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States of a [TCP] connection. These can be seen with netstat -at ;CLOSED:A theoretical state where a [TCP] connection doesn't exist yet. ;LISTEN:A state where [TCP] connection is waiting for a connection to be made to it. ;SYN_RCVD: A [SYN] has been received, a SYN ACK has been sent, and we are waiting for an [ACK] ;SYN_SENT: A [SYN] has been sent to start a connection, the SYN ACK hasn't yet been received. ;ESTABLISHED: A connection has been established ;CLOSE_WAIT:After a [TCP] connection has been closed, the spec says you should make sure that the connection is not reopened for a few minutes to prevent packets that were delayed in the Internet causing the connection to be reopened. These will disappear by themselves after a while. This can cause problems on servers that have lots of very short connections (such as a web server) by filling up the OS's connection table. See also [EADDRINUSE] ;LAST_ACK:A FIN has been received, we've sent our own FIN, but we are waiting on the ACK before we can move to the closed state. ;FIN_WAIT_1:A FIN has been sent, waiting for a ACK or FIN ;FIN_WAIT_2:A FIN has been sent, and an ACK has been received. More data can be received. This connection is half open. ;CLOSING:A FIN has been sent, and one received, the FIN hasn't been ACK'd yet.
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TcpStates
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FIN
SYN
RST
TCP