TCP port occupation: Why is the port still occupied after the server program exits and how to solve it?

Python TCP server port occupation problem: The port is still occupied after the program exits and the solution

When developing a TCP server in Python, a common problem is that after the server program is closed, the target port is still in an occupied state, resulting in the inability to restart the server immediately. This article will explore this problem in depth and provide effective solutions.

Problem: Developers use socket.socket() to create a TCP server and combine multiprocessing.pool to process client requests. After the server terminated unexpectedly, lsof -i :6001 does not show that port 6001 is occupied, but OSError: [Errno 98] Address already in use error occurred during restart. However, netstat -anp | grep 6001 shows a large number of connections in TIME_WAIT state.

Cause: This is caused by the TIME_WAIT status of the TCP connection. When the server exits abnormally, some TCP connections may not be closed correctly and enter TIME_WAIT state. The operating system retains these ports for a period of time (mins to tens of minutes) to ensure data transmission reliability. lsof focuses on the association between the process and the file, while the TIME_WAIT connection is not directly held by any process, so lsof cannot be detected; netstat can display the network connection status, so you can see the TIME_WAIT connection.

Solution: Set the SO_REUSEADDR option before the server program binds the port. This option allows the port to be bound immediately when it is in the TIME_WAIT state. The modified code is as follows:

 serversocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
serversocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) # Add serversocket.bind before bind(('0.0.0.0', port))
# ...Other codes...

Adding serversocket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) can solve the port occupation problem caused by TIME_WAIT status. Linux 3.9 and later kernels have added the SO_REUSEPORT option, which is more refined than SO_REUSEADDR . It is recommended to set these two options at the same time. Windows systems may also need to set the SO_EXCLUSIVEADDRUSE option.

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