This post will provide details on TCP connections reporting ‘CLOSE_WAIT’ states. The possible state values for TCP sockets are as follows:

BOUND Bound, ready to connect or listen.
CLOSED Closed. The socket is not being used.
CLOSING Closed, then remote shutdown; awaiting acknowledgment.
CLOSE_WAIT Remote shutdown; waiting for the socket to close.
ESTABLISHED Connection has been established.
FIN_WAIT_1 Socket closed; shutting down connection.
FIN_WAIT_2 Socket closed; waiting for shutdown from remote.
IDLE Idle, opened but not bound.
LAST_ACK Remote shutdown, then closed; awaiting acknowledgment.
LISTEN Listening for incoming connections.
SYN_RECEIVED Active/initiate synchronization received and the connection under way
SYN_SENT Actively trying to establish connection.
TIME_WAIT Wait after close for remote shutdown retransmission.

‘CLOSE_WAIT‘ state means the other end of the connection has been closed while the local end is still waiting for the application to close.

Details

‘CLOSE_WAIT’ state on tcp connections occurs if the system has not received a close system call from the application, after having received notification (‘FIN’ packet) from the other system that it has closed its endpoint. In other words it means that the local end of the connection has received ‘FIN’ from the other end, but the OS is waiting for the program at the local end to actually close its connection.

The problem is that a program running on the local machine is not closing the socket. It is not a TCP tuning issue. A connection can stay in ‘CLOSE_WAIT’ forever while the program holds the connection open. So mostly this issue happens due to an application bug. However if TCP/IP parameters are not properly set, closed TCP/IP connections will stay for a very long time in various ‘CLOSE’ states which take file descriptors from the process. In order to solve this problem, sometimes it is necessary to tune TCP/IP parameters in /etc/sysctl.conf (such as those net.ipv4.tcp_xxx parameters), so that TCP/IP connections are closed in a short amount of time.