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I have two servers and I used my own embedded system with LwIP to do connection to these server.

My embedded system with LwIP is the client and I have server1 and server2. I connected to server1 and end the connection before connecting to server2.

Further breakdown on the flow:

  1. Client creates New Socket with server1
  2. Client sent DNS packet to obtain server1's ip address; received ACK from AP
  3. Client send TCP SYN packet;
  4. Server1 send TCP SYN-ACK and perform some data transmission
  5. Client ends connection with server1 by sending TCP RST packet; and close socket
  6. Client creates New Socket with server2
  7. Client sent DNS packet to obtain server2's ip address; received ACK from AP
  8. Client send TCP SYN packet to server2
  9. Server2 send TCP SYN-ACK and perform some data transmission
  10. Client ends connection with server2 by sending TCP RST packet; and close socket

However, sometimes server2 did not response to client's SYN Packet which is in Step 9. Its only happens sometime. I checked several forum like:

[1] Why would a server not send a SYN/ACK packet in response to a SYN packet

[2] Server not sending a SYN/ACK packet in response to a SYN packet

My code does not enable window scaling. I cannot check the server as its a private server, so I am not very sure if it was dropped. My environment is quite noisy and busy with many routers plus communication devices. This problem only happens in noisy environment but not in a cleaner environment.

What can I do as a client to fix this problem?

Sue Koh
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1 Answers1

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Somes ideas on why sometimes your server doesn't answer with SYN-ACK :

  • 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. When the server app is stopped or crashed, the tcp listening socket is closed, so the OS doesn't answer.
  • You server has some troubles to find your client back. The can be some routing issue or asymetric routing (this break firewall).
  • 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.
  • Packet loss in the network : You SYN or SYN ACK packet is dropped.
  • The server is overwhelm and sometime cannot answer you SYN packet.

There can be a large number of thing that can make server doesn't answer SYN packet.

What I will do :

  • 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.
  • If you don't have access to the server, doing the same thing on the closest router/firewall.
  • Contact someone that has access on the server.
Nicox
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  • Thank you for the awesome feedback. I will try to get access to the server, if possible. I have another question. Because I am using lwip and apparently I failed to get syn-ack from server, my lwip_connect will stop and doesn't return a value (most probably blocking). Also there is no retry in the SYN packet, any idea why is that so? The port I'm using is 8883 for secure mqtt. – Sue Koh Apr 02 '22 at 15:19
  • Unfortunately I don't have any knowledge on "lwip". What I can say though, is that when trying to send a SYN packet and you don't have any answer, the client is supposed to retransmit that SYN packet multiple times before giving up. This seams more like an issue with lwip. – Nicox Apr 02 '22 at 17:23
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