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Why my server does not respond to client's [SYN]?

asked 2018-11-27 13:48:47 +0000

anonymous user


updated 2018-11-28 03:33:11 +0000

I have connected a Client ECU to my Server. I am trying to establish is a 3 way handshake. When I simulate my configuration the client ECU sends me the first message of 3 way handshake (SYN) message.

Client address: 00:18:23:11:xx:xx
Server address: 10:E7:C6:0C:xx:xx

Source Port (Client)= 49152
Destination Port (Server): 443

Client [Source]= fe80::xxx:xxxx:fe11:0
Server [Destination]=fe80::3c7b:72ca:xxxx:xxxx

I am not able to respond to the client with a SYN ACK. I am not sure what could be the issue. what are the possible solutions ? All the data is captured by me on wireshark on port443. I am not able to upload any file as of now.

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From the posts I have read here, understood that it might be the firewall blocking my server from responding to the client. Still not sure what might be the problem.

Nikhil gravatar imageNikhil ( 2018-11-28 03:34:18 +0000 )edit

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answered 2018-11-28 13:16:26 +0000

sindy gravatar image

Wireshark tells you what has happened, but rarely why it has happened. So you have seen that the client initiates the session by sending SYN; the next step should be to run Wireshark or tcpdump on the server to see whether the SYN packet has arrived there. If yes, there is a firewall on the server itself or the application (http server) either does not listen at all or it has some internal whitelist or blacklist which doesn't accept requests from the IP address of the client, or there may be a routing problem as the server may lack a route for the client address, so it may send it out using the default route which cannot deliver the response to the client, or nowhere at all if none of the existing routes on the server matches the IP address of the client.

If the SYN doesn't reach the server, there is some firewall or a plain dysfunction somewhere between the client and the server.

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Hi @sindy, I found the cause why the server was not able to send it. It was the wrong IP address assigned by me on server side. Now I am able to receive TLS 3 way handshake and I receive a client hello message. Now it's this time, I am not able to send server hello to client. Any suggestions ?

Nikhil gravatar imageNikhil ( 2018-11-29 07:29:53 +0000 )edit

As the TCP session has been successfully established and the server has received data on it, the network layer seems to be fine (routes are routing in both directions, firewalls permit data to flow through).So I would look into the log file of the server application for explanation. Does the server actively terminate the connection after receiving the client hello, does it respond with something else than server hello, or it simply doesn't respond at all? Anything from incompatible TLS mode or invalid/misconfigred certificate chain to server side bug may be the root cause.

sindy gravatar imagesindy ( 2018-11-29 12:41:45 +0000 )edit

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Asked: 2018-11-27 13:48:47 +0000

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Last updated: Nov 28 '18