CLOSEWAIT

CLOSE_WAIT is a state in the Transmission Control Protocol (TCP) connection lifecycle. It occurs when the remote peer has sent a FIN to indicate it wants to close its end of the connection, and the local endpoint has acknowledged that request but has not yet closed its own end. In this state, the connection is half-closed: data can still be received from the remote side, but the local side must complete its closure.

The transition into CLOSE_WAIT happens after the local endpoint processes the remote’s FIN, often while the application using the socket is still running. The important responsibility during CLOSE_WAIT is for the local application to finish processing any remaining data and then close the socket. Until the application closes, resources associated with the connection (such as file descriptors and buffers) remain allocated.

From a practical standpoint, CLOSE_WAIT should be temporary. If a process remains in CLOSE_WAIT for an extended period, it can indicate that the application is not closing the socket promptly, potentially leading to resource leaks or exhaustion on the host. In diagnostics, netstat or other tools may show the state in the local endpoint’s connection table.

CLOSE_WAIT is one of several TCP states used to describe the lifecycle of a connection. It follows the receipt of a FIN and precedes the final termination steps after the local end closes its side. Understanding CLOSE_WAIT helps diagnose issues related to improper or delayed socket closure.