So yes, it is possible for the application code (using normal BSD-style socket API under Linux) to cause this RST packet: by using a nonblocking socket and closing the connection while the three-way handshake is still in progress. The three message mechanism is designed so that two computers that want to pass information back and forth to each other can negotiate the parameters of the connection before transmitting data such as HTTP browser requests. That's why it would reply with a RST packet to the server. The three messages transmitted by TCP to negotiate and start a TCP session are nicknamed SYN, SYN-ACK, and ACK for SYNchronize, SYNchronize-ACKnowledgement, and ACKnowledge respectively. When the delayed reply finally arrived at the client host, the kernel would see the socket was already closed, and would treat the SYN,ACK reply as belonging to an invalid socket. In some rare cases the SYN,ACK packets from the server would be delayed by several hundred milliseconds on their way through the network. This is useful to scan a large number of hosts/networks. GitHub - tevino/tcp-shaker: Perform TCP handshake. The -iL option allows you to read the list of target systems using a text file. Known as the 'SYN, SYN-ACK, ACK handshake,' computer A transmits a SYNchronize packet to computer B, which sends. heartbeat: Perform TCP handshake without ACK in Go, useful for health check, that is SYN, SYN-ACK, RST. Even though, this packet is not carrying any data but connection settings, the acknowledgement number is increased by 1 which tells the client it has received the SYN packet while it sets its sequence number to zero. Looks like the capture was taken at the server and the server respopnds to the income connection attempt SYN with a SYN, ACK, but the client fails to complete the TCP 3-way handshake with an ACK, instead the client resends the SYN. Unfortunately the client code was very impatient: if the connection wasn't established after a few hundred milliseconds, the client would call close() on the socket. The procedure that takes place between two TCP/IP nodes to establish a connection. The second packet (it is also called SYN/ACK packet) from the server to the client is pretty similar to the first packet, except ACK flag being set to 1 this time. Analysis by screenshot is always tricky, much better to share the capture. By default, the threshold is 512 connections from any single IP address. After the number of connections from the same IP address reaches the SYN-ACK-ACK proxy threshold, the NetScreen device rejects further connection requests from that IP address. The client in this case uses nonblocking sockets, so the connect() call will return immediately while the kernel keeps waiting for the SYN,ACK packet from server. To prevent such an attack, you can enable the SYN-ACK-ACK proxy protection SCREEN option. For closure: the problem was a stupid bug in my code (in the client).
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