Packet sender tcp response data3/17/2023 ![]() If you don't have access to the server, doing the same thing on the closest router/firewall.Doing some tcpdump on the server to verify the network : Does the server receive the SYN packet or there is an issue in the network beforehand ? Does the server generate and send a SYN ACK packet ? And work from there. ![]() There can be a large number of thing that can make server doesn't answer SYN packet. The server is overwhelm and sometime cannot answer you SYN packet.Packet loss in the network : You SYN or SYN ACK packet is dropped.If the Client and server are on the same network, this can be some issue with Layer 2 issue like Spanning Tree protocol or ARP.The can be some routing issue or asymetric routing (this break firewall). You server has some troubles to find your client back.When the server app is stopped or crashed, the tcp listening socket is closed, so the OS doesn't answer. Application on the server is stopped when the SYN packet is sent : This can be a crash and then manage to work again with a auto restart mecanism of the program.Somes ideas on why sometimes your server doesn't answer with SYN-ACK : What can I do as a client to fix this problem? This problem only happens in noisy environment but not in a cleaner environment. My environment is quite noisy and busy with many routers plus communication devices. I cannot check the server as its a private server, so I am not very sure if it was dropped. Server not sending a SYN/ACK packet in response to a SYN packet Why would a server not send a SYN/ACK packet in response to a SYN packet However, sometimes server2 did not response to client's SYN Packet which is in Step 9. Client ends connection with server2 by sending TCP RST packet and close socket. ![]()
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