Limitations: If you're seeing a large number of CLOSE_WAIT connections and your system is getting overloaded, you might need to review the system limits, such as the number of open file descriptors allowed (ulimit -n), or fine-tune other operating system parameters related to networking.

In Summary:

The CLOSE_WAIT state indicates that the remote side has requested the termination of the connection, and the local system is waiting for the application to close the socket. It’s typically a sign that the application is not properly closing its connections, so you'll need to check your application logic and ensure that sockets are closed correctly after use.