A RST should be validated first, invalid RST is ignored. In all states except SYN-SENT, the sequence number should be in the receive window. And in the SYN-SENT state, the RST is invalid if the ACK field acknowledges the SYN ( since it's the first packet from another side, any sequence number is OK).
If the receiver is in SYN-RECEIVED and came from LISTEN, then it will go to LISTEN if got valid RST. For other state, the receiver will abort the connection and go to CLOSED state.
- connect to a port that has no server listen on it
- firewall may reject the connection via RST (you can config iptables to reject)
- routers in the middle may send RST
- userspace close socket without read received buffer in the kernel
- send to a closed socket
You can use the SO_LINER option to send a RST on close instead of FIN. Following are demo of python and c:
s.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER, struct.pack('ii', 1, 0))
s.close() struct linger lg = { .l_onoff = 1, .l_linger= 0};
if (setsockopt(sockfd, SOL_SOCKET, SO_LINGER, &lg, sizeof(lg)) != 0) {
// failed to set SO_LINGER
}
close(sockfd);